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• How to Write a Literature Review | Guide, Examples, & Templates

How to Write a Literature Review | Guide, Examples, & Templates

Published on January 2, 2023 by Shona McCombes . Revised on September 11, 2023.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research that you can later apply to your paper, thesis, or dissertation topic .

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates, and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarize sources—it analyzes, synthesizes , and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

What is the purpose of a literature review, examples of literature reviews, step 1 – search for relevant literature, step 2 – evaluate and select sources, step 3 – identify themes, debates, and gaps, step 4 – outline your literature review’s structure, step 5 – write your literature review, free lecture slides, other interesting articles, frequently asked questions, introduction.

• Quick Run-through
• Step 1 & 2

When you write a thesis , dissertation , or research paper , you will likely have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and its scholarly context
• Develop a theoretical framework and methodology for your research
• Position your work in relation to other researchers and theorists
• Show how your research addresses a gap or contributes to a debate
• Evaluate the current state of research and demonstrate your knowledge of the scholarly debates around your topic.

Writing literature reviews is a particularly important skill if you want to apply for graduate school or pursue a career in research. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research problem and questions .

Make a list of keywords

Start by creating a list of keywords related to your research question. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list as you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some useful databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can also use boolean operators to help narrow down your search.

Make sure to read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

You likely won’t be able to read absolutely everything that has been written on your topic, so it will be necessary to evaluate which sources are most relevant to your research question.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models, and methods?
• Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible , and make sure you read any landmark studies and major theories in your field of research.

You can use our template to summarize and evaluate sources you’re thinking about using. Click on either button below to download.

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It is important to keep track of your sources with citations to avoid plagiarism . It can be helpful to make an annotated bibliography , where you compile full citation information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

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To begin organizing your literature review’s argument and structure, be sure you understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly visual platforms like Instagram and Snapchat—this is a gap that you could address in your own research.

There are various approaches to organizing the body of a literature review. Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarizing sources in order.

Try to analyze patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organize your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text , your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, you can follow these tips:

• Summarize and synthesize: give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: don’t just paraphrase other researchers — add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically evaluate: mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: use transition words and topic sentences to draw connections, comparisons and contrasts

In the conclusion, you should summarize the key findings you have taken from the literature and emphasize their significance.

When you’ve finished writing and revising your literature review, don’t forget to proofread thoroughly before submitting. Not a language expert? Check out Scribbr’s professional proofreading services !

This article has been adapted into lecture slides that you can use to teach your students about writing a literature review.

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If you want to know more about the research process , methodology , research bias , or statistics , make sure to check out some of our other articles with explanations and examples.

• Sampling methods
• Simple random sampling
• Stratified sampling
• Cluster sampling
• Likert scales
• Reproducibility

 Statistics

• Null hypothesis
• Statistical power
• Probability distribution
• Effect size
• Poisson distribution

Research bias

• Optimism bias
• Cognitive bias
• Implicit bias
• Hawthorne effect
• Anchoring bias
• Explicit bias

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarize yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your thesis or dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

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How to write a literature review introduction (+ examples)

The introduction to a literature review serves as your reader’s guide through your academic work and thought process. Explore the significance of literature review introductions in review papers, academic papers, essays, theses, and dissertations. We delve into the purpose and necessity of these introductions, explore the essential components of literature review introductions, and provide step-by-step guidance on how to craft your own, along with examples.

Why you need an introduction for a literature review

When you need an introduction for a literature review, what to include in a literature review introduction, examples of literature review introductions, steps to write your own literature review introduction.

A literature review is a comprehensive examination of the international academic literature concerning a particular topic. It involves summarizing published works, theories, and concepts while also highlighting gaps and offering critical reflections.

In academic writing , the introduction for a literature review is an indispensable component. Effective academic writing requires proper paragraph structuring to guide your reader through your argumentation. This includes providing an introduction to your literature review.

It is imperative to remember that you should never start sharing your findings abruptly. Even if there isn’t a dedicated introduction section .

Instead, you should always offer some form of introduction to orient the reader and clarify what they can expect.

There are three main scenarios in which you need an introduction for a literature review:

• Academic literature review papers: When your literature review constitutes the entirety of an academic review paper, a more substantial introduction is necessary. This introduction should resemble the standard introduction found in regular academic papers.
• Literature review section in an academic paper or essay: While this section tends to be brief, it’s important to precede the detailed literature review with a few introductory sentences. This helps orient the reader before delving into the literature itself.
• Literature review chapter or section in your thesis/dissertation: Every thesis and dissertation includes a literature review component, which also requires a concise introduction to set the stage for the subsequent review.

You may also like: How to write a fantastic thesis introduction (+15 examples)

It is crucial to customize the content and depth of your literature review introduction according to the specific format of your academic work.

In practical terms, this implies, for instance, that the introduction in an academic literature review paper, especially one derived from a systematic literature review , is quite comprehensive. Particularly compared to the rather brief one or two introductory sentences that are often found at the beginning of a literature review section in a standard academic paper. The introduction to the literature review chapter in a thesis or dissertation again adheres to different standards.

Here’s a structured breakdown based on length and the necessary information:

Academic literature review paper

The introduction of an academic literature review paper, which does not rely on empirical data, often necessitates a more extensive introduction than the brief literature review introductions typically found in empirical papers. It should encompass:

• The research problem: Clearly articulate the problem or question that your literature review aims to address.
• The research gap: Highlight the existing gaps, limitations, or unresolved aspects within the current body of literature related to the research problem.
• The research relevance: Explain why the chosen research problem and its subsequent investigation through a literature review are significant and relevant in your academic field.
• The literature review method: If applicable, describe the methodology employed in your literature review, especially if it is a systematic review or follows a specific research framework.
• The main findings or insights of the literature review: Summarize the key discoveries, insights, or trends that have emerged from your comprehensive review of the literature.
• The main argument of the literature review: Conclude the introduction by outlining the primary argument or statement that your literature review will substantiate, linking it to the research problem and relevance you’ve established.
• Preview of the literature review’s structure: Offer a glimpse into the organization of the literature review paper, acting as a guide for the reader. This overview outlines the subsequent sections of the paper and provides an understanding of what to anticipate.

By addressing these elements, your introduction will provide a clear and structured overview of what readers can expect in your literature review paper.

Regular literature review section in an academic article or essay

Most academic articles or essays incorporate regular literature review sections, often placed after the introduction. These sections serve to establish a scholarly basis for the research or discussion within the paper.

In a standard 8000-word journal article, the literature review section typically spans between 750 and 1250 words. The first few sentences or the first paragraph within this section often serve as an introduction. It should encompass:

• An introduction to the topic: When delving into the academic literature on a specific topic, it’s important to provide a smooth transition that aids the reader in comprehending why certain aspects will be discussed within your literature review.
• The core argument: While literature review sections primarily synthesize the work of other scholars, they should consistently connect to your central argument. This central argument serves as the crux of your message or the key takeaway you want your readers to retain. By positioning it at the outset of the literature review section and systematically substantiating it with evidence, you not only enhance reader comprehension but also elevate overall readability. This primary argument can typically be distilled into 1-2 succinct sentences.

In some cases, you might include:

• Methodology: Details about the methodology used, but only if your literature review employed a specialized method. If your approach involved a broader overview without a systematic methodology, you can omit this section, thereby conserving word count.

By addressing these elements, your introduction will effectively integrate your literature review into the broader context of your academic paper or essay. This will, in turn, assist your reader in seamlessly following your overarching line of argumentation.

Introduction to a literature review chapter in thesis or dissertation

The literature review typically constitutes a distinct chapter within a thesis or dissertation. Often, it is Chapter 2 of a thesis or dissertation.

Some students choose to incorporate a brief introductory section at the beginning of each chapter, including the literature review chapter. Alternatively, others opt to seamlessly integrate the introduction into the initial sentences of the literature review itself. Both approaches are acceptable, provided that you incorporate the following elements:

• Purpose of the literature review and its relevance to the thesis/dissertation research: Explain the broader objectives of the literature review within the context of your research and how it contributes to your thesis or dissertation. Essentially, you’re telling the reader why this literature review is important and how it fits into the larger scope of your academic work.
• Primary argument: Succinctly communicate what you aim to prove, explain, or explore through the review of existing literature. This statement helps guide the reader’s understanding of the review’s purpose and what to expect from it.
• Preview of the literature review’s content: Provide a brief overview of the topics or themes that your literature review will cover. It’s like a roadmap for the reader, outlining the main areas of focus within the review. This preview can help the reader anticipate the structure and organization of your literature review.
• Methodology: If your literature review involved a specific research method, such as a systematic review or meta-analysis, you should briefly describe that methodology. However, this is not always necessary, especially if your literature review is more of a narrative synthesis without a distinct research method.

By addressing these elements, your introduction will empower your literature review to play a pivotal role in your thesis or dissertation research. It will accomplish this by integrating your research into the broader academic literature and providing a solid theoretical foundation for your work.

Comprehending the art of crafting your own literature review introduction becomes significantly more accessible when you have concrete examples to examine. Here, you will find several examples that meet, or in most cases, adhere to the criteria described earlier.

Example 1: An effective introduction for an academic literature review paper

To begin, let’s delve into the introduction of an academic literature review paper. We will examine the paper “How does culture influence innovation? A systematic literature review”, which was published in 2018 in the journal Management Decision.

The entire introduction spans 611 words and is divided into five paragraphs. In this introduction, the authors accomplish the following:

• In the first paragraph, the authors introduce the broader topic of the literature review, which focuses on innovation and its significance in the context of economic competition. They underscore the importance of this topic, highlighting its relevance for both researchers and policymakers.
• In the second paragraph, the authors narrow down their focus to emphasize the specific role of culture in relation to innovation.
• In the third paragraph, the authors identify research gaps, noting that existing studies are often fragmented and disconnected. They then emphasize the value of conducting a systematic literature review to enhance our understanding of the topic.
• In the fourth paragraph, the authors introduce their specific objectives and explain how their insights can benefit other researchers and business practitioners.
• In the fifth and final paragraph, the authors provide an overview of the paper’s organization and structure.

In summary, this introduction stands as a solid example. While the authors deviate from previewing their key findings (which is a common practice at least in the social sciences), they do effectively cover all the other previously mentioned points.

Example 2: An effective introduction to a literature review section in an academic paper

The second example represents a typical academic paper, encompassing not only a literature review section but also empirical data, a case study, and other elements. We will closely examine the introduction to the literature review section in the paper “The environmentalism of the subalterns: a case study of environmental activism in Eastern Kurdistan/Rojhelat”, which was published in 2021 in the journal Local Environment.

The paper begins with a general introduction and then proceeds to the literature review, designated by the authors as their conceptual framework. Of particular interest is the first paragraph of this conceptual framework, comprising 142 words across five sentences:

“ A peripheral and marginalised nationality within a multinational though-Persian dominated Iranian society, the Kurdish people of Iranian Kurdistan (a region referred by the Kurds as Rojhelat/Eastern Kurdi-stan) have since the early twentieth century been subject to multifaceted and systematic discriminatory and exclusionary state policy in Iran. This condition has left a population of 12–15 million Kurds in Iran suffering from structural inequalities, disenfranchisement and deprivation. Mismanagement of Kurdistan’s natural resources and the degradation of its natural environmental are among examples of this disenfranchisement. As asserted by Julian Agyeman (2005), structural inequalities that sustain the domination of political and economic elites often simultaneously result in environmental degradation, injustice and discrimination against subaltern communities. This study argues that the environmental struggle in Eastern Kurdistan can be asserted as a (sub)element of the Kurdish liberation movement in Iran. Conceptually this research is inspired by and has been conducted through the lens of ‘subalternity’ ” ( Hassaniyan, 2021, p. 931 ).

In this first paragraph, the author is doing the following:

• The author contextualises the research
• The author links the research focus to the international literature on structural inequalities
• The author clearly presents the argument of the research
• The author clarifies how the research is inspired by and uses the concept of ‘subalternity’.

Thus, the author successfully introduces the literature review, from which point onward it dives into the main concept (‘subalternity’) of the research, and reviews the literature on socio-economic justice and environmental degradation.

While introductions to a literature review section aren’t always required to offer the same level of study context detail as demonstrated here, this introduction serves as a commendable model for orienting the reader within the literature review. It effectively underscores the literature review’s significance within the context of the study being conducted.

Examples 3-5: Effective introductions to literature review chapters

The introduction to a literature review chapter can vary in length, depending largely on the overall length of the literature review chapter itself. For example, a master’s thesis typically features a more concise literature review, thus necessitating a shorter introduction. In contrast, a Ph.D. thesis, with its more extensive literature review, often includes a more detailed introduction.

Numerous universities offer online repositories where you can access theses and dissertations from previous years, serving as valuable sources of reference. Many of these repositories, however, may require you to log in through your university account. Nevertheless, a few open-access repositories are accessible to anyone, such as the one by the University of Manchester . It’s important to note though that copyright restrictions apply to these resources, just as they would with published papers.

Master’s thesis literature review introduction

The first example is “Benchmarking Asymmetrical Heating Models of Spider Pulsar Companions” by P. Sun, a master’s thesis completed at the University of Manchester on January 9, 2024. The author, P. Sun, introduces the literature review chapter very briefly but effectively:

PhD thesis literature review chapter introduction

The second example is Deep Learning on Semi-Structured Data and its Applications to Video-Game AI, Woof, W. (Author). 31 Dec 2020, a PhD thesis completed at the University of Manchester . In Chapter 2, the author offers a comprehensive introduction to the topic in four paragraphs, with the final paragraph serving as an overview of the chapter’s structure:

PhD thesis literature review introduction

The last example is the doctoral thesis Metacognitive strategies and beliefs: Child correlates and early experiences Chan, K. Y. M. (Author). 31 Dec 2020 . The author clearly conducted a systematic literature review, commencing the review section with a discussion of the methodology and approach employed in locating and analyzing the selected records.

Having absorbed all of this information, let’s recap the essential steps and offer a succinct guide on how to proceed with creating your literature review introduction:

• Contextualize your review : Begin by clearly identifying the academic context in which your literature review resides and determining the necessary information to include.
• Outline your structure : Develop a structured outline for your literature review, highlighting the essential information you plan to incorporate in your introduction.
• Literature review process : Conduct a rigorous literature review, reviewing and analyzing relevant sources.
• Summarize and abstract : After completing the review, synthesize the findings and abstract key insights, trends, and knowledge gaps from the literature.
• Craft the introduction : Write your literature review introduction with meticulous attention to the seamless integration of your review into the larger context of your work. Ensure that your introduction effectively elucidates your rationale for the chosen review topics and the underlying reasons guiding your selection.

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• UConn Library
• Literature Review: The What, Why and How-to Guide
• Introduction

Literature Review: The What, Why and How-to Guide — Introduction

• Getting Started
• How to Pick a Topic
• Strategies to Find Sources
• Evaluating Sources & Lit. Reviews
• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings

What are Literature Reviews?

So, what is a literature review? "A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available, or a set of summaries." Taylor, D.  The literature review: A few tips on conducting it . University of Toronto Health Sciences Writing Centre.

Goals of Literature Reviews

What are the goals of creating a Literature Review?  A literature could be written to accomplish different aims:

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 

Baumeister, R. F., & Leary, M. R. (1997). Writing narrative literature reviews .  Review of General Psychology , 1 (3), 311-320.

What kinds of sources require a Literature Review?

• A research paper assigned in a course
• A thesis or dissertation
• A grant proposal
• An article intended for publication in a journal

All these instances require you to collect what has been written about your research topic so that you can demonstrate how your own research sheds new light on the topic.

Types of Literature Reviews

What kinds of literature reviews are written?

Narrative review: The purpose of this type of review is to describe the current state of the research on a specific topic/research and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weakness, and gaps are identified. The review ends with a conclusion section which summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.

• Example : Predictors and Outcomes of U.S. Quality Maternity Leave: A Review and Conceptual Framework:  10.1177/08948453211037398  

Systematic review : "The authors of a systematic review use a specific procedure to search the research literature, select the studies to include in their review, and critically evaluate the studies they find." (p. 139). Nelson, L. K. (2013). Research in Communication Sciences and Disorders . Plural Publishing.

• Example : The effect of leave policies on increasing fertility: a systematic review:  10.1057/s41599-022-01270-w

Meta-analysis : "Meta-analysis is a method of reviewing research findings in a quantitative fashion by transforming the data from individual studies into what is called an effect size and then pooling and analyzing this information. The basic goal in meta-analysis is to explain why different outcomes have occurred in different studies." (p. 197). Roberts, M. C., & Ilardi, S. S. (2003). Handbook of Research Methods in Clinical Psychology . Blackwell Publishing.

• Example : Employment Instability and Fertility in Europe: A Meta-Analysis:  10.1215/00703370-9164737

Meta-synthesis : "Qualitative meta-synthesis is a type of qualitative study that uses as data the findings from other qualitative studies linked by the same or related topic." (p.312). Zimmer, L. (2006). Qualitative meta-synthesis: A question of dialoguing with texts .  Journal of Advanced Nursing , 53 (3), 311-318.

• Example : Women’s perspectives on career successes and barriers: A qualitative meta-synthesis:  10.1177/05390184221113735

Literature Reviews in the Health Sciences

• UConn Health subject guide on systematic reviews Explanation of the different review types used in health sciences literature as well as tools to help you find the right review type
• << Previous: Getting Started
• Next: How to Pick a Topic >>
• Last Updated: Sep 21, 2022 2:16 PM
• URL: https://guides.lib.uconn.edu/literaturereview

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Writing a Literature Review

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A literature review is a document or section of a document that collects key sources on a topic and discusses those sources in conversation with each other (also called synthesis ). The lit review is an important genre in many disciplines, not just literature (i.e., the study of works of literature such as novels and plays). When we say “literature review” or refer to “the literature,” we are talking about the research ( scholarship ) in a given field. You will often see the terms “the research,” “the scholarship,” and “the literature” used mostly interchangeably.

Where, when, and why would I write a lit review?

There are a number of different situations where you might write a literature review, each with slightly different expectations; different disciplines, too, have field-specific expectations for what a literature review is and does. For instance, in the humanities, authors might include more overt argumentation and interpretation of source material in their literature reviews, whereas in the sciences, authors are more likely to report study designs and results in their literature reviews; these differences reflect these disciplines’ purposes and conventions in scholarship. You should always look at examples from your own discipline and talk to professors or mentors in your field to be sure you understand your discipline’s conventions, for literature reviews as well as for any other genre.

A literature review can be a part of a research paper or scholarly article, usually falling after the introduction and before the research methods sections. In these cases, the lit review just needs to cover scholarship that is important to the issue you are writing about; sometimes it will also cover key sources that informed your research methodology.

Lit reviews can also be standalone pieces, either as assignments in a class or as publications. In a class, a lit review may be assigned to help students familiarize themselves with a topic and with scholarship in their field, get an idea of the other researchers working on the topic they’re interested in, find gaps in existing research in order to propose new projects, and/or develop a theoretical framework and methodology for later research. As a publication, a lit review usually is meant to help make other scholars’ lives easier by collecting and summarizing, synthesizing, and analyzing existing research on a topic. This can be especially helpful for students or scholars getting into a new research area, or for directing an entire community of scholars toward questions that have not yet been answered.

What are the parts of a lit review?

Most lit reviews use a basic introduction-body-conclusion structure; if your lit review is part of a larger paper, the introduction and conclusion pieces may be just a few sentences while you focus most of your attention on the body. If your lit review is a standalone piece, the introduction and conclusion take up more space and give you a place to discuss your goals, research methods, and conclusions separately from where you discuss the literature itself.

Introduction:

• An introductory paragraph that explains what your working topic and thesis is
• A forecast of key topics or texts that will appear in the review
• Potentially, a description of how you found sources and how you analyzed them for inclusion and discussion in the review (more often found in published, standalone literature reviews than in lit review sections in an article or research paper)
• Summarize and synthesize: Give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: Don’t just paraphrase other researchers – add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically Evaluate: Mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: Use transition words and topic sentence to draw connections, comparisons, and contrasts.

Conclusion:

• Summarize the key findings you have taken from the literature and emphasize their significance
• Connect it back to your primary research question

How should I organize my lit review?

Lit reviews can take many different organizational patterns depending on what you are trying to accomplish with the review. Here are some examples:

• Chronological : The simplest approach is to trace the development of the topic over time, which helps familiarize the audience with the topic (for instance if you are introducing something that is not commonly known in your field). If you choose this strategy, be careful to avoid simply listing and summarizing sources in order. Try to analyze the patterns, turning points, and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred (as mentioned previously, this may not be appropriate in your discipline — check with a teacher or mentor if you’re unsure).
• Thematic : If you have found some recurring central themes that you will continue working with throughout your piece, you can organize your literature review into subsections that address different aspects of the topic. For example, if you are reviewing literature about women and religion, key themes can include the role of women in churches and the religious attitude towards women.
• Qualitative versus quantitative research
• Empirical versus theoretical scholarship
• Divide the research by sociological, historical, or cultural sources
• Theoretical : In many humanities articles, the literature review is the foundation for the theoretical framework. You can use it to discuss various theories, models, and definitions of key concepts. You can argue for the relevance of a specific theoretical approach or combine various theorical concepts to create a framework for your research.

What are some strategies or tips I can use while writing my lit review?

Any lit review is only as good as the research it discusses; make sure your sources are well-chosen and your research is thorough. Don’t be afraid to do more research if you discover a new thread as you’re writing. More info on the research process is available in our "Conducting Research" resources .

As you’re doing your research, create an annotated bibliography ( see our page on the this type of document ). Much of the information used in an annotated bibliography can be used also in a literature review, so you’ll be not only partially drafting your lit review as you research, but also developing your sense of the larger conversation going on among scholars, professionals, and any other stakeholders in your topic.

Usually you will need to synthesize research rather than just summarizing it. This means drawing connections between sources to create a picture of the scholarly conversation on a topic over time. Many student writers struggle to synthesize because they feel they don’t have anything to add to the scholars they are citing; here are some strategies to help you:

• It often helps to remember that the point of these kinds of syntheses is to show your readers how you understand your research, to help them read the rest of your paper.
• Writing teachers often say synthesis is like hosting a dinner party: imagine all your sources are together in a room, discussing your topic. What are they saying to each other?
• Look at the in-text citations in each paragraph. Are you citing just one source for each paragraph? This usually indicates summary only. When you have multiple sources cited in a paragraph, you are more likely to be synthesizing them (not always, but often
• Read more about synthesis here.

The most interesting literature reviews are often written as arguments (again, as mentioned at the beginning of the page, this is discipline-specific and doesn’t work for all situations). Often, the literature review is where you can establish your research as filling a particular gap or as relevant in a particular way. You have some chance to do this in your introduction in an article, but the literature review section gives a more extended opportunity to establish the conversation in the way you would like your readers to see it. You can choose the intellectual lineage you would like to be part of and whose definitions matter most to your thinking (mostly humanities-specific, but this goes for sciences as well). In addressing these points, you argue for your place in the conversation, which tends to make the lit review more compelling than a simple reporting of other sources.

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• What is a Literature Review? | Guide, Template, & Examples

What is a Literature Review? | Guide, Template, & Examples

Published on 22 February 2022 by Shona McCombes . Revised on 7 June 2022.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research.

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarise sources – it analyses, synthesises, and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

Why write a literature review, examples of literature reviews, step 1: search for relevant literature, step 2: evaluate and select sources, step 3: identify themes, debates and gaps, step 4: outline your literature review’s structure, step 5: write your literature review, frequently asked questions about literature reviews, introduction.

• Quick Run-through
• Step 1 & 2

When you write a dissertation or thesis, you will have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and scholarly context
• Develop a theoretical framework and methodology for your research
• Position yourself in relation to other researchers and theorists
• Show how your dissertation addresses a gap or contributes to a debate

You might also have to write a literature review as a stand-alone assignment. In this case, the purpose is to evaluate the current state of research and demonstrate your knowledge of scholarly debates around a topic.

The content will look slightly different in each case, but the process of conducting a literature review follows the same steps. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research objectives and questions .

If you are writing a literature review as a stand-alone assignment, you will have to choose a focus and develop a central question to direct your search. Unlike a dissertation research question, this question has to be answerable without collecting original data. You should be able to answer it based only on a review of existing publications.

Make a list of keywords

Start by creating a list of keywords related to your research topic. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list if you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can use boolean operators to help narrow down your search:

Read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

To identify the most important publications on your topic, take note of recurring citations. If the same authors, books or articles keep appearing in your reading, make sure to seek them out.

You probably won’t be able to read absolutely everything that has been written on the topic – you’ll have to evaluate which sources are most relevant to your questions.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models and methods? Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• How does the publication contribute to your understanding of the topic? What are its key insights and arguments?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible, and make sure you read any landmark studies and major theories in your field of research.

You can find out how many times an article has been cited on Google Scholar – a high citation count means the article has been influential in the field, and should certainly be included in your literature review.

The scope of your review will depend on your topic and discipline: in the sciences you usually only review recent literature, but in the humanities you might take a long historical perspective (for example, to trace how a concept has changed in meaning over time).

Remember that you can use our template to summarise and evaluate sources you’re thinking about using!

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It’s important to keep track of your sources with references to avoid plagiarism . It can be helpful to make an annotated bibliography, where you compile full reference information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

You can use our free APA Reference Generator for quick, correct, consistent citations.

Prevent plagiarism, run a free check.

To begin organising your literature review’s argument and structure, you need to understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly-visual platforms like Instagram and Snapchat – this is a gap that you could address in your own research.

There are various approaches to organising the body of a literature review. You should have a rough idea of your strategy before you start writing.

Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarising sources in order.

Try to analyse patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organise your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text, your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

If you are writing the literature review as part of your dissertation or thesis, reiterate your central problem or research question and give a brief summary of the scholarly context. You can emphasise the timeliness of the topic (“many recent studies have focused on the problem of x”) or highlight a gap in the literature (“while there has been much research on x, few researchers have taken y into consideration”).

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, make sure to follow these tips:

• Summarise and synthesise: give an overview of the main points of each source and combine them into a coherent whole.
• Analyse and interpret: don’t just paraphrase other researchers – add your own interpretations, discussing the significance of findings in relation to the literature as a whole.
• Critically evaluate: mention the strengths and weaknesses of your sources.
• Write in well-structured paragraphs: use transitions and topic sentences to draw connections, comparisons and contrasts.

In the conclusion, you should summarise the key findings you have taken from the literature and emphasise their significance.

If the literature review is part of your dissertation or thesis, reiterate how your research addresses gaps and contributes new knowledge, or discuss how you have drawn on existing theories and methods to build a framework for your research. This can lead directly into your methodology section.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a dissertation , thesis, research paper , or proposal .

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarise yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your  dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

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Literature Reviews

• What is a literature review?
• Steps in the Literature Review Process
• Define your research question
• Determine inclusion and exclusion criteria
• Choose databases and search
• Review Results
• Synthesize Results
• Analyze Results
• Librarian Support

What is a Literature Review?

A literature or narrative review is a comprehensive review and analysis of the published literature on a specific topic or research question. The literature that is reviewed contains: books, articles, academic articles, conference proceedings, association papers, and dissertations. It contains the most pertinent studies and points to important past and current research and practices. It provides background and context, and shows how your research will contribute to the field. 

A literature review should: 

• Provide a comprehensive and updated review of the literature;
• Explain why this review has taken place;
• Articulate a position or hypothesis;
• Acknowledge and account for conflicting and corroborating points of view

From  S age Research Methods

Purpose of a Literature Review

A literature review can be written as an introduction to a study to:

• Demonstrate how a study fills a gap in research
• Compare a study with other research that's been done

Or it can be a separate work (a research article on its own) which:

• Organizes or describes a topic
• Describes variables within a particular issue/problem

Limitations of a Literature Review

Some of the limitations of a literature review are:

• It's a snapshot in time. Unlike other reviews, this one has beginning, a middle and an end. There may be future developments that could make your work less relevant.
• It may be too focused. Some niche studies may miss the bigger picture.
• It can be difficult to be comprehensive. There is no way to make sure all the literature on a topic was considered.
• It is easy to be biased if you stick to top tier journals. There may be other places where people are publishing exemplary research. Look to open access publications and conferences to reflect a more inclusive collection. Also, make sure to include opposing views (and not just supporting evidence).

Source: Grant, Maria J., and Andrew Booth. “A Typology of Reviews: An Analysis of 14 Review Types and Associated Methodologies.” Health Information & Libraries Journal, vol. 26, no. 2, June 2009, pp. 91–108. Wiley Online Library, doi:10.1111/j.1471-1842.2009.00848.x.

Meryl Brodsky : Communication and Information Studies

Hannah Chapman Tripp : Biology, Neuroscience

Carolyn Cunningham : Human Development & Family Sciences, Psychology, Sociology

Larayne Dallas : Engineering

Janelle Hedstrom : Special Education, Curriculum & Instruction, Ed Leadership & Policy ​

Susan Macicak : Linguistics

Imelda Vetter : Dell Medical School

For help in other subject areas, please see the guide to library specialists by subject .

Periodically, UT Libraries runs a workshop covering the basics and library support for literature reviews. While we try to offer these once per academic year, we find providing the recording to be helpful to community members who have missed the session. Following is the most recent recording of the workshop, Conducting a Literature Review. To view the recording, a UT login is required.

• October 26, 2022 recording
• Last Updated: Oct 26, 2022 2:49 PM
• URL: https://guides.lib.utexas.edu/literaturereviews

Libraries | Research Guides

Literature reviews, what is a literature review, learning more about how to do a literature review.

• Planning the Review
• The Research Question
• Choosing Where to Search
• Organizing the Review
• Writing the Review

A literature review is a review and synthesis of existing research on a topic or research question. A literature review is meant to analyze the scholarly literature, make connections across writings and identify strengths, weaknesses, trends, and missing conversations. A literature review should address different aspects of a topic as it relates to your research question. A literature review goes beyond a description or summary of the literature you have read. 

• Sage Research Methods Core Collection This link opens in a new window SAGE Research Methods supports research at all levels by providing material to guide users through every step of the research process. SAGE Research Methods is the ultimate methods library with more than 1000 books, reference works, journal articles, and instructional videos by world-leading academics from across the social sciences, including the largest collection of qualitative methods books available online from any scholarly publisher. – Publisher

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• Last Updated: Jan 17, 2024 10:05 AM
• URL: https://libguides.northwestern.edu/literaturereviews

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Introduction to Literature Reviews

Introduction.

• Step One: Define
• Step Two: Research
• Step Three: Write
• Suggested Readings

A literature review is a written work that :

• Compiles significant research published on a topic by accredited scholars and researchers;
• —Surveys scholarly articles, books, dissertations, conference proceedings, and other sources;
• —Examines contrasting perspectives, theoretical approaches, methodologies, findings, results, conclusions.
• —Reviews critically, analyzes, and synthesizes existing research on a topic; and,
• Performs a thorough “re” view, “overview”, or “look again” of past and current works on a subject, issue, or theory.

From these analyses, the writer then offers an overview of the current status of a particular area of knowledge from both a practical and theoretical perspective.

Literature reviews are important because they are usually a  required  step in a thesis proposal (Master's or PhD). The proposal will not be well-supported without a literature review. Also, literature reviews are important because they help you learn important authors and ideas in your field. This is useful for your coursework and your writing. Knowing key authors also helps you become acquainted with other researchers in your field.

Look at this diagram and imagine that your research is the "something new." This shows how your research should relate to major works and other sources.

Olivia Whitfield | Graduate Reference Assistant | 2012-2015

• Next: Step One: Define >>
• Last Updated: Jun 28, 2023 5:49 PM
• URL: https://libraryguides.missouri.edu/literaturereview

What is a Literature Review? How to Write It (with Examples)

A literature review is a critical analysis and synthesis of existing research on a particular topic. It provides an overview of the current state of knowledge, identifies gaps, and highlights key findings in the literature. 1 The purpose of a literature review is to situate your own research within the context of existing scholarship, demonstrating your understanding of the topic and showing how your work contributes to the ongoing conversation in the field. Learning how to write a literature review is a critical tool for successful research. Your ability to summarize and synthesize prior research pertaining to a certain topic demonstrates your grasp on the topic of study, and assists in the learning process. 

Table of Contents

• What is the purpose of literature review? 
• a. Habitat Loss and Species Extinction: 
• b. Range Shifts and Phenological Changes: 
• c. Ocean Acidification and Coral Reefs: 
• d. Adaptive Strategies and Conservation Efforts: 
• How to write a good literature review 
• Choose a Topic and Define the Research Question: 
• Decide on the Scope of Your Review: 
• Select Databases for Searches: 
• Conduct Searches and Keep Track: 
• Review the Literature: 
• Organize and Write Your Literature Review: 
• Frequently asked questions 

What is a literature review?

A well-conducted literature review demonstrates the researcher’s familiarity with the existing literature, establishes the context for their own research, and contributes to scholarly conversations on the topic. One of the purposes of a literature review is also to help researchers avoid duplicating previous work and ensure that their research is informed by and builds upon the existing body of knowledge.

What is the purpose of literature review?

A literature review serves several important purposes within academic and research contexts. Here are some key objectives and functions of a literature review: 2  

• Contextualizing the Research Problem: The literature review provides a background and context for the research problem under investigation. It helps to situate the study within the existing body of knowledge. 
• Identifying Gaps in Knowledge: By identifying gaps, contradictions, or areas requiring further research, the researcher can shape the research question and justify the significance of the study. This is crucial for ensuring that the new research contributes something novel to the field. 
• Understanding Theoretical and Conceptual Frameworks: Literature reviews help researchers gain an understanding of the theoretical and conceptual frameworks used in previous studies. This aids in the development of a theoretical framework for the current research. 
• Providing Methodological Insights: Another purpose of literature reviews is that it allows researchers to learn about the methodologies employed in previous studies. This can help in choosing appropriate research methods for the current study and avoiding pitfalls that others may have encountered. 
• Establishing Credibility: A well-conducted literature review demonstrates the researcher’s familiarity with existing scholarship, establishing their credibility and expertise in the field. It also helps in building a solid foundation for the new research. 
• Informing Hypotheses or Research Questions: The literature review guides the formulation of hypotheses or research questions by highlighting relevant findings and areas of uncertainty in existing literature. 

Literature review example

Let’s delve deeper with a literature review example: Let’s say your literature review is about the impact of climate change on biodiversity. You might format your literature review into sections such as the effects of climate change on habitat loss and species extinction, phenological changes, and marine biodiversity. Each section would then summarize and analyze relevant studies in those areas, highlighting key findings and identifying gaps in the research. The review would conclude by emphasizing the need for further research on specific aspects of the relationship between climate change and biodiversity. The following literature review template provides a glimpse into the recommended literature review structure and content, demonstrating how research findings are organized around specific themes within a broader topic. 

Literature Review on Climate Change Impacts on Biodiversity:

Climate change is a global phenomenon with far-reaching consequences, including significant impacts on biodiversity. This literature review synthesizes key findings from various studies: 

a. Habitat Loss and Species Extinction:

Climate change-induced alterations in temperature and precipitation patterns contribute to habitat loss, affecting numerous species (Thomas et al., 2004). The review discusses how these changes increase the risk of extinction, particularly for species with specific habitat requirements. 

b. Range Shifts and Phenological Changes:

Observations of range shifts and changes in the timing of biological events (phenology) are documented in response to changing climatic conditions (Parmesan & Yohe, 2003). These shifts affect ecosystems and may lead to mismatches between species and their resources. 

c. Ocean Acidification and Coral Reefs:

The review explores the impact of climate change on marine biodiversity, emphasizing ocean acidification’s threat to coral reefs (Hoegh-Guldberg et al., 2007). Changes in pH levels negatively affect coral calcification, disrupting the delicate balance of marine ecosystems. 

d. Adaptive Strategies and Conservation Efforts:

Recognizing the urgency of the situation, the literature review discusses various adaptive strategies adopted by species and conservation efforts aimed at mitigating the impacts of climate change on biodiversity (Hannah et al., 2007). It emphasizes the importance of interdisciplinary approaches for effective conservation planning. 

How to write a good literature review

Writing a literature review involves summarizing and synthesizing existing research on a particular topic. A good literature review format should include the following elements. 

Introduction: The introduction sets the stage for your literature review, providing context and introducing the main focus of your review. 

• Opening Statement: Begin with a general statement about the broader topic and its significance in the field. 
• Scope and Purpose: Clearly define the scope of your literature review. Explain the specific research question or objective you aim to address. 
• Organizational Framework: Briefly outline the structure of your literature review, indicating how you will categorize and discuss the existing research. 
• Significance of the Study: Highlight why your literature review is important and how it contributes to the understanding of the chosen topic. 
• Thesis Statement: Conclude the introduction with a concise thesis statement that outlines the main argument or perspective you will develop in the body of the literature review. 

Body: The body of the literature review is where you provide a comprehensive analysis of existing literature, grouping studies based on themes, methodologies, or other relevant criteria. 

• Organize by Theme or Concept: Group studies that share common themes, concepts, or methodologies. Discuss each theme or concept in detail, summarizing key findings and identifying gaps or areas of disagreement. 
• Critical Analysis: Evaluate the strengths and weaknesses of each study. Discuss the methodologies used, the quality of evidence, and the overall contribution of each work to the understanding of the topic. 
• Synthesis of Findings: Synthesize the information from different studies to highlight trends, patterns, or areas of consensus in the literature. 
• Identification of Gaps: Discuss any gaps or limitations in the existing research and explain how your review contributes to filling these gaps. 
• Transition between Sections: Provide smooth transitions between different themes or concepts to maintain the flow of your literature review. 

Conclusion: The conclusion of your literature review should summarize the main findings, highlight the contributions of the review, and suggest avenues for future research. 

• Summary of Key Findings: Recap the main findings from the literature and restate how they contribute to your research question or objective. 
• Contributions to the Field: Discuss the overall contribution of your literature review to the existing knowledge in the field. 
• Implications and Applications: Explore the practical implications of the findings and suggest how they might impact future research or practice. 
• Recommendations for Future Research: Identify areas that require further investigation and propose potential directions for future research in the field. 
• Final Thoughts: Conclude with a final reflection on the importance of your literature review and its relevance to the broader academic community. 

Conducting a literature review

Conducting a literature review is an essential step in research that involves reviewing and analyzing existing literature on a specific topic. It’s important to know how to do a literature review effectively, so here are the steps to follow: 1  

Choose a Topic and Define the Research Question:

• Select a topic that is relevant to your field of study. 
• Clearly define your research question or objective. Determine what specific aspect of the topic do you want to explore? 

Decide on the Scope of Your Review:

• Determine the timeframe for your literature review. Are you focusing on recent developments, or do you want a historical overview? 
• Consider the geographical scope. Is your review global, or are you focusing on a specific region? 
• Define the inclusion and exclusion criteria. What types of sources will you include? Are there specific types of studies or publications you will exclude? 

Select Databases for Searches:

• Identify relevant databases for your field. Examples include PubMed, IEEE Xplore, Scopus, Web of Science, and Google Scholar. 
• Consider searching in library catalogs, institutional repositories, and specialized databases related to your topic. 

Conduct Searches and Keep Track:

• Develop a systematic search strategy using keywords, Boolean operators (AND, OR, NOT), and other search techniques. 
• Record and document your search strategy for transparency and replicability. 
• Keep track of the articles, including publication details, abstracts, and links. Use citation management tools like EndNote, Zotero, or Mendeley to organize your references. 

Review the Literature:

• Evaluate the relevance and quality of each source. Consider the methodology, sample size, and results of studies. 
• Organize the literature by themes or key concepts. Identify patterns, trends, and gaps in the existing research. 
• Summarize key findings and arguments from each source. Compare and contrast different perspectives. 
• Identify areas where there is a consensus in the literature and where there are conflicting opinions. 
• Provide critical analysis and synthesis of the literature. What are the strengths and weaknesses of existing research? 

Organize and Write Your Literature Review:

• Literature review outline should be based on themes, chronological order, or methodological approaches. 
• Write a clear and coherent narrative that synthesizes the information gathered. 
• Use proper citations for each source and ensure consistency in your citation style (APA, MLA, Chicago, etc.). 
• Conclude your literature review by summarizing key findings, identifying gaps, and suggesting areas for future research. 

The literature review sample and detailed advice on writing and conducting a review will help you produce a well-structured report. But remember that a literature review is an ongoing process, and it may be necessary to revisit and update it as your research progresses. 

Frequently asked questions

A literature review is a critical and comprehensive analysis of existing literature (published and unpublished works) on a specific topic or research question and provides a synthesis of the current state of knowledge in a particular field. A well-conducted literature review is crucial for researchers to build upon existing knowledge, avoid duplication of efforts, and contribute to the advancement of their field. It also helps researchers situate their work within a broader context and facilitates the development of a sound theoretical and conceptual framework for their studies.

Literature review is a crucial component of research writing, providing a solid background for a research paper’s investigation. The aim is to keep professionals up to date by providing an understanding of ongoing developments within a specific field, including research methods, and experimental techniques used in that field, and present that knowledge in the form of a written report. Also, the depth and breadth of the literature review emphasizes the credibility of the scholar in his or her field.  

Before writing a literature review, it’s essential to undertake several preparatory steps to ensure that your review is well-researched, organized, and focused. This includes choosing a topic of general interest to you and doing exploratory research on that topic, writing an annotated bibliography, and noting major points, especially those that relate to the position you have taken on the topic. 

Literature reviews and academic research papers are essential components of scholarly work but serve different purposes within the academic realm. 3 A literature review aims to provide a foundation for understanding the current state of research on a particular topic, identify gaps or controversies, and lay the groundwork for future research. Therefore, it draws heavily from existing academic sources, including books, journal articles, and other scholarly publications. In contrast, an academic research paper aims to present new knowledge, contribute to the academic discourse, and advance the understanding of a specific research question. Therefore, it involves a mix of existing literature (in the introduction and literature review sections) and original data or findings obtained through research methods. 

Literature reviews are essential components of academic and research papers, and various strategies can be employed to conduct them effectively. If you want to know how to write a literature review for a research paper, here are four common approaches that are often used by researchers.  Chronological Review: This strategy involves organizing the literature based on the chronological order of publication. It helps to trace the development of a topic over time, showing how ideas, theories, and research have evolved.  Thematic Review: Thematic reviews focus on identifying and analyzing themes or topics that cut across different studies. Instead of organizing the literature chronologically, it is grouped by key themes or concepts, allowing for a comprehensive exploration of various aspects of the topic.  Methodological Review: This strategy involves organizing the literature based on the research methods employed in different studies. It helps to highlight the strengths and weaknesses of various methodologies and allows the reader to evaluate the reliability and validity of the research findings.  Theoretical Review: A theoretical review examines the literature based on the theoretical frameworks used in different studies. This approach helps to identify the key theories that have been applied to the topic and assess their contributions to the understanding of the subject.  It’s important to note that these strategies are not mutually exclusive, and a literature review may combine elements of more than one approach. The choice of strategy depends on the research question, the nature of the literature available, and the goals of the review. Additionally, other strategies, such as integrative reviews or systematic reviews, may be employed depending on the specific requirements of the research.

The literature review format can vary depending on the specific publication guidelines. However, there are some common elements and structures that are often followed. Here is a general guideline for the format of a literature review:  Introduction:   Provide an overview of the topic.  Define the scope and purpose of the literature review.  State the research question or objective.  Body:   Organize the literature by themes, concepts, or chronology.  Critically analyze and evaluate each source.  Discuss the strengths and weaknesses of the studies.  Highlight any methodological limitations or biases.  Identify patterns, connections, or contradictions in the existing research.  Conclusion:   Summarize the key points discussed in the literature review.  Highlight the research gap.  Address the research question or objective stated in the introduction.  Highlight the contributions of the review and suggest directions for future research.

Both annotated bibliographies and literature reviews involve the examination of scholarly sources. While annotated bibliographies focus on individual sources with brief annotations, literature reviews provide a more in-depth, integrated, and comprehensive analysis of existing literature on a specific topic. The key differences are as follows: 

References 

• Denney, A. S., & Tewksbury, R. (2013). How to write a literature review.  Journal of criminal justice education ,  24 (2), 218-234. 
• Pan, M. L. (2016).  Preparing literature reviews: Qualitative and quantitative approaches . Taylor & Francis. 
• Cantero, C. (2019). How to write a literature review.  San José State University Writing Center . 

Paperpal is an AI writing assistant that help academics write better, faster with real-time suggestions for in-depth language and grammar correction. Trained on millions of research manuscripts enhanced by professional academic editors, Paperpal delivers human precision at machine speed.  

Try it for free or upgrade to  Paperpal Prime , which unlocks unlimited access to premium features like academic translation, paraphrasing, contextual synonyms, consistency checks and more. It’s like always having a professional academic editor by your side! Go beyond limitations and experience the future of academic writing.  Get Paperpal Prime now at just US$19 a month!

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• Empirical Research: A Comprehensive Guide for Academics 
• How to Write a Scientific Paper in 10 Steps 
• Life Sciences Papers: 9 Tips for Authors Writing in Biological Sciences
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What is a literature review?

A literature review is an integrated analysis -- not just a summary-- of scholarly writings and other relevant evidence related directly to your research question.  That is, it represents a synthesis of the evidence that provides background information on your topic and shows a association between the evidence and your research question.

A literature review may be a stand alone work or the introduction to a larger research paper, depending on the assignment.  Rely heavily on the guidelines your instructor has given you.

Why is it important?

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Discovers relationships between research studies/ideas.
• Identifies major themes, concepts, and researchers on a topic.
• Identifies critical gaps and points of disagreement.
• Discusses further research questions that logically come out of the previous studies.

APA7 Style resources

APA Style Blog - for those harder to find answers

1. Choose a topic. Define your research question.

Your literature review should be guided by your central research question.  The literature represents background and research developments related to a specific research question, interpreted and analyzed by you in a synthesized way.

• Make sure your research question is not too broad or too narrow.  Is it manageable?
• Begin writing down terms that are related to your question. These will be useful for searches later.
• If you have the opportunity, discuss your topic with your professor and your class mates.

2. Decide on the scope of your review

How many studies do you need to look at? How comprehensive should it be? How many years should it cover? 

• This may depend on your assignment.  How many sources does the assignment require?

3. Select the databases you will use to conduct your searches.

Make a list of the databases you will search. 

Where to find databases:

• use the tabs on this guide
• Find other databases in the Nursing Information Resources web page
• More on the Medical Library web page
• ... and more on the Yale University Library web page

4. Conduct your searches to find the evidence. Keep track of your searches.

• Use the key words in your question, as well as synonyms for those words, as terms in your search. Use the database tutorials for help.
• Save the searches in the databases. This saves time when you want to redo, or modify, the searches. It is also helpful to use as a guide is the searches are not finding any useful results.
• Review the abstracts of research studies carefully. This will save you time.
• Use the bibliographies and references of research studies you find to locate others.
• Check with your professor, or a subject expert in the field, if you are missing any key works in the field.
• Ask your librarian for help at any time.
• Use a citation manager, such as EndNote as the repository for your citations. See the EndNote tutorials for help.

Review the literature

Some questions to help you analyze the research:

• What was the research question of the study you are reviewing? What were the authors trying to discover?
• Was the research funded by a source that could influence the findings?
• What were the research methodologies? Analyze its literature review, the samples and variables used, the results, and the conclusions.
• Does the research seem to be complete? Could it have been conducted more soundly? What further questions does it raise?
• If there are conflicting studies, why do you think that is?
• How are the authors viewed in the field? Has this study been cited? If so, how has it been analyzed?

Tips: 

• Review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.
• Create a matrix of the studies for easy analysis, and synthesis, across all of the studies.
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Literature reviews

What this guide covers, what is a literature review, literature review resources, types of literature reviews, what is the difference between a literature review and a systematic review, related information and guides, further help.

• Conducting your search
• Store and organise the literature
• Evaluate and critique the literature
• Different subject areas
• Find literature reviews

Reusing content from this guide

Attribute our work under a Creative Commons Attribution-NonCommercial 4.0 International License.

The literature review process involves a number of steps.

This guide focuses on:

• evaluating.

A literature review is a survey and critical analysis of what has been written on a particular topic, theory, question or method.

"In writing the literature review, the purpose is to explore what knowledge and ideas have been established on a topic, what approaches and viewpoints have been adopted, and what are their strengths and weaknesses."

Source: "Focus and frame". (2008). In Eriksson, P. & Kovalainen, A. Introducing Qualitative Methods: Qualitative methods in business research (pp. 44) . London: SAGE Publications Ltd. doi: 10.4135/9780857028044.

Get an overview on doing a literature review:

• Sage research methods online - Literature review methods map Information on the literature review methodology with links to further resources - the Project Planner, books, articles, videos and more.
• Ten simple rules for writing a literature review Gives 10 tips on how to approach and carry out a literature review. By Pautasso M (2013) Ten Simple Rules for Writing a Literature Review. PLoS Comput Biol9(7): e1003149.
• The literature review. In: Doing your undergraduate program This chapter looks at the purpose of literature reviews, how it is done, setting the boundaries of your search and more.

• More books on literature reviews A selection of literature review books available via UQ Library Search.

The type of literature review you do will depend on a variety of factors:

• Your discipline
• The purpose - undergraduate assessment, PHD thesis, journal article?
• Your lecturer or supervisor's requirements

Always follow the guidelines outlined by your lecturer or supervisor or consult the instructions for authors (for journal articles), when conducting your literature review.

• is an overview of the significant literature on a topic
• typically includes a critical analysis of each work included
• demonstrates the reviewers knowledge of the topic
• is a list of citations of research sources (books, journal articles, websites etc) on a topic
• includes a brief summary and analysis or evaluation of each citation = the annotation
• a critical assessment of all research studies on a particular research question
• has specific criteria for collecting and evaluating the literature
• includes a synthesis of the findings of the included studies
• This method developed by Griffith University's School of Environment bridges the gap between traditional narrative review methods and meta-analyses to enable students to produce results that are reliable, quantifiable and reproducible.

The requirements of narrative literature reviews are usually quite different than systematic reviews . However, you may be required to adopt some of the characteristics of a systematic approach when doing your literature review. Check the guidelines or criteria that have been set by your supervisor so you know what is expected of you.

Characteristics of reviews

• Meeting the review family: Exploring review types and associated information retrieval requirements This article defines different review types and discusses appropriate search methods for each type.
• Writing literature reviews - Student Support Student Support provides information on how to write effective literature reviews.
• Writing skills Learn strategies for good writing from the Graduate School.
• Systematic reviews An overview of systematic reviews and resources to support producing one.
• Subject guides See recommended resources in different subject areas.
• Grey literature Find literature that is not available in traditional channels of publishing and distribution.
• How to find guides Techniques and resources to find specific information formats.

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An Introduction to Literature Reviews

• Watching now: Chapter 1: How Do You Conduct a Literature Review? Start time: 00:00:00 End time: 00:04:40
• Chapter 2: How Do You Find and Organize Sources of Information? Start time: 00:04:41 End time: 00:08:24
• Chapter 3: How Do You Assess These Sources of Information? Start time: 00:08:25 End time: 00:10:15
• Chapter 4: How Do You Write up Your Findings? Start time: 00:10:16 End time: 00:12:19
• Chapter 5: How Do You Identify Gaps in Literature? Start time: 00:12:20 End time: 00:15:08

Video Type: Tutorial

Jensen, E. (Academics). (2017). An introduction to literature reviews [Video]. Sage Research Methods. https:// doi. org/10.4135/9781473992283

Jensen, Eric. "An Introduction to Literature Reviews." In Sage Video . London: SAGE Publications Ltd, 2017. Video, 00:15:08. https:// doi. org/10.4135/9781473992283.

Jensen, E., 2017. An Introduction to Literature Reviews , Sage Video. [Streaming Video] London: Sage Publications Ltd. Available at: <https:// doi. org/10.4135/9781473992283 & gt; [Accessed 2 Apr 2024].

Jensen, Eric. An Introduction to Literature Reviews . Online video clip. SAGE Video. London: SAGE Publications, Ltd., 30 Sep 2016. doi: https:// doi. org/10.4135/9781473992283. 2 Apr 2024.

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Dr. Eric Jensen, Professor of Sociology at the University of Warwick, and Dr. Charles Laurie, Director of Research at Verisk Maplecroft, explain how to write a literature review, and why researchers need to do so. Literature reviews can be stand-alone research or part of a larger project. They communicate the state of academic knowledge on a given topic, specifically detailing what is still unknown.

Chapter 1: How Do You Conduct a Literature Review?

• Start time: 00:00:00
• End time: 00:04:40

Chapter 2: How Do You Find and Organize Sources of Information?

• Start time: 00:04:41
• End time: 00:08:24

Chapter 3: How Do You Assess These Sources of Information?

• Start time: 00:08:25
• End time: 00:10:15

Chapter 4: How Do You Write up Your Findings?

• Start time: 00:10:16
• End time: 00:12:19

Chapter 5: How Do You Identify Gaps in Literature?

• Start time: 00:12:20
• End time: 00:15:08
• Product: Sage Research Methods Video
• Type of Content: Tutorial
• Title: An Introduction to Literature Reviews
• Publisher: SAGE Publications Ltd
• Publication year: 2017
• Online pub date: September 30, 2016
• Discipline: Anthropology
• Methods: Literature review
• Duration: 00:15:08
• DOI: https:// doi. org/10.4135/9781473992283
• Keywords: accuracy in communication , awareness , gap problem , knowledge bases , knowledge creation , practices, strategies, and tools , relevance (education) , Social processes , Specific / diffuse , terminology as topic , understanding (epistemology) , voice and visibility , writing (communication) , writing (composition) Show all Show less

Academic: Eric Jensen

Practitioner: Charles Laurie

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How To Write An A-Grade Literature Review

3 straightforward steps (with examples) + free template.

By: Derek Jansen (MBA) | Expert Reviewed By: Dr. Eunice Rautenbach | October 2019

Quality research is about building onto the existing work of others , “standing on the shoulders of giants”, as Newton put it. The literature review chapter of your dissertation, thesis or research project is where you synthesise this prior work and lay the theoretical foundation for your own research.

Long story short, this chapter is a pretty big deal, which is why you want to make sure you get it right . In this post, I’ll show you exactly how to write a literature review in three straightforward steps, so you can conquer this vital chapter (the smart way).

Overview: The Literature Review Process

• Understanding the “ why “
• Finding the relevant literature
• Cataloguing and synthesising the information
• Outlining & writing up your literature review
• Example of a literature review

But first, the “why”…

Before we unpack how to write the literature review chapter, we’ve got to look at the why . To put it bluntly, if you don’t understand the function and purpose of the literature review process, there’s no way you can pull it off well. So, what exactly is the purpose of the literature review?

Well, there are (at least) four core functions:

• For you to gain an understanding (and demonstrate this understanding) of where the research is at currently, what the key arguments and disagreements are.
• For you to identify the gap(s) in the literature and then use this as justification for your own research topic.
• To help you build a conceptual framework for empirical testing (if applicable to your research topic).
• To inform your methodological choices and help you source tried and tested questionnaires (for interviews ) and measurement instruments (for surveys ).

Most students understand the first point but don’t give any thought to the rest. To get the most from the literature review process, you must keep all four points front of mind as you review the literature (more on this shortly), or you’ll land up with a wonky foundation.

Okay – with the why out the way, let’s move on to the how . As mentioned above, writing your literature review is a process, which I’ll break down into three steps:

• Finding the most suitable literature
• Understanding , distilling and organising the literature
• Planning and writing up your literature review chapter

Importantly, you must complete steps one and two before you start writing up your chapter. I know it’s very tempting, but don’t try to kill two birds with one stone and write as you read. You’ll invariably end up wasting huge amounts of time re-writing and re-shaping, or you’ll just land up with a disjointed, hard-to-digest mess . Instead, you need to read first and distil the information, then plan and execute the writing.

Step 1: Find the relevant literature

Naturally, the first step in the literature review journey is to hunt down the existing research that’s relevant to your topic. While you probably already have a decent base of this from your research proposal , you need to expand on this substantially in the dissertation or thesis itself.

Essentially, you need to be looking for any existing literature that potentially helps you answer your research question (or develop it, if that’s not yet pinned down). There are numerous ways to find relevant literature, but I’ll cover my top four tactics here. I’d suggest combining all four methods to ensure that nothing slips past you:

Method 1 – Google Scholar Scrubbing

Google’s academic search engine, Google Scholar , is a great starting point as it provides a good high-level view of the relevant journal articles for whatever keyword you throw at it. Most valuably, it tells you how many times each article has been cited, which gives you an idea of how credible (or at least, popular) it is. Some articles will be free to access, while others will require an account, which brings us to the next method.

Method 2 – University Database Scrounging

Generally, universities provide students with access to an online library, which provides access to many (but not all) of the major journals.

So, if you find an article using Google Scholar that requires paid access (which is quite likely), search for that article in your university’s database – if it’s listed there, you’ll have access. Note that, generally, the search engine capabilities of these databases are poor, so make sure you search for the exact article name, or you might not find it.

Method 3 – Journal Article Snowballing

At the end of every academic journal article, you’ll find a list of references. As with any academic writing, these references are the building blocks of the article, so if the article is relevant to your topic, there’s a good chance a portion of the referenced works will be too. Do a quick scan of the titles and see what seems relevant, then search for the relevant ones in your university’s database.

Method 4 – Dissertation Scavenging

Similar to Method 3 above, you can leverage other students’ dissertations. All you have to do is skim through literature review chapters of existing dissertations related to your topic and you’ll find a gold mine of potential literature. Usually, your university will provide you with access to previous students’ dissertations, but you can also find a much larger selection in the following databases:

• Open Access Theses & Dissertations
• Stanford SearchWorks

Keep in mind that dissertations and theses are not as academically sound as published, peer-reviewed journal articles (because they’re written by students, not professionals), so be sure to check the credibility of any sources you find using this method. You can do this by assessing the citation count of any given article in Google Scholar. If you need help with assessing the credibility of any article, or with finding relevant research in general, you can chat with one of our Research Specialists .

Alright – with a good base of literature firmly under your belt, it’s time to move onto the next step.

Need a helping hand?

Step 2: Log, catalogue and synthesise

Once you’ve built a little treasure trove of articles, it’s time to get reading and start digesting the information – what does it all mean?

While I present steps one and two (hunting and digesting) as sequential, in reality, it’s more of a back-and-forth tango – you’ll read a little , then have an idea, spot a new citation, or a new potential variable, and then go back to searching for articles. This is perfectly natural – through the reading process, your thoughts will develop , new avenues might crop up, and directional adjustments might arise. This is, after all, one of the main purposes of the literature review process (i.e. to familiarise yourself with the current state of research in your field).

As you’re working through your treasure chest, it’s essential that you simultaneously start organising the information. There are three aspects to this:

• Logging reference information
• Building an organised catalogue
• Distilling and synthesising the information

I’ll discuss each of these below:

2.1 – Log the reference information

As you read each article, you should add it to your reference management software. I usually recommend Mendeley for this purpose (see the Mendeley 101 video below), but you can use whichever software you’re comfortable with. Most importantly, make sure you load EVERY article you read into your reference manager, even if it doesn’t seem very relevant at the time.

2.2 – Build an organised catalogue

In the beginning, you might feel confident that you can remember who said what, where, and what their main arguments were. Trust me, you won’t. If you do a thorough review of the relevant literature (as you must!), you’re going to read many, many articles, and it’s simply impossible to remember who said what, when, and in what context . Also, without the bird’s eye view that a catalogue provides, you’ll miss connections between various articles, and have no view of how the research developed over time. Simply put, it’s essential to build your own catalogue of the literature.

I would suggest using Excel to build your catalogue, as it allows you to run filters, colour code and sort – all very useful when your list grows large (which it will). How you lay your spreadsheet out is up to you, but I’d suggest you have the following columns (at minimum):

• Author, date, title – Start with three columns containing this core information. This will make it easy for you to search for titles with certain words, order research by date, or group by author.
• Categories or keywords – You can either create multiple columns, one for each category/theme and then tick the relevant categories, or you can have one column with keywords.
• Key arguments/points – Use this column to succinctly convey the essence of the article, the key arguments and implications thereof for your research.
• Context – Note the socioeconomic context in which the research was undertaken. For example, US-based, respondents aged 25-35, lower- income, etc. This will be useful for making an argument about gaps in the research.
• Methodology – Note which methodology was used and why. Also, note any issues you feel arise due to the methodology. Again, you can use this to make an argument about gaps in the research.
• Quotations – Note down any quoteworthy lines you feel might be useful later.
• Notes – Make notes about anything not already covered. For example, linkages to or disagreements with other theories, questions raised but unanswered, shortcomings or limitations, and so forth.

If you’d like, you can try out our free catalog template here (see screenshot below).

2.3 – Digest and synthesise

Most importantly, as you work through the literature and build your catalogue, you need to synthesise all the information in your own mind – how does it all fit together? Look for links between the various articles and try to develop a bigger picture view of the state of the research. Some important questions to ask yourself are:

• What answers does the existing research provide to my own research questions ?
• Which points do the researchers agree (and disagree) on?
• How has the research developed over time?
• Where do the gaps in the current research lie?

To help you develop a big-picture view and synthesise all the information, you might find mind mapping software such as Freemind useful. Alternatively, if you’re a fan of physical note-taking, investing in a large whiteboard might work for you.

Step 3: Outline and write it up!

Once you’re satisfied that you have digested and distilled all the relevant literature in your mind, it’s time to put pen to paper (or rather, fingers to keyboard). There are two steps here – outlining and writing:

3.1 – Draw up your outline

Having spent so much time reading, it might be tempting to just start writing up without a clear structure in mind. However, it’s critically important to decide on your structure and develop a detailed outline before you write anything. Your literature review chapter needs to present a clear, logical and an easy to follow narrative – and that requires some planning. Don’t try to wing it!

Naturally, you won’t always follow the plan to the letter, but without a detailed outline, you’re more than likely going to end up with a disjointed pile of waffle , and then you’re going to spend a far greater amount of time re-writing, hacking and patching. The adage, “measure twice, cut once” is very suitable here.

In terms of structure, the first decision you’ll have to make is whether you’ll lay out your review thematically (into themes) or chronologically (by date/period). The right choice depends on your topic, research objectives and research questions, which we discuss in this article .

Once that’s decided, you need to draw up an outline of your entire chapter in bullet point format. Try to get as detailed as possible, so that you know exactly what you’ll cover where, how each section will connect to the next, and how your entire argument will develop throughout the chapter. Also, at this stage, it’s a good idea to allocate rough word count limits for each section, so that you can identify word count problems before you’ve spent weeks or months writing!

PS – check out our free literature review chapter template…

3.2 – Get writing

With a detailed outline at your side, it’s time to start writing up (finally!). At this stage, it’s common to feel a bit of writer’s block and find yourself procrastinating under the pressure of finally having to put something on paper. To help with this, remember that the objective of the first draft is not perfection – it’s simply to get your thoughts out of your head and onto paper, after which you can refine them. The structure might change a little, the word count allocations might shift and shuffle, and you might add or remove a section – that’s all okay. Don’t worry about all this on your first draft – just get your thoughts down on paper.

Once you’ve got a full first draft (however rough it may be), step away from it for a day or two (longer if you can) and then come back at it with fresh eyes. Pay particular attention to the flow and narrative – does it fall fit together and flow from one section to another smoothly? Now’s the time to try to improve the linkage from each section to the next, tighten up the writing to be more concise, trim down word count and sand it down into a more digestible read.

Once you’ve done that, give your writing to a friend or colleague who is not a subject matter expert and ask them if they understand the overall discussion. The best way to assess this is to ask them to explain the chapter back to you. This technique will give you a strong indication of which points were clearly communicated and which weren’t. If you’re working with Grad Coach, this is a good time to have your Research Specialist review your chapter.

Finally, tighten it up and send it off to your supervisor for comment. Some might argue that you should be sending your work to your supervisor sooner than this (indeed your university might formally require this), but in my experience, supervisors are extremely short on time (and often patience), so, the more refined your chapter is, the less time they’ll waste on addressing basic issues (which you know about already) and the more time they’ll spend on valuable feedback that will increase your mark-earning potential.

Literature Review Example

In the video below, we unpack an actual literature review so that you can see how all the core components come together in reality.

Let’s Recap

In this post, we’ve covered how to research and write up a high-quality literature review chapter. Let’s do a quick recap of the key takeaways:

• It is essential to understand the WHY of the literature review before you read or write anything. Make sure you understand the 4 core functions of the process.
• The first step is to hunt down the relevant literature . You can do this using Google Scholar, your university database, the snowballing technique and by reviewing other dissertations and theses.
• Next, you need to log all the articles in your reference manager , build your own catalogue of literature and synthesise all the research.
• Following that, you need to develop a detailed outline of your entire chapter – the more detail the better. Don’t start writing without a clear outline (on paper, not in your head!)
• Write up your first draft in rough form – don’t aim for perfection. Remember, done beats perfect.
• Refine your second draft and get a layman’s perspective on it . Then tighten it up and submit it to your supervisor.

Psst… there’s more!

This post is an extract from our bestselling Udemy Course, Literature Review Bootcamp . If you want to work smart, you don't want to miss this .

You Might Also Like:

38 Comments

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You’re welcome, Yinka. Thank you for the kind words. All the best writing your literature review.

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A Step-by-Step Guide to Writing a Stellar Literature Review (with Help from AI)

Table of contents

Aren’t all of us mini versions of Sherlock Holmes when browsing data and archives for a research piece? As we go through the process, a comprehensive literature review is an essential toolkit to make your research shine.

A literature review consists of scholarly sources that validate the content. Its primary objective is to offer a concise summary of the research and to let you explore relevant theories and methodologies. Through this review, you can identify gaps in the existing research and bridge them with your contribution. 

The real challenge is how to write an excellent literature review. Let’s learn.

What is the purpose of a literature review?

A literature review is an introduction to your research. It helps you put your perspective to the table, along with a summary of the theme.

What does my literature review communicate?

• Explanation of your research: how the information was collected, the research method, the justification of the chosen data sources, and an overview of the data analysis.
• Framework: the journey from where the concept began and how it is presented.
• Connects the previous and current research: 

It presents the broader scope of your research by connecting it to the existing data and debates and underlining how your content fits the prevailing studies. 

In an era of information overload, a literature review must be well-structured. 

Let’s learn all about the structure and style of a literature review that’ll help you strengthen your research.

Literature review– structure and style

Begin with a question and end it with the solution– the key to structuring a literature review. It resembles an essay’s format, with the first paragraph introducing the readers to the topic and the following explaining the research in-depth.

The conclusion reiterates the question and summarizes the overall insights of your research. There’s no word count restriction. —it depends on the type of research. For example, a dissertation demands lengthy work, whereas a short paper needs a few pages. 

In a literature review, maintaining high quality is vital, with a focus on academic writing style. Informal language should be avoided in favor of a more formal tone. 

The content avoids contractions, clearly differentiating between previous and current research through the use of past and present tense. Wordtune assists in establishing a formal tone, enhancing your work with pertinent suggestions. This AI-powered tool ensures your writing remains genuine, lucid, and engaging. 

The option of refining the tonality offers multiple possibilities for rephrasing a single sentence. Thus, pick the best and keep writing.

Get Wordtune for free > Get Wordtune for free >

Your friendly step-by-step guide to writing a literary review (with help from AI)

Do you find it challenging to begin the literature review? Don’t worry! We’re here to get you started with our step-by-step guide.

1. Narrow down the research scope

Simply begin with the question: What am I answering through my research?

Whether it’s cooking or painting, the real challenge is the prep-up for it rather than performing the task. Once you’re done, it smoothly progresses. Similarly, for your literature review, prepare the groundwork by narrowing down the research scope.

Browse and scoop out relevant data inclining well with your research. While you can’t cover every aspect of your research, pick a topic that isn’t too narrow nor too broad to keep your literature review well-balanced. 

2. Hunt relevant literature

The next question: Does this data align with the issue I’m trying to address?

As you review sources of information, hunt out the best ones. Determine which findings help in offering a focused insight on your topic. The best way to pick primary sources is to opt for the ones featured in reliable publications. You can also choose secondary sources from other researchers from a reasonable time frame and a relevant background.

For example, if your research focuses on the Historical Architecture of 18th-century Europe, the first-hand accounts and surveys from the past would hold more weight than the new-age publications. 

3. Observe the themes and patterns in sources

Next comes: What is the core viewpoint in most of the research? Has it stayed constant over time, or have the authors differed in their points of view?

Ensure to scoop out the essential aspects of what each source represents. Once you have collected all this information, combine it and add your interpretations at the end. This process is known as synthesis.

Synthesize ideas by combining arguments, findings and forming your new version.

4. Generate an outline

The next question: How can I organize my review effectively? When navigating multiple data sources, you must have noticed a structure throughout the research. Develop an outline to make the process easier. An outline is a skeletal format of the review, helping you connect the information more strategically.

Here are the three different ways to organize an outline– Chronologically, Thematically, or by Methodology.You can develop the outline chronologically, starting from the older sources and leading to the latest pieces. Another way of organizing is to thematically divide the sections and discuss each under the designated sub-heading.

You can even organize it per the research methods used by the respective authors. The choice of outline depends on the subject. For example, in the case of a science paper, you can divide the information into sections like introduction, types of equipment, method, procedure, findings, etc. In contrast, it’s best to present it in divisions based on timelines like Ancient, Middle Ages, Industrial revolutions, etc., for a history paper.

If you’re confused about how to structure the data, work with Wordtune. 

With the Generate with AI feature, you can mention your research topic and let Wordtune curate a comprehensive outline for your study.

Having a precise prompt is the key to getting the best results.

5. Start filling!

Your next question must be: Am I ready to compose all the parts of the literature review?

Once you’re ready with the basic outline and relevant sources, start filling in the data. Go for an introductory paragraph first to ensure your readers understand the topic and how you will present it. Ensure you clearly explain the section in the first sentence.

However, if beginning from the first paragraph seems intimidating, don’t worry! Add the main body content to the sub-headings, then jump to the introduction. 

Add headings wherever possible to make it more straightforward and guide your readers logically through different sources. Lastly, conclude your study by presenting a key takeaway and summarizing your findings. To make your task easier, work with Wordtune. It helps align your content with the desired tone and refine the structure.

6. Give attention to detail and edit

The last question: Am I satisfied with the language and content written in the literature review? Is it easy to understand?

Once you’re done writing the first draft of a literature review, it’s time to refine it. Take time between writing and reading the draft to ensure a fresh perspective. It makes it easier to spot errors when you disconnect from the content for some time. Start by looking at the document from a bird's eye to ensure the formatting and structure are in order. 

After reviewing the content format, you must thoroughly check your work for grammar, spelling, and punctuation. One of the best approaches to editing and proofreading is to use Wordtune . It helps simplify complex sentences, enhance the content quality, and gain prowess over the tonality.

The dos and don’ts of writing a literature review

Writing a stellar literature review requires following a few dos and don'ts. Just like Sherlock Holmes would never overlook a hint, you must pay attention to every minute detail while writing a perfect narrative. To help you write, below are some dos and don'ts to remember.

Composing a literature review demands a holistic research summary, each part exhibiting your understanding and approach. As you write the content, make sure to cover the following points:

• Keep a historical background of the field of research. Highlight the relevant relation between the old studies and your new research.
• Discuss the core issue, question, and debate of your topic.
• Theories lay the foundation of research. While you’re writing a literature review, make sure to add relevant concepts and ideas to support your statements.
• Another critical thing to keep in mind is to define complex terminologies. It helps the readers understand the content with better clarity. 

Examples of comprehensive literature reviews

Aren’t good examples the best way to understand a subject? Let’s look into a few examples of literature reviews and analyze what makes them well-written.

1. Critical Thinking and Transferability: A Review of the Literature (Gwendolyn Reece)

An overview of scholarly sources is included in the literature review, which explores critical thinking in American education. The introduction stating the subject’s importance makes it a winning literature review. Following the introduction is a well-defined purpose that highlights the importance of research.

As one keeps reading, there is more clarity on the pros and cons of the research. By dividing information into parts with relevant subheadings, the author breaks a lengthy literature review into manageable chunks, defining the overall structure.

Along with other studies and presented perspectives, the author also expresses her opinion. It is presented with minimal usage of ‘I,’ keeping it person-poised yet general. Toward the conclusion, the author again offers an overview of the study. A summary is further strengthened by presenting suggestions for future research as well. 

2. The Use of Technology in English Language Learning: A Literature Review

This literature review is thematically organized on how technology affects language acquisition. The study begins with an introduction to the topic with well-cited sources. It presents the views of different studies to help readers get a sense of different perspectives. After giving these perspectives, the author offers a personalized opinion.

One of the critical aspects that makes this a good literature review is a dedicated paragraph for definitions. It helps readers proceed further with a clear understanding of the crucial terminologies. There’s a comparison of the modern and previous studies and approaches to give an overall picture of the research.

Once the main body is composed, the author integrates recommendations for action-based tips. Thus, the literature review isn’t just summarizing the sources but offering actions relevant to the topics. Finally, the concluding paragraph has a brief overview with key takeaways.

Wordtune: your writing buddy!

A literature review demands the right balance of language and clarity. You must refine the content to achieve a formal tone and clear structure. Do you know what will help you the most? Wordtune !. 

The real-time grammar checker leaves no scope for errors and lets you retain precision in writing. This writing companion is all you need for stress-free working and comprehensive literature review development.

Let the narrative begin

A literary review isn't just about summarizing sources; it's about seamlessly bringing your perspective to the table. Always remember to set a narrative for added interest and a brilliant composition. With structure and style being the pillars of a stellar literature review, work with Wordtune to ensure zero compromises on the quality.

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Research Skills

Introduction and literature review.

This section is the beginning of the article, but don’t expect it to contain any sort of position or argument. In academic articles, this section has one, overarching purpose: to demonstrate that the authors are familiar with all previous relevant research on the issue they are writing about. Therefore, this section is usually the most “citation-heavy” section of the paper. It is not uncommon to have one or more citations at the end of each sentence. You will likely also encounter a number of compound citations: parentheticals in which not one source, but two or more are cited at one time. Each sentence that precedes a citation in this section is typically a very brief paraphrase of the relevant methods or applicable findings of the other articles that have come before. This review of prior studies is a very important exercise for scholars because it demonstrates the depth of their understanding. None of the articles you read occur in a vacuum; they are usually part of an evolving web of scholarship. Each new article picks up the thread (or, usually, several threads) left by articles published recently. Another important thing to realize is that, in a very real sense, the authors have not really begun; they do not make an argument or say much that is new in this section. It is designed to provide an academic history and theoretical context for the topic of discussion.

At the very end of every literature review section, however, the authors do something important. After having demonstrated their familiarity with previous research, authors indicate that, even though much research has been done, there are still gaps in the research that need filling. You should try to find language such as, “While many studies have examined this subject, no one has looked at this particular issue in this way.” The authors then announce their intention to address that gap in knowledge with the research that follows. This rhetorical move always appears at the end of this section, and often gives the reader the clearest and most detailed description of what exactly the authors are looking at—and why. This is not a thesis, however. Academic articles are not like the essays you may be used to writing, in which the thesis appears at the end of the introduction. The research gap is more akin to a hypothesis than a thesis. It does not make an argument, which comes much later—usually in the discussion or conclusion.

There are also articles that are stand-alone literature reviews; these are sometimes called “Review Articles” or “Meta-analyses.” Rather than engaging in original research, these articles, if they are recent and on point, can provide you with the bibliographic information of all the important, recent sources on your topic. There are many ways to find sources that don’t involve a search engine of any kind. Look at your articles’ references lists to see if they contain any relevant-sounding articles that you haven’t found by other means. You can save a great deal of time this way.

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Literature Review Tips for the Introduction and Discussion Sections

A literature review is a summary of studies related to a particular area of research. It identifies and summarizes all the relevant research conducted on a particular topic. It is important that your literature review is focused . Therefore, you should choose a limited number of studies that are central to your topic rather than trying to collect a wide range of studies that might not be closely connected.

Literature reviews help you accomplish the following:

• Evaluate past research  Collecting relevant resources will help you see what research has already been done. This will also help avoid duplication.
• Identify experts It is important to identify credible researchers who have knowledge in a given field, in order to seek their help if you get stuck with certain aspects of your research.
• Identify key questions  Your ultimate aim is to bring something new to the conversation. Collecting resources will help you determine the important questions that need to be addressed.
• Determine methodologies used in past studies Knowing how others have approached a particular topic will give you the opportunity to identify problems and find new ways to research and study a topic. If the reported methodology was successful, you can use it and save time that you would otherwise be spending on optimization.

Presenting Literature Review in the Introduction and Discussion Sections

There are many benefits to presenting literature reviews in the introduction and discussion sections of your manuscripts . However, there are differences in how you can present literature reviews in each section.

What Should be Included in the Literature Review of the Introduction Section?

The literature reviewed in the introduction should:

• Introduce the topic
• Establish the significance of the study
• Provide an overview of the relevant literature
• Establish a context for the study using the literature
• Identify knowledge gaps
• Illustrate how the study will advance knowledge on the topic

As you can see, literature review plays a significant role in the introduction section. However, there are some things that you should avoid doing in this section. These include:

• Elaborating on the studies mentioned in the literature review
• Using studies from the literature review to aggressively support your research
• Directly quoting studies from the literature review

It is important to know how to integrate the literature review into the introduction in an effective way. Although you can mention other studies, they should not be the focus. Instead, focus on using the literature review to aid in setting a foundation for the manuscript.

What Goes in the Literature Review of the Discussion Section?

Literature reviews play an important role in the discussion section of a manuscript . In this section, your findings should be the focus, rather than those of other researchers. Therefore, you should only use the studies mentioned in the literature review as support and evidence for your study.

There are three ways in which you can use literature reviews in the discussion section:

• To Provide Context for Your Study Using studies from the literature review helps to set the foundation for how you will reveal your findings and develop your ideas.
• Compare your Findings to Other Studies You can use previous literature as a backdrop to compare your new findings. This helps describe and also advance your ideas.
• State the Contribution of Your Study In addition to developing your ideas, you can use literature reviews to explain how your study contributes to the field of study.

However, there are three common mistakes that researchers make when including literature reviews in the discussion section. First, they mention all sorts of studies, some of which are not even relevant to the topic under investigation. Second, instead of citing the original article, they cite a related article that mentions the original article. Lastly, some authors cite previous work solely based on the abstract, without even going through the entire paper.

We hope this article helps you effectively present your literature review in both the introduction as well as the discussion section of your manuscript. You can also mention any other tips that will add to this article in the comments section below.

References:

[1]  http://www.math.montana.edu/jobo/phdprep/documents/phd6.pdf 

[2]  https://libguides.unf.edu/c.php?g=177129&p=1163732

This Is a Very Useful Information… thank you. It helped me a lot. It is explained clearfully.

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Writing: Literature Review Basics

• What is Synthesis?
• Organizing Your Research
• Paraphrasing, Summary, or Direct Quotation?
• Introductions
• Conclusions
• All Writing Guides: Home
• CORE Library Home

The Most Important Thing

The best time to write an introduction is AFTER you write the body of your paper.

Well, how do you know what to introduce until after you've figured out what you want to say?

The best time to write an introduction is as one of the last things you do.

Basic Introduction Template

For any other sort of scholarly writing, the following basic structure works well for an introduction:

• What has been said or done on this topic?  
• What is the problem with what has been said or done?
• What will you offer to solve the problem?  (The answer to this is your thesis statement.)
• How does your solution address necessary change?

Writing an Introduction

The job of an introduction is to preview what you are going to say so the audience knows what is coming.  A good introduction starts out generally and works towards a specific statement of what you intend to discuss in your writing. 

The introduction explains the focus and establishes the importance of the subject. It discusses what kind of work has been done on the topic and identifies any controversies within the field or any recent research which has raised questions about earlier assumptions. It may provide background or history, and it indicates why the topic is important, interesting, problematic, or relevant in some way.  It concludes with a purpose or thesis statement. In a stand-alone literature review, this statement will sum up and evaluate the state of the art in this field of research; in a review that is an introduction or preparatory to a larger work, such as the Culminating Project, it will suggest how the review findings will lead to the research the writer proposes to undertake.

In a literature review, an introduction may contain the following:

• A concise definition of a topic under consideration (this may be a descriptive or argumentative thesis, or proposal), as well as the scope of the related literature being investigated. (Example: If the topic under consideration is ‘women’s wartime diaries’, the scope of the review may be limited to published or unpublished works, works in English, works from a particular location, time period, or conflict, etc.)  
• The introduction should also note what topics are being included and what are intentional exclusions. (Example: “This review will not explore the diaries of adolescent girls.”)
• A final sentence should signal the list of key topics that will be used to discuss the selected sources.

Many theories have been proposed to explain what motivates human behavior. Although the literature covers a wide variety of such theories, this review will focus on five major themes which emerge repeatedly throughout the literature reviewed. These themes are incorporation of the self-concept into traditional theories of motivation, the influence of rewards on motivation, the increasing importance of internal forces of motivation, autonomy and self-control as sources of motivation, and narcissism as an essential component of motivation. Although the literature presents these themes in a variety of contexts, this paper will primarily focus on their application to self-motivation.

Let's break that apart.

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A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

It is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the field.

In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews. Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to make summary claims of the sort found in systematic reviews [see below].

Integrative Review Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review A review does not always focus on what someone said [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection, and data analysis. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own study.

Systematic Review This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

NOTE : Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

Cook, Kathleen E. and Elise Murowchick. “Do Literature Review Skills Transfer from One Course to Another?” Psychology Learning and Teaching 13 (March 2014): 3-11; Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . London: SAGE, 2011; Literature Review Handout. Online Writing Center. Liberty University; Literature Reviews. The Writing Center. University of North Carolina; Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: SAGE, 2016; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012; Randolph, Justus J. “A Guide to Writing the Dissertation Literature Review." Practical Assessment, Research, and Evaluation. vol. 14, June 2009; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016; Taylor, Dena. The Literature Review: A Few Tips On Conducting It. University College Writing Centre. University of Toronto; Writing a Literature Review. Academic Skills Centre. University of Canberra.

Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

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• Review Article
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• Published: 20 May 2023

A review of glass corrosion: the unique contribution of studying ancient glass to validate glass alteration models

• Roberta Zanini   ORCID: orcid.org/0000-0002-3190-9191 1 , 2   na1 ,
• Giulia Franceschin   ORCID: orcid.org/0000-0003-1817-2962 1   na1 ,
• Elti Cattaruzza   ORCID: orcid.org/0000-0003-0643-0266 2 &
• Arianna Traviglia   ORCID: orcid.org/0000-0002-4508-1540 1  

npj Materials Degradation volume  7 , Article number:  38 ( 2023 ) Cite this article

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• Structure of solids and liquids

Glass has been used in widespread applications within several sectors since ancient times and it has been systematically studied under different perspectives. However, its thermodynamic properties and the variety of its compositions, several aspects related to its durability and its alteration mechanisms remain still open to debate. This literature review presents an overview of the most relevant studies on glass corrosion and the interaction between glass and the environment. The review aims to achieve two objectives. On one hand, it aims to highlight how far research on glass corrosion has come by studying model systems created in the laboratory to simulate different alteration conditions and glass compositions. On the other, it seeks to point out what are the critical aspects that still need to be investigated and how the study of ancient, altered glass can add to the results obtained in laboratory models. The review intends also to demonstrate how advanced analytical techniques commonly used to study modern and technical glass can be applied to investigate corrosion marks on ancient samples.

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Introduction

Throughout history, glass has played a central role in the production of goods for widespread use, making it a material that has been extensively studied from various perspectives. However, due to its thermodynamic properties and the vast range of existing compositions, there are still several aspects of its nature that are not fully understood. For instance, evaluating glass durability and alteration mechanisms remains a challenge, as it requires taking into account numerous factors, some of which are difficult to replicate in laboratory experiments. One of the most prominent challenges is the time required for the transformation of the glass structure, which is directly responsible for the kinetics and dynamics of the processes involved.

Technological innovation has recently introduced new tools for studying the durability of glass and shedding light on the deep connections in extremely complex materials between compositions, structure, and the surrounding environment. As an example, a recent review 1 reported about the possibility of employing mechanistic models to study aqueous glass alteration. Thanks to the availability of such novel techniques, glass alteration mechanisms and kinetics can be hypothesised using non-conventional experimental methods (simulations) and without direct experimental validations. In parallel, the long-term perspective provided by studying ancient vitreous objects can inform and underpin the predictions obtained through simulation and laboratory methods 2 , 3 , 4 , 5 , 6 .

Most of the studies performed to understand glass corrosion are based on artificial ageing experiments that are designed to monitor, step by step, the process of glass alteration and the physicochemical evolution of the glass structure 7 , 8 , 9 , 10 . Because of the great variability of the experimental conditions involved in these studies, however, results are not yet conclusive, and the glass transformation process is still only partially described in literature.

Standard approaches to the study of glass corrosion are limited in that they are valid in some specific experimental conditions, but not in others. Indeed, most of the available research focuses on the corrosion mechanisms of glass with specific compositions (lead-rich or borosilicate families), or under conditions of saturated humidity (relative humidity, RH ≥ 100%) 11 , 12 , 13 . In both cases, the atomic diffusion occurring in the material alteration processes is assisted by two key parameters, which are the presence of ionic species with high mobility and the presence of liquid water on the surface, respectively. Only an handful of published works 2 , 7 , 8 –focused on ageing under conditions of RH < 100% and on a wider variety of glass compositions, like those of the silica-soda-lime (SSL) glass family– are available nowadays in literature: they represent valuable experimental approaches to determine and separate the relative influence of different parameters on the chemical and environmental durability of glass.

In view of the above, this review focuses on the literature available on glass alteration and related structural investigations that have contributed to the comprehension of the modifications occurring in the glass network when glass degrades. In addition, an examination of the existing –but limited– literature on the analysis of ancient glass will bring to the attention of the chemistry community a vast amount of unexplored information that is relevant also for the study of contemporary glass types, to validate the prediction of glass behaviour obtained through laboratory experiments. The most advanced techniques to study alteration on heritage glass will be described with the aim of highlighting how deepening the understanding of altered ancient glass properties is key for a full understanding of the phenomenon of natural ageing of both modern and ancient glass objects.

For better clarity, the term a rchaeological glass will be used to refer to glass specimens that have aged in soil or marine environments for centuries, whereas the term historical glass will be used when speaking about glass aged under the effect of environmental moisture fluctuations (in museums or private collections). The term ancient glass is more generally used to refer to both the above mentioned cathegories.

By using a combination of comprehensive characterization of ancient samples and laboratory-based ageing experiments, researchers can obtain valuable information about glass alterations that have occurred over hundreds of years while also monitoring the alteration process step-by-step. This integrated approach offers a unique opportunity to consolidate and supplement current theories of glass corrosion and validate predictive models using real-world examples of long-term alteration. Overall, this approach provides a more complete understanding of the complex process of glass corrosion and can help guide the development of more durable glass materials.

Glass structure and its dissolution prediction: key highlights

Glass is an amorphous material that shares structural features at the atomic level with a supercooled liquid, while exhibiting mechanical properties typical of solids.

Zachariasen 14 was the first to describe the amorphous structure of glass in 1932. His studies led him to conclude that there is no long-range atomic periodicity in glass structure. He also defined the requirements for a particular oxide to exist in the vitreous state. Silicon dioxide, also known as silica, is the primary network former in both ancient as well as in modern glass.

In silica-based glass, the bonding forces involved in the formation of the glass network are those of crystalline SiO 2 , which has a tetrahedral structure with 4 oxygen atoms located at the corners of a tetrahedron and a Si atom placed in the middle of the structure 15 (Fig. 1 ). The glass network can be figured as a built-up of such tetrahedral elements, which share each corner with neighbouring tetrahedra, one per corner. The remaining corners are available to form other chemical bonds. SiO 2 is considered a primary network former, but other oxide types exist with these same properties, such as B 2 O 3 or P 2 O 5 . The addition of alkali oxides, such as Na 2 O, K 2 O, Li 2 O, to the glass network former is responsible for breaking some of the Si-O-Si bonds bridging silicon atoms and for including the alkaline cations into the glass structure. These network modifiers generate the formation of non-bridging oxygens (NBOs) in addition to the bridging oxygens (BOs) of the silica network. NBOs hold a negative charge that is locally compensated by one highly mobile alkali cation with a positive charge (i.e., Na + ). The main effect of such network modifiers is to decrease the viscosity of the glass melt, thus facilitating its workability at lower temperatures. Alkaline earth oxides like CaO are added to the glass batch as network stabilisers 16 . They connect to two NBOs and are commonly considered as lower mobility ions (i.e., Ca 2+ ) that are possibly effective in inhibiting the diffusion of other cations across the silica network, hence improving the chemical resistance of the glass (Fig. 2 ). It is now clear why 90% of commercial glass is made of a mixture of silica (as network former), sodium and calcium oxide (as network modifiers). Nevertheless, the positive influence of alkaline earths in increasing glass stability against alteration has not been fully demonstrated yet, and several works indicated that Ca 2+ and Mg 2+ ions are at least as mobile as alkalis within the hydrated layer in atmospheric conditions 17 , 18 .

Schematic representation of a tetrahedral silica unit (not to scale).

For clarity’s sake, only the BOs and NBOs helping to identify the Q n configuration in the silica network are coloured in red for BO and in yellow for NBO.

The structure of glass is not in thermodynamic equilibrium. The durability of glass depends on both the kinetic and thermodynamic stability of its oxide components. In a state of thermodynamic equilibrium, the chemical potential of the species on the glass surface and those in solution are equal, and as a result, no net mass transfer occurs. However, glass does not exist in a state of thermodynamic equilibrium and is therefore prone to undergo chemical reactions that can result in degradation over time. The understanding of the kinetic and thermodynamic stability of glass components is crucial for developing more durable and long-lasting glass materials.

An essential factor to be taken into consideration to predict glass dissolution is the knowledge of the relative concentration of bridging and non-bridging oxygen atoms 19 . The latter are bonded to only one silicon atom and their quantity within the glass network is proportional to the concentration of modifier ions.

Considering the two possible configurations for an oxygen atom (BO and NBO), the silicon atom may be found in five different tetrahedral arrangements: Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 , where the subscript indicates the number of bridging oxygens (Fig. 2 ). The structure of the glass network is the result of the distribution of the rings and voids regulated by the interconnection between these different silicate tetrahedra. The size of the voids in the network controls the rate of water diffusion, which is kinetically favoured when the dimension of the voids is comparable to the diameter of the water molecule (0.28 nm). In complex glass (mixed alkali glass), modifier cations can totally or partially fill the voids, but when the material is exposed to high relative humidity conditions, these alkali ions are leached from the glass surface and replaced by hydrogen ions as part of molecular water. The ion-exchange reaction drives the hydrolysis of the glass network with kinetics depending on both the distribution of local structural units (Q n ) and the modifier content. In addition, the exchange of high-radius cations as K + from the bulk is considered to leave a bigger void in the glass network compared to the exchange of smaller cation as Na + , thus facilitating the entrance of water molecules into the deeper areas. In general, it can be said that the higher the concentration of NBO, the higher the number of ion exchange sites available and the rate of ion-exchange and network hydrolysis, following the reaction trend: Q 1  > Q 2  > Q 3  > Q 4 19 , 20 . For this reason, in the discussion about the kinetics of the processes of ionic exchange and hydrolysis reactions it is essential to take into consideration the chemical composition of glass.

Taking into account what has been said above, knowing the chemical composition of complex glass and its Q n concentration and distribution is fundamental for the appreciation of its chemical stability and leaching resistance, so as to adopt an adequate preventive conservation strategy. Nevertheless, the glass reactivity does not depend on Q n species only. Understanding the correlation between local structural features of the glass and the activation energies of individual bonds is also crucial to predict the dissolution mechanism of the glass network. Potential Mean Force (PMF) calculations estimated the activation barrier for Si dissolution in presence of aluminium (Al). They revealed that Al is easily dissociated from glass network, but Si dissociation is hindered when Al is present as a second neighbour 21 . As a result, Al causes opposing effects on glass durability if added at low and high concentrations: the addition of Al in small concentration increases the durability by reinforcing the strength of Si and increasing the polymerisation of the glass network, while at high Al concentration, the preferential release of Al results in the weakening of the silicate network. This predicting method can be extended to understand the role of Na, Mg, or B in more complex glass compositions.

Since the prediction of dissolution of glass network is a complicated topic of research, many recent works 22 , 23 , 24 reported the use of machine learning based approaches to account for the percentage of bridging oxygen species, network connectivity, average ring size, as well as the composition modification due to the preferential release of modifier cations during the incongruent dissolution.

Several glass studies demonstrated the usefulness of Raman spectroscopy as an analytical technique to discriminate the characteristic vibrational modes of each Q n configuration 25 , 26 , 27 , 28 . Through the deconvolution of the Raman bands typically associated to the glass network it is possible to determine the single Q n distribution and associate the variation of the area of the related Q n band with the chemical composition of the sample analysed 29 , 30 . When using this analytical approach, Raman spectroscopy can be adopted as a technique to distinguish a stable glass from an unstable one by means of the rigorous deconvolution of the vibrational bands of the glass network 29 , 31 , 32 .

Nowadays, an analytical protocol that combines the potentialities of Raman spectroscopy described above with the advantages of using portable instrumentation to evaluate the chemical stability of glass, to predict the glass network dissolution for preventive conservation purposes, or even to establish the suitability of glass as storage material for nuclear waste is not available. Moreover, the in-depth spatial resolution of Raman spectroscopy is inadequate if one wants to determine the layered structures on an altered surface, whose features, in fact, sometimes vary on the nanometre scale. The association of other complementary analytical techniques would be ideal to optimise the reliability of Raman spectroscopy results.

X-ray absorption near edge structure (XANES) spectroscopy at the Si K-edge is another interesting technique that has been used to study the polymerisation degree of SiO 4 tetrahedra silica glass. In XANES spectra, position and structure of the absorption edge are largely determined by the charge of the absorber atom and by geometry of the first coordination shell, which depends on the coordination of the nearest neighbour atoms, the degree of polymerisation, and the presence of network modifiers and network substitutes 33 . A study conducted on silicate glass reported that, with the increase in the polymerisation degree of the silica tetrahedra, the Si K-edge shifts towards higher energy, while it shifts towards lower energy when Si is substituted for another network former (Al) 34 .

The chemical changes occurring at the surface of a corroding glass often cause an alteration of the local environment of metal atoms, especially of the metal-oxygen pair distribution. These characteristics have been measured using conventional and glancing angle extended X-ray absorption fine structure (EXAFS) techniques, able to give information about number and distance of the atoms surrounding the absorber one. Examining the modifiers distribution in the vicinity of the surface gives important hints on how the surface is modified as corrosion advances 35 .

As transition metal cations are generally of interest for the application of X-ray absorption techniques, their study can be easily exploited to monitor the decay of ancient glass. The modifications of the chemical environment of chromophore species (i.e., transition metal cations) can be recorded using both XANES and EXAFS techniques by monitoring the spectra of a selected metal species in an altered glass sample. By means of this method, it was possible to establish a relationship between the oxidation state of Fe and Cu cations during glass decay. Whereas, the Mn oxidation state was not directly correlated with the glass decay of the samples studied 36 .

XANES and EXAFS analysis are not common and straightforward techniques, since they require the preparation of tailored samples from analysed objects (which is not always possible when dealing with cultural items) and the access to a synchrotron facility. However, this type of analysis provides important information to help the identification of structural and chemical changes in altered glass samples when investigating the molecular changes around cations with high field strength and well-defined short-range order.

In the following section, the main models of glass corrosion will be explored: they are based on the observation of the interactions between glass structure and external environment and on the different mechanisms in place during the process of glass alteration.

Exploring the mechanisms of glass alteration through the different existing models

The terminology used in the published works is imprecise. The terms corrosion , alteration , degradation , and deterioration are often found to be frequently used interchangeably as synonyms, despite having different shades of meaning. For the sake of clarity, all these terms will be used here as synonyms when talking about the phenomena that induce a change in glass physicochemical properties, regardless of the intrinsic or environmental factors that have determined such change. The term dissolution will be, instead, used to refer only to the rupture of the Si-O covalent bonds and the breakup of the structural silica network, and the term leaching will be used to identify the step of degradation that consists in the loss of alkali and alkaline earth metals ionically bonded to the silica network that precedes the network dissolution 37 . Even if leaching is often associated with the initial steps of glass degradation, the loss of alkalis can be a secondary phenomenon in specific conditions of high temperature (RH < 100%) and unstable glass compositions 16 . In other particular conditions, the degradation process occurs without any loss of alkalis, because the water penetrated into the glass network is unable to solvate them 8 , 9 , 18 .

The interaction between water and glass can activate two different degradation phenomena, i.e., leaching and network brakeage, mainly depending on the pH of the solution in contact with the surface. During leaching, the aqueous solution in contact with the surface typically has pH<9 38 , 39 . In this condition (pH<9), ion exchange that involves alkalis (Na, K) and alkaline earth metals (Ca, Mg) occurs, forming ionic bonds with the oxygen of the glass network and the H + ion from the aqueous solution 16 . This is a diffusive phenomenon and the thickness of the glass region involved in the reaction (indicatively a few microns) depends on the glass composition and on the time and temperature of exposition. The altered layers that form on the surface can act as a diffusion barrier to further extraction, even if hazardous cracks that allow the penetration of water molecules into the pristine glass may form. The alteration due to leaching does not affect the Si atoms: the network distribution does not change, only the Si-O-M bonds do.

On the other hand, the ions interdiffusion during leaching leads to a pH increase (eventually above 9) due to the formation of Na + OH - species in solution and Si-OH acid from the reaction between Si-O-Na and H 2 O. An alkaline environment results in more aggressive attacks to the glass network, since it promotes the dissolution of the Si–O bonds 40 . The reaction with the hydroxyl ions (OH – ) breaks the Si-O–Si bonds and silanol groups Si–OH are formed.

The sites left free by the leaching of cations from the glass surface can be easily filled by hydrogen ions, which have small ionic radius. The hydrolysis process induces the introduction of H 2 O molecules and OH - ions into the opened silica structure, thus increasing the rate of the hydration process and the ion exchange.

The interactions and reactions that occur between aqueous solution and glass have been over the years the subject matter of extensive research focusing on the mechanism involved in this process at different scales of observation, research that has been key to support the formulation of the classic theory of alteration and Interfacial Dissolution Precipitation model for dissolution of vitreous materials.

The Classic Inter-Diffusion (CID) model of glass corrosion is based on diffusion-controlled hydrolysis and ion exchange reactions 41 , which lead to the formation of structurally and chemically distinct zones (Fig. 3 ).

The latter consists of a hydrated, cation-depleted layer resulting from a selective cation release. Courtesy of Gin et al. 48 .

The classic theory of glass corrosion is underpinned by different formulated models of ion-exchange that take into consideration the effects of the preferential dissolution of more soluble cations during the initial part of the leaching process 42 , 43 .

Many experimental and theoretical results obtained from observation in liquid conditions (and not in unsaturated humidity ones) have reported that the leaching mechanism involves the preferential release of alkali and alkali-earth ions rather than that of network formers, such as Si or Al ions, with a consequent formation of alumina/silicate-rich layer on the glass surface 38 , 43 .

In general, the concept of preferential leaching is based on the thermodynamic and kinetic stability of the different glass modifiers. At lower temperatures and for ions with the same charge, the diffusion of larger ions (for example Ba 2+ or Ca 2+ ) becomes energetically unfavoured, while the smaller ions (for example Mg 2+ ) can move more easily through the glass network 44 . In any case, double-charged ions usually show less diffusivity in SSL glass than single-charged ones, mainly because of the marked effect of the very intense local electric fields acting on them. If the leaching mechanism proceeds, the solution becomes richer and richer in OH – and its pH increases favouring the dissolution of silica through the break of O-Si-O network. Preferential leaching supports the selective removal of specific cations (non-stoichiometric release) and designs a theory about incongruent dissolution of glass that explains the formation of altered surface layer.

The technological developments of the last decades have made it possible to reveal increasingly more detailed evidence on the process that controls glass corrosion 11 . The chemical reactions proposed in the general mechanism of glass corrosion (hydration, hydrolysis, and interdiffusion) are still considered valid in the most recent studies. However, over the last years the attention of the scientific community has been focusing more on understanding how these reactions evolve kinetically and thermodynamically during the alteration process, and how they influence the structural and microstructural properties of the alteration layer at the atomic scale 1 , 45 .

In 2015, a nanometre-scale study 46 of glass corrosion was performed using a combination of high mass and spatial resolution techniques, proposing a revised theory of glass corrosion called the Interfacial Dissolution-Reprecipitation (IDP) model. The IDP model is based on the congruent dissolution of silicate glass coupled in space and time with the reprecipitation of amorphous spherical silica aggregates of variable size. In opposition to the traditional glass alteration model, this recent theory supports the stoichiometric dissolution of glass without interdiffusion-controlled ion-exchange mechanisms at the glass reaction front 46 .

Hellmann et al. validated this model through the study of artificially aged borosilicate glass altered at 50 °C in deionised water, using a unique combination of techniques with high spatial and mass resolution 46 . By following the mobility of the major constituent elements of complex borosilicate glass, an identical release behaviour was noticed for modifier and former ions, regardless of their charge. These results validate two processes at the basis of this novel corrosion mechanism, i.e., the stoichiometric release of all the glass elements and the precipitation of amorphous silica with the formation of an altered surface layer. Furthermore, the interface between pristine glass and altered zone was demonstrated to be chemically and structurally well defined, with elemental gradients in the nano- to sub-nano-metric range.

A schematic representation of the IDP model is presented in Fig. 4 according to the results of oxygen and silicon isotope tracer experiments in ternary borosilicate glass 47 .

a Initial congruent dissolution of glass is the first step occurring at the glass-water interface. This stage continues until the amorphous silica solution is supersaturated and etching pits are formed on the surface; ( b ) Si-rich interfacial solution layer is formed depending on the ratio of silica in solution to silica released during glass dissolution. Under this condition, the localised saturation of silica in solution promotes condensation and nucleation reactions that lead to the polymerisation of monomeric silica to form dimers and oligomers; ( c ) Precipitation of silica in the form of spheres on the dissolving glass surface occurs after silica supersaturation and nucleation in the solution; ( d ) Formation of SAL (Surface Alteration Layer) composed of altered amorphous silica proceeds along with congruent glass dissolution and further diffusion of dissolved species through the developing SAL; ( e ) Diffusive transport of water and dissolved species through the corrosion rim continues depending on the porosity of the SAL, i.e., higher porosity values correspond to higher diffusion rates; ( f ) Precipitation of secondary minerals like zeolites and clays sometimes occurs within and at the silica surface.

As described above, the IDP model entails the formation of alteration layer through the precipitation of hydrated species from the thin film of water to the hydrolysis front, with high degree of liberty to reorganise (Fig. 5 ). This leads to a sharp concentration profile of highly soluble cations and an interstitial water layer that would allow an easy separation of the altered layer from the pristine glass 48 .

Courtesy of Gin et al. 48 .

The rate of the reactions involved in the glass corrosion (ion-exchange, hydration, and dissolution) depends on factors such as glass composition, temperature, and pH. All of them may occur simultaneously during process of alteration in contact with liquid water and can be rate-limiting in function of the experimental conditions. In conditions of unsaturated humidity, studies demonstrating the existence of a rate drop followed by a residual regime are lacking. For these reasons, both the two models presented in literature (CID and IDP) are not able to describe universally the distribution of mobile ions and hydrous species inside the alteration layer.

A work published in 2017 49 reported the in-depth characterisation using atom probe tomography (ATP), transmission electron microscopy (TEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) of the alteration layer formed under close-to-saturation conditions. The results revealed the presence of an alteration layer with a more complex structure, made of three different sub-layers (Fig. 6 ): (i) close to the pristine glass, a thin hydrated layer containing all the glass components, (ii) moving towards the surface, a passivating layer with constant concentration of glass formers (Si, Al) and decreasing concentration of modifiers (Na, Ca) which is delimitated by a rough interface where alkaline and alkaline earths are preferentially leached out, and (iii) an external nanometric layer where Si undergoes hydrolysis and condensation reactions 49 . These results contradict the IDP model of glass corrosion recently designed, which highlights that many gaps are still present in the explanation of the mechanism of glass alteration.

Courtesy of Gin et al. 49 .

The authors of this review would like to emphasize that the universal application of the IDP model and other intermediate models to all silicate glass types is still an open question. This is because most recent research has focused solely on the artificial alteration of borosilicate glass, and the applicability of these models to other types of silicate glass remains uncertain. The characterisation of synthetic and geological glass altered over long time periods can be key to understand both the complex mechanisms responsible of the long-term transformation of glass network and the physicochemical features of the material transformed by the alteration. Multi analytical and high spatial and mass resolution investigation performed on surface of ancient glass provide clues about the characteristics of the altered layers and about how they reorganised their internal structure during long time periods. Instead, looking at the interface between the alteration layer and the pristine glass provides information about the mechanisms of glass alteration 2 . In this way, this approach promises to be efficient in validating the above mentioned debated kinetic models. The knowledge of the long-term behaviour of glass structure is also pivotal to predict the network dissolution in burial conditions in nuclear waste management studies 4 , 50 .

Evidence of degradation on ancient glass

Archaeological glass shows multiple and clearly visible symptoms of deterioration that can help identify well-distinguished classes of glass alteration 51 depending on the conditions it aged in (i.e., in soil, underwater, under extreme environmental conditions) for several centuries. Dulling, iridescence, weeping, pitting, discolouration and cracking of the surface affecting the specimens are common phenomena that can be observed on glass that has been recovered from archaeological excavation 52 . The formation of one or the other of them depends both on the physicochemical properties of the glass and on the environmental factors it has been exposed to. More than one of the manifestations of alteration can be found in a single glass object, thus making it sometimes difficult to identify the most appropriate strategy for conservation and/or stabilisation.

The atmospheric deterioration of glass appears markedly different from the degradation that occurs when glass objects have been buried for centuries, for example under soil, and it is rarely observed on this type of glass that has remained interred up to discovery and recovery. Crizzling , also known as glass sickness or glass disease, has been identified as the major alteration symptom for glass objects stored in museums and in private collections.

Alteration of ancient glass in archaeological stratigraphic contexts

The term dulling is used to refer to the loss of clarity and transparency typically observed in ancient glass and caused by the formation of layers of alteration products on the glass surface 52 .

As discussed above in the section dedicated to the general mechanism of glass corrosion (Section “Exploring the mechanisms of glass alteration through the different existing models”), in presence of neutral or acidic conditions, elements like alkalis are typically leached out from the first glass layers onto the surface. These leached species, reacting with humidity and moisture of the environment, tend to form corrosion products (like salts) that build up on the object’s surface and determine at first the loss of the original clarity 53 . In addition, at advanced stage of alteration, also hydrated silica (silica-gel) particles can also reprecipitate on the glass surface, leading to the formation of thicker alteration layers and causing an additional loss of glass transparency and the appearance of translucency 54 . This phenomenon is due to a combination of effects occurring between the local presence of water and the composition of glass, which determines the diffusion of ionic species from the first atomic layers under the surface to the environment and the consequent reprecipitation of hydrated silica and other alkali-derived compounds. The extent of the visual effect is much more considerable as the ion exchange proceeds and the thickness of the deposited layers on surface increases.

In more advanced stages, dulling can lead to the formation of iridescence patinas (Fig. 7 ) that may eventually detach in the form of crusts from the original glass substrate. In 1863 Brewster 55 demonstrated that this iridescent effect is due to the diffraction of incident light from layers of weathering products containing metal oxides formed after ion leaching. The rays of light are reflected from thin alternating layers of air and weathered glass crusts. These densely overlapping layers gradually penetrate deeper into the glass and they eventually change in colour towards darker hues 56 . The cationic species leached from the glass are often prone to reacting with the anionic species derived from reaction between OH - (especially in basic conditions) and atmospheric acid gases, thus forming salts with hygroscopic properties on the surface. This generates a phenomenon called weeping , which was first described by Organ in 1956. Weeping can lead to the formation of crystals or solutions of salts, depending on their deliquescence relative humidity 57 .

Images collected with Olympus BX43F optical microscope, x10 magnification. A detail of the indented rim where the overlapping of thin layers of altered patina is clearly visible (right panel).

The alteration phenomenon called pitting , is described as micro, small or large based on pit size, and it can occur simultaneously at different individual sites that later merge into interconnected complex structures producing an altered top layer, which causes the loss of glass transparency 52 .

Contrary to dulling, pitting is a visible mark of the weathering process, which occurs in alkaline solution 58 and commonly found on excavated glass. As described in Section “Exploring the mechanisms of glass alteration through the different existing models”, during the alteration process in alkaline solution, the prevailing deterioration mechanism is the dissolution of the silica network through the breaking of Si-O-Si bonds. Subsequently, the prolonged presence of a layer of moisture on the surface of glass causes an increase in the pH of the attacking solution, and ultimately pits are formed as a result of local dissolution of the silica network 52 .

A model that explains the formation of altered pits was recently developed by observing the decay process of different silicate glasses in river and marine aquatic environments 59 . The experimental results showed that the alteration of SSL glass is characterised by a two-step mechanism. The first step, called “hydration period” , is short and causes the formation of isolate fissures, while the second step, called “pit development period” , involves the creation of basic species (OH - ) during the dealkalinisation process that progressively break the silica network, thus widening the fissures to form pits. According to the results of this work 59 , the formation of pits is correlated to a dynamic loss of mass, i.e., the slow rate of the first step of hydration allows the diffusion of solution and the consequent basic attack inside the fissures, causing local network dissolution.

Figure 8 shows the surface of a Roman archaeological sample affected by pitting.

Image collected with Olympus BX43F optical microscope, x10 magnification.

Discolouration can be often seen on archaeological glass surface in combination with other types of weathering phenomena described above. It is closely related to darkening, which occurs when the oxidation of specific leached ions, such as iron, manganese, and copper, changes the colour of the weathering crusts, or to the production of hydrogen sulphide by sulphur-reducing bacteria in anaerobic environment and the formation of lead sulphide 53 . The latter case occurs only when the glass contains a high concentration of Pb oxide, and it is buried under anaerobic condition. In other cases, the presence of manganese and iron causes the darkening of glass with the formation of brownish pits 60 , as those visible in Fig. 9 . Ancient glass contains these elements in the form of impurities present in raw materials (sand and wood ash) or as a result of their deliberate addition as chromophores or/and decolourant agents in the form of minerals (i.e., pyrolusite) 61 .

In the case of archaeological glass that has been interred for centuries, cracks may be present on the surface of the glass fragments: these are caused by the shrinkage of the alkali-deficient layer due to temperature and humidity changes. Typically, these cracks are filled with mineralised material, likely originating from the soil in which it was buried. The mechanism of their formation is currently being studied since it has not yet been fully understood.

Alteration of glass in atmospheric conditions

Crizzling has been identified as the appearance of minute cracks on the surface of glass (Fig. 10 ) developing over time. These cracks penetrate deeper in the body of the object, ultimately resulting in its physical collapse. This phenomenon is due to two main factors: the unstable composition and the storage in fluctuating humidity environments 62 . Better storage conditions can slow down, but not stop the deterioration, because the role of glass composition remains a key factor in the evolution of the crizzling alteration 63 .

Object exposed at the Museo Civico di Modena (Italy). Picture by Renaud Bernadet.

Early publications reported the chemical effects causing crizzling through experiments that reproduced the condition of glass alteration in the laboratory. The results showed that crizzling is mostly associated with glass compositions characterised by high alkali and low CaO contents, and/or a high K/Ca ratio 64 .

In 1975 Brill 63 first used the term crizzled to describe glass with a decrease in its transparency due to the formation of fine cracks on the surface. He noted that certain glass objects that were in contact with water for centuries do not exhibit a high degree of degradation; however, once they are exposed to museum storage conditions (light, low RH and temperature), they display the formation of crizzling. This alteration mechanism is due to the dehydration of the glass surface, i.e., the low RH in museum display cases (15-20 %) causes a loss (up to twenty percent in weight) of the water that penetrated the gel layer of altered glass, bringing on a significant loss of volume in the gel layer itself which ultimately results in the cracking of the glass surface 62 , 63 . In general, the cracking of the hydrated gel layer formed on the surface of unstable glass can be attributed to several factors, including the dehydration of the gel layer itself - as mentioned above - the leaching process, which can lead to network contraction after the replacement of larger alkali ions (Na + ) by smaller hydrogen ions (H + ), and the different coefficient of expansion of the bulk glass and the gel layer 16 .

The guidelines of the Corning Museum of Glass 52 describes the process of crizzling indicating five stages. In the first stage (Initial stage) the glass has a blurred appearance due to the presence of leached alkali on the surface. During this phase, it is still possible to wash the surface and the glass can return to its original appearance. Conversely, in the second stage (Incipient crizzling) the haziness remains also after washing and the glass surface exhibits fine cracks like tiny silvery lines. Cracking progresses in stages three (Full-blow crizzling) and four (Advanced crizzling) until it gets to the deepest regions, leading to the loss of small fragments. Eventually, crizzling is so deep that the glass loses its structural integrity, even without any external contribution (Fragmentation stage).

Often crizzled glass has a pinkish hue. When alkaline leaching occurs and the glass structure is open, the manganese ions present in the surface cracks oxidise, yielding a pink colour 62 . This phenomenon is more evident in ancient glass which contains manganese as a decolourant.

To limit the evolution of the crizzling process, preventive conservation is an essential strategy for the safety of museum glass objects. The Corning guidelines set the optimal RH range for glass conservation between 35 to 65%, however crizzled glass or glass with a particularly fragile composition require specific individually controlled cases with a stable RH condition in the range of 40–50% inside 65 .

In addition to preventive conservation, the development of an innovative and compatible consolidation treatment for the protection or repair of cracked glass is an open challenge for the scientific community of cultural heritage conservation.

Intrinsic and extrinsic parameters influencing glass deterioration

As already mentioned, atmospheric conditions, such as temperature, the pH of the environment, salts and ions concentration, relative pressure under burial or marine conditions, and the presence of water in liquid form (RH ≥ 100%) or vapour (RH < 100%), strongly influence the kinetic of the glass surface alteration and its chemical transformation.

Many published papers of archaeological interest use the term weathering to refer to the typical degradation process that affects archaeological glass that has been exposed to particularly unfavourable environmental conditions (especially in burial and underwater contexts) 52 . Weathering is a degradation process occurring through contact with water in the environment, both in the vapour and in the liquid state, whereas the term atmospheric deterioration is used to describe glass that aged under the effect of water in the form of moisture (especially in protected environments like museum display cases), which means in the form of vapour state interaction.

The study of extrinsic factors, that are closely linked to the environmental conditions acting during the alteration process over centuries requires the development of appropriate artificial ageing protocols, which allow the modelling of the phenomenon as a function of these parameters. Real cases of glass degradation are the result of the combined actions of the intrinsic and extrinsic factors mentioned above, which generate entangled mechanisms of ionic interdiffusion from the glass network to the environment and vice versa. From an experimental point of view, studying this interconnected process of ionic interdiffusion and the formation and growth of novel phases implies the need of relying on simplified model systems to investigate the effect of specific variables to the detriment of others, which are kept constant.

The study of the effect of the various factors on glass degradation has attracted the curiosity of researchers since the beginning of the XX century. Already in 1925, G. W. Morey 66 stated that the subject was still in an empirical state, despite the considerable number of works carried out. At that time, to understand the effect of water on the alteration process many experiments were conducted by varying temperature, the pH of the environment and glass composition.

The following paragraphs describe the effect of various parameters that have a primary influence on the process of glass alteration. The studies there cited use ancient glass samples as evidence of the long-term effect of these parameters.

Effect of glass composition

Many ancient finds of SSL glass are macroscopically preserved intact in their shape, despite the physicochemical alterations caused by the burial environment over the centuries. Currently, 90% of globally manufactured glass is still based on the SSL composition, which has been kept largely consistent over the centuries except for a few modifications introduced at the beginning of the 20 th century to enhance chemical durability and resistance to devitrification 67 . Besides SSL glass, many other types of ancient glass exist, such as potash lime glass (K 2 O – CaO – SiO 2 ), lead silicate glass (PbO – SiO 2 ), or potash lead silica glass (K 2 O – PbO – SiO 2 ) 68 . The optimal preservation of certain ancient samples and the complete collapse of others is the result of a complex interplay between their intrinsic material properties and the extrinsic factors acting on them.

Intrinsically, glass physicochemical properties play a significant role in determining its degradation behaviour. Such properties typically correspond to the chemical composition of glass, the nature of its surface, the presence of impurities, inclusions, inhomogeneity, and phase separations. In particular, the concentration of silica, alkali (soda, potash), stabiliser (lime, lead), as well as the inclusions of trace elements and additives like metal oxides, added into glass as chromophores, opacifiers and decolourants all strongly affect material durability 53 . Small variations in the concentration of these components determine strong variations in glass durability. Diffusion through the leached layer is more likely to occur for smaller ions, such as Na, Mg, Li, rather than for larger ones, i.e., Ca or Ba. Although all types of alkaline silicate glass are susceptible to weathering degradation, from a thermodynamic point of view stability increases as in the following: K 2 SiO 3  < Na 2 SiO 3  < Li 2 SiO 3 38 .

Silica rich glass, such as Roman SSL glass, is more durable than poor silica glass, like medieval glass 69 . In addition, K + , which is contained in medieval glass as a monovalent cation modifier, is more susceptible to leaching out from the deeper region of the glass network during weathering alteration than bivalent cations such as Ca 2+ , which are present in Roman glass 70 , 71 . This behaviour is due to the bivalent cations forming stronger bonds with non-bridging oxygen, as previously mentioned. The preferential leaching of K + over Ca 2+ during the weathering process was also confirmed by an experimental study that investigated the weathering phenomena on naturally weathered potash-lime-silica-glass 72 . This behaviour of K + cations was also observed during leaching experiments in aqueous acidic solution.

The study of the varieties of composition of ancient glass and their resistance in RH > 100% have made it possible to identify the presence of compositions that are more chemically stable than others and to define general conditions to discriminate between stable and unstable glass 73 .

In 1978 74 , Hench systematically studied the surface of glass exposed to pure water and distinguished between 6 main types of surfaces with increasing inclination to deterioration in relation to their composition. The surface layers of the different glass types may have protective or non-protective properties for the glass substrate, depending on the capacity of reducing ion leaching and glass dissolution. In this work Hench distinguished stable and unstable glass based on the different compositions of the alteration products that form the first surface layers when glass is altered in liquid conditions. However, the protective character of hydrated layer has not been demonstrated in unsaturated humidity conditions yet; and, moreover, a work of Sessegolo et al . 75 studied medieval stained-glass windows in unsaturated conditions with isotopic water experiment and demonstrated that the alteration layer is not protective against vapour transport and interdiffusion.

A further way to discriminate a stable glass from an unstable one could be to use the ternary diagram of Fig. 11 , which was formulated in 1975 by Newton et al . 76 with a view to help predict the weathering behaviour of different types of glass. Plotting the concentration (mol. %) of network stabilisers (RO), network modifiers (R 2 O), and silica (SiO 2 ) determines the chemical stability of a given glass composition, i.e., highly durable glasses are placed near the centre. This diagnostic model may work well when binary or ternary glass is considered, however complications may occur when classifying ancient glass, which has a more complex composition, because it may be necessary to consider the combination of multiple formers (SiO 2 ad Al 2 O 3 ), monovalent oxides (R 2 O) and divalent oxides (RO).

R 2 O represent the content of network stabilisers (monovalent oxides), RO the content of network modifiers (divalent oxides), and SiO 2 the content of silica.

By using electron microprobe analysis (EMPA) and hydration-dehydration experiments on ancient glass Brill observed that a deficit of stabiliser (CaO content less than 4 wt%) combined with an excess of alkali (over 20 wt%) in SSL glass easily leads to an extensive surface deterioration known as crizzling 63 . He defined as unstable glass the one with total alkali oxide concentration over 20%; this composition range determines silica network configurations that are open enough to facilitate the migration of monovalent cations. In line with these findings, other studies reported that an increase in the CaO to SiO 2 ratio increases glass stability, while a concentration of calcium oxide over 15 wt% entails rapid glass instability 38 . Recently, a comparison in terms of glass chemical stability was made using artificial mock-ups with different glass compositions which were exposed to high humidity environment and different levels of formic acid 77 . The results showed that glass with a higher content of stabilisers exhibits greater stability, especially glass with calcium.

The presence of potash as a modifier and the low silica content in the composition make glass particularly fragile and susceptible to chemical alteration. Numerous works reported the considerable damage that affects medieval glass windows with Si-K-Ca-based composition, which is considered one of the most unstable 78 , 79 , 80 . Typical alteration marks are mainly pitting 81 or the formation of a corrosion crust 71 on the surface due to the combined attack of water and increased pollution in the air. The weathering crust is often very heterogeneous and fractured because of the wetting-drying cycle conditions and generally consists of calcite, gypsum and/or syngenite formed by the reaction between the alkaline and alkaline-earth elements released during ion-exchange and sulphur dioxide in the atmosphere 6 .

Effect of minor components in glass composition

In addition to the major elements discussed above, other minor elements may be present in the composition of ancient glass as decolorising, colorising or impurities of the raw materials used for their production. These minor elements can in turn contribute to the alteration process of the glass itself by giving rise to distinctive corrosion marks.

Several authors have remarked that manganese that accumulated between the altered layers may have originated from the burial environment 82 , 83 . Dark deposits have been detected inside the dealkalinisation layer on the surface of Roman glass samples 84 . Secondary electron images have uncovered that these deposits are formed from the interconnection of spherical particles of about 2 µm in size, with iron and manganese oxides as their major components. These studies clarify that during the leaching process Fe (II) and Mn (II) ions are hydrated and oxidised, giving rise to the formation of dark amorphous products that precipitate into the pores of the leached silica film.

It is well-known that glass technologists used to add Mn, as well as Sb, to the glass melt as a decolorising agent, but it is also known that, in this type of glass, the appearance of brownish areas on the altered surface is due to an oxidation process of Mn(II) to higher oxidation states. It is generally observed that in the dark areas Mn is present in +IV oxidation state 85 , 86 . Nevertheless, a recent work performed using synchrotron radiation X-ray absorption spectroscopy (XAS) analysis on historical stained-glass windows demonstrated that the most extended brown altered areas mainly contain Mn mainly under a +III oxidation state 87 .

Through the analysis of 14th–17th century window glass, Schalm et al. 82 concluded that the formation of Mn-rich inclusions takes place simultaneously with the growing of leached layers settling along their interfaces and their concentration is mainly caused by the environment (soil) in which the glass was buried for several centuries.

Although brown/black staining has mostly been attributed to manganese compounds, iron (and titanium) compounds were often reported in association with the manganese 53 . Other elements which are often present in historical glass network as impurities can improve the chemical stability of glass. For example, low amounts of ZrO 2 (2 %wt) increase the acid and alkaline durability of glass 88 , and a hydrated ZrO 2 surface can act as a barrier to further dissolution of other ionic species. More recently, it was demonstrated that the substituting an insoluble oxides like zirconia to a fraction of silica slows the glass dissolution kinetics, but prevents the alteration gel reorganisation inhibiting the pore closure mechanisms and leading to greater degree of corrosion 89 .

These results show that it is essential to know the exact composition of an ancient object to determine a range of the aging behaviour that it is prone to. To do that, it is important to also consider the presence of elements in lower or trace concentrations to fully describe the corrosion phenomenon and formulate models that are closer to real cases of ancient glass alteration.

Effect of liquid vs atmospheric water

Water was observed to be the primary environmental ageing factor that causes glass deterioration by Lavoisier already in the early 1770s. Lavoisier also indicated two different mechanisms of water penetration into the glass network, i.e., through network voids between oxygen atoms in molecular form, and by hydrolysis and condensation reactions with the metal-oxygen bonds 19 .

The alteration of silicate glass differs when it occurs in liquid-phase or in vapour-phase regime. In contact with water the glass surface undergoes chemical attack through ion-interdiffusion and hydrolysis of metal-oxygen bonds 6 . In vapour conditions (RH < 100%) the hydration process does not release elements into the fluid, but it involves a redistribution of elements in the alteration layer (expect for volatised elements) 90 . The formed hydrated layer has chemical composition and porosity that are different from those obtained in liquid conditions, thus resulting in the glass durability properties specific for liquid or vapour-phase regimes. Even if the molecular process is the same, the interplay between the intrinsic and extrinsic variable changes affecting the macroscopic transformation of the material 45 .

During the phenomenon of water diffusion, no matter in which RH condition, the glass surface is covered by different monolayers of water, depending on the water partial pressure in the atmosphere. As soon as the silica network of a freshly made glass encounters moisture from the environment the chemisorption of water molecules starts from the first layers of the surface, which is consequentially more subjected to physical and chemical transformation. Water is chemisorbed on the glass surface and a thin film of water of few molecular layers builds up onto the chemisorbed layer, depending on RH condition.

One of the first characterisations of altered glass surface was conducted by Hench in 1975 56 , using a combination of analytical techniques that enables to obtain physicochemical information from different depths of the altered glass surface. He systematically studied the glass surface altered in liquid water, using techniques like Auger electron spectroscopy (AES), infrared reflection spectroscopy (IRRS), and electron microprobe (EMP), and distinguished 6 main types of surfaces with increasing inclination to deterioration (Table 1 ). Of these, only Type I is stable and shows no main differences with bulk glass. The other types are considered unstable and prone to deterioration over time. Type II and type IV are of interest for historical deterioration. Type II is formed when the glass contains a high level of network formers, and it involves the formation of a silica-rich layer on the surface that acts as a protective film, preventing the leaching of alkali ions and the rupture of the glass network (Si-O-Si). On the other hand, no protective film is formed for Type IV and so the leaching of alkali can proceed. This latter situation is typical of unstable glass.

According to Hench’s classification, ancient glass has a surface that corresponds to Type II and Type IV surfaces, which are characterised respectively by the presence of silica-rich surface protective layers (when the concentration of network formers is high enough) and the presence of non-protective surface layers that allow the alkali leaching to proceed (when the soda to silica ratio is high enough) 56 .

In the case of discontinuous contact with aqueous media, which is typical of the atmospheric alteration, the formation of laminated structure on the altered glass surface has been explained by the intermittent water supply 6 . Through the observation of ancient glass surface by TEM, a sequence of bands of different thickness has been recently distinguished 91 , i.e., thinner bands called lamellae (20-50 nm) and thicker ones called laminations (0.1-4 µm), which consist in groups of laminae with the same orientation. The amorphous laminae, which are depleted of alkaline ions, are formed from the local rearrangement of glass elements resulting from the repetition of several cycles of interdiffusion and glass dissolution processes. Moreover, cracks are developed perpendicularly and parallelly to the laminae and laminations. Based on this study, two main processes take place within the cracks, i.e., the migration of atmospheric solutions deeper into the bulk glass, thus moving the alteration front further, and the precipitation of secondary mineral phases which favours the mechanic separation and the loss of glass fragments.

In 2016, a study of the altered layers observed in ancient glass proposed a model to explain the formation process of this laminated degradation 92 . Thanks to optical microscope, field emission-scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDX) analyses, it was possible to describe surface lamellae as an alternation of random amorphous silica nanoparticles with different packing densities and with thickness between 0.1 and 10 µm. Moreover, the growth of nanosized silica particles on the surface of the altered glass was observed to be one of the by-products of the leaching process of glass components that occurs under alkaline conditions.

While glass alteration in contact with water in a liquid state was widely studied during the 1900s, testing approaches in unsaturated atmosphere have been seldom explored until more recently 93 . In 2020, Majerus et al. 7 published an overview of the experimental protocols for glass alteration tests in unsaturated atmosphere. All the tests reported were conducted under RH < 100% condition and, when a saline solution was present, their configuration ensured that the samples were never in contact with it. These results obtained under RH < 100% suggested that the glass alteration mechanism was different from the one obtained in contact with liquid water (RH > 100%). In contrast to saturated humidity conditions, in unsaturated environments, the hierarchical order of glass alteration processes involves network hydrolysis first and interdiffusion second, particularly for glass with compositions classified as unstable.

Alloteau et al. recently demonstrated that in water vapour conditions, the process of glass hydration is independent from dealkalinisation: at higher temperature (80 °C) hydrolysis predominates over diffusion processes and solvation, whilst at lower temperature the two processes proceed together in parallel. To state that, they performed ageing test in static conditions (no T and RH cycling) at controlled temperature and humidity (RH 85%) on three different types of unstable historical glass (soda-lime silicate from antiquity, mixed alkali silicate from middle-age/renaissance and potassium silicate from XVI-XVIII century). The conditions were set to avoid any liquid water flow on the samples during tests.

Sessegolo et al. 75 evaluated the respective contribution of rain, wet periods and unsaturated humidity on the kinetics of formation of the alteration layer of potash-lime-silica glass. The results demonstrated that the fluctuation of dry and wet periods (non-static condition) leads to the formation of an altered layer rich of cracks, pits, and scales, which constitute a network of pores and fractures behaving as a major vector for interdiffusion of liquid and vapour water. The characterisation of this fracture network is particularly important in case of prolonged alteration in atmospheric conditions, where liquid and vapour water phases are systematically combined in the associated diffusion processes. In another subsequent work 94 the authors proposed a model to estimate the thickness of alteration of stained glass windows subjected to liquid water (such as rain), comparing the expected value of alteration thickness extrapolated linearly over 650 year with those observed on ancient stained glass. The values predicted by the developed model match well with the alteration layer thickness observed on real ancient medieval samples 6 , 95 , 96 .

Effect of saline and burial environments

In the last two decades, several works have been done to explore the effect of ageing in burial or marine environments on the alteration mechanisms of glass and on the formation of new alteration phenomena. A considerable number of works focused on the prediction of the durability of high-level nuclear waste glass and the migration of radioactive and non-radioactive elements into the burial soil 9 , 97 , 98 .

Verney-Carron et al. 5 studied ancient Roman glass blocks naturally aged in seawater obtaining the kinetic parameters to develop a geochemical model to simulate the alteration of archaeological glass. Comparing the results obtained from the characterisation of ancient glass with those from the computer simulation, this work demonstrated the importance of archaeological glass for validating the predictive capacity of geochemical long-term model, thus bridging the gap between the results obtained from short-term experiments and long-term alteration of complex systems.

Palomar devoted many studies to weathering and to the comprehension of the environmental effect on the stages of glass alteration 59 , 99 , 100 . In particular, she reported the effect of coastal atmosphere on glass degradation 101 , which is a scarcely investigated subject. The alteration of glass surface exposed to coastal environment is mainly caused by the high presence of liquid water that covers the glass surface, thus inducing a hydrolytic attack and the dealkalisation process, and by the high wind speed, which favours the transportation and deposition of sodium and chlorine ions on the glass surface. Marine aerosol in elevated concentrations represents a hazardous agent for the chemical stability of glass, since its action could increase the hygroscopicity of the glass surface and open the glass structure, allowing the alteration to proceed deeper into the glass.

Another visible mark of degradation observed on the surface of glass altered in saline environment is the formation of peculiar chemical ring-like patterns that have been explained according to the Liesegang theory (Fig. 12 ). Diffusion, reaction, nucleation and crystal growth are all phenomena that have been used to formulate models that explain the Liesegang rings formation 102 , 103 . Dal Bianco et al . observed these weathering rings present on glass fragments from the Roman ship Iulia Felix found on the Grado lagoon, in North-East Italy, and dated back to the 2 nd century AD 104 , 105 . Their characterisation study showed a maximum diameter of the rings of about 1 mm and the interconnection of interface lines during the simultaneous growth of adjacent rings. This study did not report an exhaustive theory about the formation process of rings, but the authors observed that the structure was similar to the descriptions of Liesegang kinetic evolution of precipitates in gel found in other research works 106 , 107 . This assumption is acceptable since hardly corroded glass structure can be assumed as a gel where weakly soluble salts periodically precipitate due to the reaction between two soluble substances, one of which is dissolved in the gel medium. The final appearance of the precipitates depends on their solubility and on the initial concentration ratio of the reagents, but generally their aspect is concentric around the centre in which precipitation starts 108 .

Image collected with an Olympus BX43F optical microscope, x10 magnification. Interconnection between adjacent rings can be observed.

Nowadays, the formation of this type of rings on archaeological glass has been observed on the surface of samples recovered in submarine environment 104 , 109 . The authors of this review suggest that a saline-rich environment, whether seawater or wet soil, play a role in the formation of the rings. The soluble substances present in these environments may react with soluble substances from the aged glass forming salt precipitates on the surface of glass that act as centre of nucleation for the growth of the concentric structure.

The mechanisms that generate the formation of rings on altered glass surface have not been analytically confirmed, and the study of the evolution of the kinetics that controls ring growth on archaeological glass remains incomplete, despite the availability of modern techniques. Although the layers observed and experimentally described by Schalm 110 can be compared to the bands structure obtained from Liesegang kinetics, there is a lack of systematic experimental work in the literature to confirm whether this theory can be applied to the dissolution-precipitation mechanisms of glass.

Chemical studies on the composition of the soil where archaeological glass aged for centuries might validate the hypothesis of ionic exchange between the elements of the soil and the elements of the glass network, providing the opportunity to understand how the interaction with the external environment can drive the process of alteration in different types of glass.

Many published research studies mention that also for glass samples that altered buried in soil the most common pathology observed is the formation of dealkalinisation layers 111 , 112 , 113 . One of these performed a stratigraphical analysis using a non-invasive technique (laser induced breakdown spectroscopy, LIBS) to observe the progressive dealkalinisation of glass bulk composition, reporting an increase in the calcium and sodium intensity signals on the glass surface 84 .

In the field of ancient glass, experimental studies simulating ageing in soil are less common 114 , 115 , 116 . In a recent work, Palomar et al . tried to replicate ageing in soil in a natural burial environment to understand the corrosion mechanisms acting on different ancient glass types (Roman, medieval, lead crystal glass, common window glass) 117 . The burial tests were set up to last 300 days and were carried out at 60 °C, to accelerate the alteration processes. The pH of the burial soil and the glass composition showed to have a key role in controlling the reactions between the constituent glass elements and those from the environment.

When considering burial artificial alteration of mixed alkali glass compositions with low silica concentration, acidic or neutral conditions (pH 6.5 – 7.5) lead to the formation of micro pits and cracks; an iridescent and translucent layer is formed, instead, on the surface of glass under alkaline soil condition (8.0 – 9.0) 118 . Silica-soda-lime glass shows a different behaviour. In fact, in acidic soil the formation of both isolated and interconnected fissures may be observed. However, under neutral and alkaline soil conditions an increase in the number and depth of pits, whose rate of accretion depends on the content of alkaline oxides in the glass, can be detected, with a considerable increase in the diffusion of surface degradation under the alkaline condition 117 . This experimental evidence is a clear example of the necessity to consider glass composition and environmental factors simultaneously when approaching the understanding of the glass corrosion mechanism.

Effect of the environmental pH

A pivotal factor in determining the rate of glass corrosion is the pH of the attacking solution both in case of vapour and liquid conditions 10 . Under conditions of low pH (acidic solution), the deterioration mechanism predominantly involves the ion-exchange process due to the abundance of hydronium ions in solution and the formation of silanol groups [Si-OH], generating a hydrated gel on the surface which slows down degradation 117 . Differently, at high pH, the interaction between the glass surface and the alkaline solution leads to the dissolution of the silica network through the rupture of the Si-O-Si bonds, which implicates a more aggressive condition.

In works performed in 2016 92 and 2021 110 , Schalm et al. observed that glass surface is transformed in both acid and alkaline condition, but different morphologies can be developed depending on the pH of the solution. At pH<7 the transformed glass has a homogeneous morphology determined by the dissolution of leached silicate network and precipitation of dissolved compounds as amorphous silica forming linear and randomly branched chains of alteration product that can be described as a silica gel. At pH between 7 and 10 of the local solution the transformation process leads to the formation of silica nanoparticles packed into alternating density lamellae. The authors successfully reproduced the lamination in laboratory experiment performed around pH 10, where silica solubility is highly variable and induces the cyclical dissolution-precipitation of silica. The mechanism of formation of consecutive lamellae with alternating packing density is proposed to be dependent on the pH oscillation in the local solution during the alteration process. In extreme alkaline condition (pH>10) the fast dissolution of silica network occurs without any material precipitation.

A recent work 94 highlighted how the pH-dependency is linked to the specific composition of glass and consequently to the solubility of different glass constituents. The results show that the pH-dependency of Ca-Mg-silicates highly differs from the behaviour of aluminosilicates, and that Si-K-Ca medieval glasses have a very low pH dependency at alkaline conditions.

Effect of pollutants

Air pollution was identified as a particularly dangerous agent that enables to speed up and enhance alteration processes. In the museum context, the presence of carboxylate acids pollutants is generally the main cause of glass corrosion and of the formation on the glass surface of efflorescence salts as deterioration products 119 . With regards to historical stained glass, an experimental work carried out over a six-year period to quantify the influence of various air pollutants from different local environments (Europe and North America) on the degradation of potash-lime-silica glass, which has a similar composition to that of medieval stained glass, showed the formation, after exposure to rain and solar radiation, of crystalline carbon-rich products unlike those of samples aged under sheltered conditions 120 .

Melcher and Steiner 120 performed a 6-year experiment to evaluate the effect of acid gas and pollutant on replicas of stained glass, comparing the leaching depth formed on potash-lime-silica glass replicas to former leaching studies performed on medieval stained glass. This work demonstrated that the hypothetic leaching depth cannot be directly related to pollution data because, while the leaching depth increases with time, the leaching rate decreases. Even a paper of Robinet et al . 30 highlighted the role of the organic pollutants formic acid, acetic acid and formaldehyde in the alteration of unstable soda silicate glass. He confirmed that museum wooden cabinets emitting organic pollutants must be avoided since they foster the progressive alteration with an estimated rate of ~2 nm/day.

Advancements in monitoring ancient glass conservation state

The kinetics of glass corrosion, as well as the sequence of events involved and the prevalence of one interconnected mechanism over another, are dependent on both the chemical composition and structure of the glassy material and the environmental conditions to which it is exposed. These conditions include the amount of water that reacts, the chemistry and pH of the solution, and the duration of exposure. Given the complexity of these factors, the alteration of glass is a complex phenomenon that requires careful study. Therefore, particular attention should be paid to the analysis of ancient glass samples that have undergone modifications over time scales that cannot be replicated in the laboratory. These samples provide valuable insights into the long-term alteration mechanisms of glass and can aid in the development of more accurate predictive models for understanding the complex behaviour of glass materials in various environmental conditions.

The characterisation of altered ancient glass provides concrete evidence of the transformation of the vitreous structure, the nature of the dissolution products, and the way in which a specific glass composition reacts to a particular environment 3 , 4 , 7 , 70 , 121 , 122 . Over the course of history, the evolution of glass manufacturing has led to the production of various types of glass, such as Roman SSL glass, medieval stained glass, or Venetian crystal glass ( Cristallo ). Artefacts (and their fragments) made from all of these types of glass inherited from the past represent an exceptional opportunity to better understand the processes involved in glass corrosion on a long-term scale. This is due to their specific chemical stability, resulting from their unique chemical composition, and their aging in soil, underwater, or confined spaces with specific microclimates for centuries.

The analysis of glass surfaces is currently considered the most effective scientific approach for studying chemical and physical variations that occur at the interface between the atmosphere and the glass surface. Advanced surface analysis techniques, such as X-ray photoelectron spectroscopy (XPS) and ToF and/or dynamic SIMS, provide high-resolution data that enable the investigation of the chemical composition of the glass surface’s first nanometres and facilitate monitoring of its modification during the alteration process 67 , 123 . SIMS is widely considered one of the most suitable techniques for studying corroded glass 124 , due to its ability to detect hydrogen and investigate the glass surface up to a depth of a few microns. Another technique commonly reported in the literature for studying the weathered surface of ancient glass is laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), which allows for the acquisition of 2D and 3D elemental distribution with spatial resolution as high as the spot size used for the analysis. Only one work, published in 2013 125 , presents the procedure to obtain elemental maps to investigate surface layer phenomena on pitted ancient glass. The results of this application showed that the dealkalinisation of glass sample occurred on surface, as the direct or indirect consequence of the hydration process, resulting in the final formation of pits and of the so-called Liesegang rings on the surface of the sample. Despite the novelty of this research, the spot size of the laser beam used in this work (diameter of 80 µm) is higher than the average size of the corrosion marks present on ancient samples. The spatial resolution on the reconstructed maps can be improved by using smaller spot size of the laser beam (down to 10 µm) making it possible to obtain a resolution suitable for appreciating the chemical variability of extremely heterogeneous samples. This can potentially increase the understanding of the mechanisms of formation of these altered phases.

Critically speaking, the extensive study of glass surface composition using these advanced analytical techniques is still seldom considered when investigating the corrosion mechanism of glass both on ancient samples and through an artificial ageing approach in the laboratory 30 , 126 , 127 . Each surface technique has its own peculiar applications and limitations 128 , but a combination of different techniques allows a complementary and more comprehensive characterisation of corroded glass.

Collecting morphological, structural, and chemical information through a multi-scale analytical approach is necessary to gain a comprehensive understanding of the evolution and kinetics of glass alteration, including the modification of glass composition. Well-designed experiments are crucial due to the co-operation and mutual influence of different mechanisms involved in glass corrosion. This approach allows for investigation from a macroscopic to a nanometre scale observation. The information gathered can help determine how and when to intervene for conservation and preservation of glass objects, whether they are of industrial or historical significance.

The evaluation of storage parameters such as relative humidity, temperature, and light, as well as the condition of the glass, are crucial for preventing degradation of glass objects in museum collections or other sites of cultural interest. This approach is particularly important for understanding the influence of intrinsic and extrinsic factors in the processes of alteration and finding the best conditions for conservation. Surveys 129 , 130 , 131 conducted in various museums in Europe have shown that a significant number of glass objects are in critical condition due to extensive degradation, highlighting the need for preventive actions. For example, at the Victoria and Albert Museum, out of 6,500 glass vessels optically investigated, more than 400 showed clear signs of glass deterioration 54 . Similarly, the glass storage conditions of the Royal Palace of Madrid and the Technological Museum of Glass (Segovia) were evaluated during the period from September 2019 to November 2020, detecting a high concentration of formic acid in the display cabinets and wardrobes, mainly due to the presence of wood, which results in a higher glass surface hygroscopicity (pH=8) 132 .

Several methods have been used to categorise unstable glass from museum collections based on its appearance 65 or by analytical techniques –like X-ray fluorescence (XRF) 133 , ion beam techniques (particle -induced X-ray emission, PIXE, and particle-induced gamma ray emission, PIGE) 134 or spectroscopic techniques (Raman and Fourier-transform infrared, FTIR) 135 , 136 – in order to determine glass composition. Using these techniques to analyse cultural heritage objects can be complex due to many operative limitations, as for instance the need of carrying out micro-sampling (which is often not possible) or moving the objects to the specialised laboratories where the instruments are hosted. With the aim to distinguish stable from unstable glass in museum collection, the ideal analytical technique should provide the chemical composition of the sample, be highly sensitive, and have a very fast time of analysis to characterise as many samples as possible in a short period of time, while operating in a non-invasive way. Consequently, it can be safely stated that no scientific and straightforward approach for understanding the chemical nature and composition of unstable glass in a non-invasive way and for large museum collections of glass objects has been developed yet.

In the light of the above, it is evident that the most effective approach to study glass degradation involves the integration of two main distinct but interconnected methods:

the evaluation of the surface of ancient glass artefacts;

the evaluation of intrinsic and extrinsic causes for glass alteration.

Advanced analytical techniques can be used to characterize ancient glass surfaces and observe the products of the long-term transformation of the glass structure resulting from the alteration process. In parallel, laboratory-based aging experiments can be used to evaluate intrinsic and extrinsic causes of glass alteration, distinguishing the effects of different parameters and identifying the most relevant factors that influence glass alteration kinetics. Parameters such as the content of alkalis and stabilizers in the glass composition, as well as temperature and humidity, strongly impact the chemical durability of glass by affecting the concentration of alkalis and hydroxyl or non-bridging oxygens in the hydrated layer. It is difficult to determine which factor has the most significant influence on the process of glass alteration because the final degradation symptoms are the result of a mutual effect between all the aforementioned.

As evidenced, over the years many experiments aimed at understanding the phenomenon have been reported in literature and almost as many methods have been proposed. This abundance of results represents an outstanding opportunity for future research studies in this field, which will be able to rely on advanced analytical approaches and more accessible high-resolution techniques even for the domain of cultural heritage science. The results obtained from the study of ancient materials are pivotal to validate the long-term capacity of kinetic models -which are based on experimental data only - and to strengthen current theories of glass corrosion.

Moreover, comparing the characteristics of artificial replicas with those of ancient glass enables a better understanding of the glass corrosion phenomena, that in turn can underpin both the formulation of new protective solutions to preserve and protect glass artefacts in the long-term and the design of technologies exploiting glass properties in a variety of applications.

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The authors would like to express their gratitude to the anonymous reviewers: their insightful comments and critiques helped to significantly improve the quality and clarity of this paper. The authors are deeply grateful for their time, effort, and dedication.

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Zanini, R., Franceschin, G., Cattaruzza, E. et al. A review of glass corrosion: the unique contribution of studying ancient glass to validate glass alteration models. npj Mater Degrad 7 , 38 (2023). https://doi.org/10.1038/s41529-023-00355-4

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Boundary-Spanning Knowledge Search and Absorptive Capacity in Cooperative Innovation: A Study on Non-Core Firms in the Context of Sustainable Development

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• Dongping Yu 1 ,
• Tongyue Zhao 2 ,
• Jiuyue Chen 2 &
• Junjun Li 3 , 4  

This research paper investigates the vital role of boundary-spanning knowledge search and absorptive capacity in shaping the cooperative innovation performance of non-core firms operating in the context of sustainable development. The dynamic nature of technological innovation, exacerbated by market flattening and the challenges posed by the COVID-19 pandemic, has compelled small and medium-sized enterprises (SMEs) to seek innovative breakthroughs. Cooperative innovation emerges as a strategic avenue for SMEs to mitigate risks and enhance their willingness to innovate. However, non-core firms often encounter technological and market disadvantages, hindering innovation efforts. This study explores how boundary-spanning knowledge search, which bridges the gap between non-core firms and the external environment, influences cooperative innovation. Furthermore, the research delves into the mediating role of both potential and actual absorptive capacity in the relationship between boundary-spanning knowledge search and cooperative innovation performance. Empirical findings indicate that boundary-spanning knowledge searches positively impact cooperative innovation, with technological and market knowledge searches yield significant results. Moreover, absorptive capacity exhibits a dual mechanism, directly influencing cooperative innovation and mediating the relationship between knowledge search and performance. This paper extends existing research by focusing on non-core firms within the sustainable development context, offering valuable insights for SMEs seeking innovative strategies. It also highlights the diverse pathways through which boundary-spanning knowledge search affects cooperative innovation. These findings offer practical implications for enhancing non-core firms’ innovation capabilities, including encouraging proactive knowledge search and developing absorptive capacity-inducing mechanisms through employee training and information platforms.

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The paper was supported by the National Natural Science Foundation Project of China (Grant Number: 72264039), the Foundation Project of Yunnan Province’s Ten Thousand Talents Plan for Top Young Talents, the Yunnan Fundamental Research Projects (Grant Number: 202401AT070282) and the Scientific Research Project of the Department of Education of Yunnan Province (Grant Number: 2023Y0669). The authors thank them sincerely for supporting the paper’s funding.

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Yu, D., Zhao, T., Chen, J. et al. Boundary-Spanning Knowledge Search and Absorptive Capacity in Cooperative Innovation: A Study on Non-Core Firms in the Context of Sustainable Development. J Knowl Econ (2024). https://doi.org/10.1007/s13132-024-01930-z

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Received : 15 January 2024

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DOI : https://doi.org/10.1007/s13132-024-01930-z

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