Corrosion and Conservation of Cultural Heritage Metallic Artefacts

P Dillmann, D Watkinson, E Angelini, A Adriaens, P Dillmann, D Watkinson, E Angelini, A Adriaens

• Publisher: Woodhead Publishing
• Year: 2013
• ISBN:
  9781782421573
• Topic: Tecnologia e ingegneria
• Subtopic: Ingegneria mineraria

The conservation of metallic archaeological and historic artefacts is a major challenge whether they are ancient bronzes or relics of our more recent industrial past. Based on the work of Working Party 21 Corrosion of Archaeological and Historical Artefacts within the European Federation of Corrosion (EFC), this important book summarises key recent research on analytical techniques, understanding corrosion processes and preventing the corrosion of cultural heritage metallic artefacts.After an introductory part on some of the key issues in this area, part two reviews the range of analytical techniques for measuring and analysing corrosion processes, including time resolved spectroelectrochemistry, voltammetry and laser induced breakdown spectroscopy. Part three reviews different types of corrosion processes for a range of artefacts, whilst part four discusses on-site monitoring techniques. The final part of the book summaries a range of conservation techniques and strategies to conserve cultural heritage metallic artefacts.Corrosion and conservation of cultural heritage metallic artefacts is an important reference for all those involved in archaeology and conservation, including governments, museums as well as those undertaking research in archaeology and corrosion science.

• Summarises key research on analytical techniques for measuring and analysing corrosion processes
• Provides detailed understanding of corrosion processes and corrosion prevention
• Discusses on-site monitoring techniques

1

Introduction: conservation versus laboratory investigation in the preservation of metallic heritage artefacts

P. Dillmann,     CNRS/CEA, France

D. Watkinson,     Cardiff University, UK

E. Angelini,     Politecnico di Torino, Italy

A. Adriaens,     Ghent University, Belgium

Societies retain evidence of their history to provide context, stability and inspiration to their cultures, which contributes to their development, growth and learning. Historical objects and structures offer tangible historical milestones that can provide immediate direct links to past events and people. For these reasons their continued survival is of great importance. Metals provide many such objects and structures, but their tendency to corrode presents preservation problems. This book reports studies and methodologies that aim to elucidate understanding of how and why historical and archaeological metallic objects corrode, as well as examining the design, application and effectiveness of treatments and strategies for controlling it.

Preventing or controlling corrosion of heritage objects is not a straightforward materials science exercise, as might be seen in corrosion science. Since interpreting the meaning and defining the role of heritage objects cannot be a quantitative process, decision making within conservation is often complex, subject to controversy, differences of opinion and the influence of changing opinions within society. Ethics, aesthetics and function, as well as more mundane factors such as cost and resources, will influence conservation research and practice. The conservation discipline is, by necessity, a broad church that encompasses elements of many science and art disciplines. Identifying and developing relationships with these disciplines creates synergy that benefits conservation practice. An inescapable reality is the dependence of the conservation processes upon materials science to identify and validate procedures, processes and treatments that arrest or control the corrosion process. Existing relationships between conservation and both corrosion and materials science are identified within this book and suggestions for developing their relationships are offered.

Analytical investigation and experimental science can provide essential quantitative and qualitative data to support conservation rationale that should, as far as possible, be evidence-based. The importance of analysis cannot be overstated, but reading this volume it becomes clear that its impact on the conservation process depends upon sampling heritage objects. The reader will recognise that without invasive analysis, conservation would remain an empirical process with no understanding of past and present corrosion mechanisms. It would remove the platform from which evidence-based conservation treatments are developed, understood and assessed in order to validate their action and optimise their use. The nature of samples from heritage objects will largely dictate the extent to which analysis can contribute to these goals. While non-invasive sampling is always a desirable ideal, it is often invasive sampling that will provide the widest range, best quality and greatest quantity of data, as well as offering the broadest application for furthering understanding and conservation practice. Insight into the nature of this analytical data, its generation and its role in the conservation process, with comment on its usefulness, application and, where appropriate, its shortcomings is provided. Overall, the book illustrates the methodology for linking understanding of metallic corrosion to the generation and design of preservation processes.

Clearly, invasive sampling involves clashes of ideals that can potentially limit understanding of the corrosion, conservation and technology of heritage metals. Ethics, aesthetics and pragmatism must form the core of the discussion for developing a sampling rationale. Immovable statements that are either for or against sampling are not the way forward. Adopting a view that all heritage metals should not intentionally be interfered with, changed or damaged will not improve conservation practice, and this will be to the detriment of all heritage metal objects. Neither is a cavalier attitude that readily encourages all kinds of sampling acceptable. Making a decision to destructively sample must take into account context, outcome and benefit. For invasive sampling to be effective and ethical requires good communication between owners, curators, conservators and analytical scientists, with a shared goal of wishing to develop new proven conservation processes.

Many of the advances in conservation and understanding of corrosion reported in this volume are possible because of invasive sampling. Metal-lographic analysis and determining the composition of metals with highly corroded exteriors are facilitated by sampling uncorroded metal interiors. Research investigating the effectiveness of chemically aided washing methods, which aim to remove chloride corrosion accelerators from archaeological iron, requires digestion of treated objects to determine their posttreatment chloride content. Determining how archaeological and historical iron have corroded informs treatment design and will require objects to be sectioned to maximise data and understanding, even though analytical techniques used to characterise corrosion products thus exposed may be non-invasive. Invasive sampling must be seen in terms of outcomes that will increase longevity of objects, aid management of collections and ensure many thousands or, in the case of archaeological iron, millions of objects survive for longer. Even where the use of analogues in test procedures is possible, their production will be informed by invasive analysis that has identified corrosion profiles, mechanisms and patterns.

Reading this book clearly reveals that invasive sampling of heritage objects advances conservation methodology. Heritage science differs from corrosion science in that, normally, it can only make major advances in understanding and procedure by selective destruction of unique irreplaceable objects. Developing a stigma about this finite action will retard research and limit improvement in treatment design. The way forward is to weigh the advantages against the disadvantages of invasive analysis and testing, with a clear eye on the greater good and the global picture of conservation. With the support of the custodians of heritage objects, analytical science can provide a major contribution to advancing preservation methods and longevity of heritage objects. Scientists can do much to ensure that invasive sampling is an accepted option by offering clear and distinct research designs and goals, which can be readily interpreted by object custodians and used in the reasoning process that will determine if invasive analysis is appropriate and acceptable. Quality of research, dissemination of outcomes and the impact of research on management practice are all important factors in this decision-making process. A brief overview of the structure and content of the book reveals the importance and success of various sampling strategies in contributing to our understanding and delivery of conservation.

The implementation of invasive analytical methods within a structured methodology, which may range from macro through micro to nano levels of investigation, can deliver information on corrosion mechanisms that will make it possible to decipher the differing parameters controlling alteration kinetics. Providing understanding of the behaviour of the system in this way informs choice of conservation treatment for comparable materials. Integrating a range of fine analytical techniques that includes methods such as time-resolved spectroelectrochemistry, voltammetry of particles and laser-induced breakdown spectroscopy (LIBS) supports such investigative analytical studies. The capability of such analytical tools can be discovered in Part II of the book, which is dedicated to analytical techniques and methodologies.

The efficiency of these stepped and integrated studies for understanding corrosion systems is provided in Part III via topics that include artistic patinas on bronzes, silver, underwater corrosion, long-term anaerobic corrosion of archaeological iron and reactivity studies within the frame of atmospheric corrosion and industrial structures. Each submission here explains particular corrosion mechanisms and how their understanding leads to diagnosis and/or selection of conservation strategies. In support of this, a specific part of the book is dedicated to the presentation of recent results or reviews of selected cultural heritage metals and their corrosion.

In addition to laboratory studies, it is necessary to generate precise information on environmental conditions and identify their impact on the behaviour of a system, as this will lead to better understanding of the corrosion behaviour of cultural heritage in-situ. Examples of long-term monitoring of archaeological sites are integrated into general studies of corrosion systems within Part III of the book; these include anoxic corrosion of iron and reactivity studies for iron corroding in the atmosphere, and Part IV is entirely dedicated to these issues. Thus, a chapter in Part IV illustrates how a new measurement system obtains electrochemical data (EIS) on site, while the importance of monitoring environmental parameters such as oxygen, relative humidity and temperature for understanding corrosion mechanisms is reported in other chapters in Part IV. This includes offering a design for a smart sensor to gather such data in specific heritage contexts like museums and archaeological sites.

Once corrosion mechanisms and the behaviour of systems are understood, conservation options and treatment designs can be addressed. Consequently, Part V is dedicated to reviews and reporting of methods and methodologies applied in conservation. These consider the conservation issues holistically to identify, report and critique treatment methods using an interdisciplinary integrated approach with scientific studies at its core. Included here are chapters on: desalination treatments; the use of subcritical fluids to stabilise archaeological ferrous artefacts; display and long-term storage of metals in museums; conservation of shipwrecks; use of protective coatings; inhibitors to protect artefacts; and the challenge of producing standards for testing in conservation science.

As this book illustrates, both laboratory and in-situ studies to investigate and understand corrosion and conservation systems are time consuming and expensive within the poorly financed conservation sector. This is why the relationship between conservators, restorers, corrosion scientists and those specialised in material analysis, should be based on integrated ongoing collaboration and not be seen as a scientific service to conservation. To maximise the potential of this collaboration, the community must prioritise issues for scientific research in metallic cultural heritage. This can and must take place within the framework of national or international committees that include representation from all interested and relevant disciplines. National opportunities for collaboration, such as the ‘Programme National de Recherches sur la Connaissance et la Conservation du patrimoine culturel’ in France exist, while representative international research groups such as ICOMCC Metals Group and the European Federation of Corrosion Working Party (WP) 21 offer further opportunity for coordinated approaches. There are also opportunities to integrate into international scientific societies to federate groups into heritage research. This type of collaboration requires proactive action in the forthcoming years if it is to become a reality. In the absence of this priority list, there is a great risk that the financial and human resources are spread so thinly that they result in inefficiency and isolated outcomes that miss the opportunity to develop a synergy that would move forward conservation within the heritage sector.

For these reasons Working Party 21 ‘Corrosion of cultural heritage metals’ within the European Federation of Corrosion has worked to bridge the gap between corrosion scientists, conservators and restorers, by organising regular sessions at the Eurocorr conferences and by sponsoring specific actions between different teams in Europe involved in research dealing with cultural heritage metals. We hope that this book, written by the most representative members of this community in Europe, will through its illustration of the benefits of collaboration and the progress that this brings, inspire the reader to action that will contribute to developing international collaboration with other relevant and interested metals heritage groups.

Part I

Conservation issues: past, present, future

2

Conservation, corrosion science and evidence-based preservation strategies for metallic heritage artefacts

D. Watkinson, Cardiff University, UK

Abstract:

Relationships between conservation and corrosion scientists are assessed and similarities, differences and synergies identified. Corrosion control as a preservation option for heritage metals is advocated as being cost-effective and pragmatic. This will require generation of data to develop predictive conservation and estimation of object lifespan as a function of their intrinsic and extrinsic variables. Methods for quantitative determination of corrosion rates of chloride infested heritage iron and techniques for scaling to heritage value are discussed. The iron hull of the ss Great Britain and an AHRC/EPSRC Heritage Science Research Programme at Cardiff University are used to illustrate the rationale behind using corrosion control in heritage.

Key words

corrosion

conservation

heritage

metals

quantitative

2.1 Introduction

This chapter explores relationships between conservation and corrosion science. There is a general overview of conservation goals and practices when preserving heritage metals and the text explores conservation definitions, structures and goals, as well as thought processes to encourage insight and collaboration. Examples show how corrosion science is knowingly or unknowingly embedded in heritage metals conservation and the chapter discusses its impact on conservation thinking, research and practice. This acts as an introduction to other contributions to this volume.

2.1.1 Corrosion science and conservation

Corrosion science is embedded in the development, design and assessment of conservation treatments for metallic heritage objects. It indirectly influences preservation strategy and policy, with corrosion control now beginning to be recognised as an acceptable conservation practice [1]. This may be due more to the impact of economic constraints and the need for pragmatism, than from recognition that ‘no corrosion’ conservation is becomin...