Corrosion of Metallic Heritage Artefacts

Investigation, Conservation and Prediction of Long Term Behaviour

P Dillmann,G Beranger,P Piccardo,H Matthiessen

• Editore: Woodhead Publishing
• Anno: 2014
• ISBN:
  9781845693015
• Argomento: Technologie et ingénierie
• Categoria: Ingénierie minière

Understanding long term corrosion processes is critical in many areas, including archaeology and conservation. This important book reviews key themes such as the processes underlying corrosion over long periods, how corrosion rates can be measured and materials conserved.After an overview of the study and conservation of metal archaeological artefacts, a group of chapters reviews long term corrosion in metals such as steel, iron and bronze. Other chapters review the impact of environmental factors on corrosion rates. The book also considers instrumental techniques for measuring corrosion such as electrochemistry and scanning electron microscopy, as well as ways of modelling corrosion processes. There is also coverage of the effectiveness of corrosion inhibitors.With its distinguished editors and contributors, Corrosion of metallic heritage artefacts improves our understanding of long term corrosion and its effects. It provides a valuable reference for those involved in archaeology and conservation, as well as those dealing with the long term storage of nuclear and other waste.

• Reviews long term corrosion in metals such as steel, iron and bronze
• Considers instumental techniques such as electrochemistry for measuring corrosion

1

Examination and conservation of historical and archaeological metal artefacts: a European overview

C. DEGRIGNY, ICOM-CC Metal Working Group, France

Publisher Summary

This chapter presents a European overview of examination and conservation of historical and archaeological metal artefacts. Experience has shown that research in the field of metal artefact conservation fluctuates. It is not healthy, but funds are not unlimited. Professionals have to consolidate the efforts made and bring new expertise that can attract greater attention and funding. Since the peak of activity during the Fifth Framework Program the funding of the EU projects related to the cultural field has decreased, but there is hope that the situation will improve and the community of professionals involved in metal conservation has to be prepared to respond to the availability of funds. Informing professionals of current opportunities, networking them, and promoting their skills and expertise are key actions that should sustain the dynamism of these professionals. Professionals working in the European area also have to realize the opportunities they have to apply for funds that do not exist in other parts of the world. The international conservation community envies us for the support we can receive to perform top-quality research. It is essential that we make the best use of these funds for the benefit of research in the conservation field.

1.1 Introduction

The investigation of metal artefacts and their conservation has been through major improvements in the last decades. In the past large samples were required to perform an accurate investigative analysis, while today only a small amount of metal is needed. Some techniques are even non-invasive (no sample taken). As regards conservation practices, the integrity of the artefact was previously not respected when cleaning the metal surface of its corrosion layers. As a result the original surface¹ (OS) was often lost. Today, no intervention is possible without respecting the OS and preserving any archaeological or historical information that can be retrieved during the cleaning process.

Surely all professionals consider the investigation and conservation of metal artefacts as being of equal importance. But when looking at the financial support they can benefit from, it is obvious that these two disciplines do not attract the same attention. Scientific examination and archaeometric studies (for dating and provenance) are the noble tasks in conservation science and still constitute most of the labours. In comparison, the design of conservation techniques and the definition of treatment parameters have been quite neglected. Even if the situation is improving, it seems that once the artefact has appeared to reveal all possible artistic, technological and historical information, we pay less attention to the possible loss of information that could occur during the conservation treatment. We know today that confidence in conservation practices suffered serious damage from over-mechanical cleaning of artefacts, the use of improper chemical treatments and various other practices that were not sufficiently controlled.

However, these mistakes had positive consequences, since professionals realised that any intervention needs to be monitored thoroughly. If, in the past, artefacts were given to trained technicians, it is commonly accepted that today special skills and knowledge are required to appreciate the complexity of artefacts and to treat them in a suitable manner.

Conservation science is a multidisciplinary field. The integrity of the artefact can only be respected if all specialists (archaeologists, art historians, conservators, conservation scientists and researchers) involved contribute to the choice of the best investigation and conservation techniques. In some countries with a long history of conservation science and a solid network of research centres, the collaboration of specialists at a national level is relatively straightforward. For other European countries that had to build their skills and knowledge from almost nothing, EU funding has given them real opportunities to set up their field of expertise and put them on the map of countries acting for the preservation of cultural heritage.

1.2 How scientific examination and archaeometric studies receive most of the funding

The need to better document artefacts that are witnesses of the development of past civilisations is universally accepted. Art historians are playing a key role here since they constantly need scientific data to support their theoretical approach. The literature in the field is quite abundant and some prominent research and cultural institutions even publish their own journals dedicated specifically to these topics (Techné² for the Centre for Research and Conservation of French Museums – UMR 171, Paris (France), and Technologische Studien³ for the Kunsthistorisches Museum, Vienna (Austria)). Other journals such as ArtMatters⁴ in the Netherlands cover topics of national interest. These new journals clearly show the need to disseminate the outcomes of research performed on artefacts not only to conservation professionals, but also to a larger audience. The general public is indeed more and more demanding as regards the available scientific information. There is no doubt, then, that everyone strongly supports the need for research laboratories dedicated to the better understanding of museum artefacts.

Ever since the discovery of X-radiography by Roentgen, researchers working on the diagnosis of museum artefacts have always promoted the use of the most innovative tools to document artworks. X-radiography was the starting point for the creation of research conservation laboratories all around the world [Mohen 1996] and these laboratories are still working on developing tools that respond to the needs and ethics in this field (non-invasive approaches or, if needed, sampling of the minimum required amount of materials) [Janssens and Van Grieken 2004]. The field is constantly changing as the analytical techniques get more sophisticated. It is understandable that highly specialised researchers are required to develop tools that are specially adapted to the needs of the conservation field.

The following techniques are both non-invasive and adapted to the study of metal artefacts:

• Neutron radiography [Flügel et al. 2004] allows structural observation that is complementary to X-radiography.

• PIXE (Proton Induced X-ray Emission) is commonly used for the analysis of bronze [Gomez-Tubio et al. 2000, Martinot et al. 2000], silver [Constantinescu et al. 2000] and gold [Perea et al. 2000, Demortier and Ruvalcaba-Sil 2000] artefacts.

• MicroXRF [Pantazis et al. 2002] is particularly adapted for portable elementary analysis.

• Neutron diffraction [Kockelmann et al. 2003] gives valuable information on phase contents and volume textures.

Other techniques are more adapted to the structural study of corrosion products:

• microRaman spectroscopy [Neff et al. 2004]

• microXRD [Dillmann et al. 2002].

These techniques often belong to exceptional research infrastructures that are primarily accessible to the national user community, but EU programmes such as the Access to Research Infrastructures (Improving the Human Research Potential and the Socio-economic Knowledge Base) have offered the possibility of finding and implementing the solutions to problems of common interest through networking activities. The Laboratories on Science and Technology for the Conservation of the European Cultural Heritage (LabS TECH⁵) is such a network. It aims to promote the sharing of knowledge, skills, expertise and resources in the field [Brunetti 2003]. The LabS TECH project is now completed and has been followed by the ARTECH⁶ (Access Research and Technology for the conservation of the European Cultural Heritage) project funded under the EU sixth Framework Programme (Structuring the European Research Area – Research Infrastructures) with similar aims to those of LabS TECH.

The networking between major research laboratories working in conservation science is one aspect; the networking between the professionals involved is another. The EU COST (European Cooperation in the Field of Scientific and Technical Research) actions have been established to respond to the need for exchange of knowledge through the regular meeting of experts, the organisation of workshops or training schools and the mobility of researchers (through short-term scientific missions). COST Action G8⁷ (and previously G1 [Demortier and Adriaens 2000]), ‘Non-destructive testing and analysis of museum objects’, has played a major role in networking conservation professionals and scientists. Its main objective was to improve the synergy between art historians, archaeologists, conservators and natural scientists. This included the exchange of knowledge about the available non-destructive techniques and the requirements to perform investigations on valuable or unique objects. In addition, museums and similar institutions obtained easy access to universities and research facilities that provide such techniques.

Although the examination of artefacts and archaeometric studies correspond to a real need and constitute activities that are easily funded, research in conservation suffers terribly from lack of funds and interest from researchers. There are many reasons for this. Firstly, conservation science is a new field. Although scientists like Rathgen designed, as early as the end of the nineteenth century, conservation protocols based on scientific approaches [Gilberg 1987], these protocols were considered more as recipes than as major scientific discoveries. Their aim was to solve a ‘technical problem’ such as the stabilisation (extraction of aggressive species) of artefacts that were suffering from active corrosion; they were not really aiming to reveal major information from the artefacts themselve...