An innovative tool


The computer-based Decision Support System (DSS) MiCorr is a new non-invasive diagnostic tool for metal objects. It is based on the comparison between corrosion forms observed on an artefact under investigation and those from case studies / corrosion models available in a database constructed by the project team and its partners. Two search engines have been developed: the first one uses interlinked key words and the second schematic representations of corrosion forms / models. The latter, based on Bertholon’s schematic description of metal corrosion[1], is the most innovative part of this application. Corrosion forms /models are described as structures of strata (metal, corroded metal, corrosion layers…), each stratum having its specific characteristics (morphology, microstructure, texture and other properties). A digital model allows the construction of a corrosion structure made from encoded building blocks (strata containing up to 30 characteristics).


The same set of analytical techniques with similar operating conditions have been used to characterize the corrosion models / case studies. They required the destructive sampling of artefacts to determine the precise nature, organization and composition of the corresponding strata. As they are characterized with the same digital construction model which is used for the corrosion forms under investigation a comparison becomes possible.


By comparing the corrosion form of an artefact under observation or a sample from the same artefact observed on cross-section with the database through the DSS, a visitor is able to find case studies / corrosion models of objects showing similar corrosion phenomena. In case this visitor is a conservation professional, this should help him/her to implement an appropriate conservation protocol.


The search engines allow the visitor to question the database but if registered he/she has also the possibility to enrich the database with new corrosion models and/or case studies, thus becoming an active contributor. The basic format of the corrosion models can be further improved with any additional analytical data and the operating conditions used.


Initiated by the Research Unit of the School of Conservation-restauration - Arc (HE-Arc CR) of the University of Applied Sciences and Arts of Western Switzerland (HES-SO) and developed by the School of Management - Arc (HEG-Arc), both located in Neuchâtel, MiCorr aims at:

  • Favouring a multidisciplinary approach for the diagnosis of metal objects. It encourages the compilation of scientific knowledge which is often dispersed throughout communities (conservators, corrosion scientists, archaeo-metallurgists…) and favours the collaboration between these different actors. Any information on existing or new corrosion models provided by the group of experts involved will allow MiCorr to become more and more efficient as a diagnostic decision support tool.
  • Facilitating access to already existing knowledge and allowing the comparison of the latter with new corrosion models under investigation. Discussion forums will further enrich old and new data.

The didactic, self-learning and open-source MiCorr tool should enable the diverse scientific communities involved in metal analysis and conservation to better apprehend existing knowledge and to contribute to research problems not solved yet.


The context of the project


A thorough understanding of alteration processes developed on metals enables us to slow down, limit and / or stop existing corrosion. In the industrial field, sampling of metals to investigate the amount of alteration observed is not unusual and the information gained provide knowledge on the corrosion developed such as stress corrosion cracking, fatigue corrosion, corrosion due to exposure to high temperature or pressure which are rarely encountered in the heritage domain. If this type of alteration does appear on historic or archaeological artefacts, it will not necessarily be similar due to the long periods of stress or weathering involved. Therefore, the examination of contemporary metals cannot solely be used to predict the long-term preservation of materials (Neff 2006[2]).


When conserving metal artefacts, conservation professionals try to carry out the most precise diagnosis in order to find an appropriate conservation treatment. This diagnosis requires a thorough description of the surface condition of the object under investigation. Based on experience and consultation of corrosion models published in the literature (Corrosion Science, Studies in Conservation…), conservation professionals try to find analogies with the observed corrosion forms. On the basis of this knowledge conservation strategies can be proposed.


But the matching between the corrosion forms investigated and those that are used for comparison is difficult to achieve. This is due to the lack of standardization in the description of the object surfaces (according to his/her experience, each conservator perceives the object in a different way) and the heterogeneity of heritage metal surfaces. The use of different analytical tools can bring further bias in the description of corrosion forms.


Professional associations of corrosion experts (NACE - the Professional society for corrosion engineers since 1943 and the European Corrosion Federation since 1955) have extensively published on the corrosion of metals. Even though corrosion mechanisms in particular environmental conditions (as well as the means to prevent them) are known, there are no online diagnostic support tools that link observed alterations with corrosion causes. Websites such as http://corrosion-doctors.org/Contents.htm provide illustrations of corrosion forms. Unlike heritage metals that display extensive and complex alteration forms, such as active or reactivated corrosion, they only show the result of the reactivity of the clean metal in a corrosive environment. In the conservation domain, only one database (www.materialspathology.com) used to exist but is not anymore available. It offered descriptions of corrosion forms found on heritage metals, but without providing a diagnosis.


Over the years much research on the diagnosis of corroded heritage metals has been carried out and regularly published: Dillmann and his team (Dillmann 2005[3]) have been investigating the alterations of archaeological and historic iron ; Robbiola (Robbiola 1998[4]) has focussed on the study of natural patinas on copper-based alloys and how they undergo changes when buried in a chlorinated soil that favours active corrosion ; Turgoose (Turgoose 1985[5]) has investigated alterations of lead artefacts in organic acid-rich environments, as found in museums showcases and storage cabinets. The outcome of all this research is dispersed in the literature due to the different backgrounds of the researchers involved (corrosion scientists, archaeometers, conservation scientists) as well as the audience to which the publications are addressed to (industry, engineering, conservation of cultural heritage).


Most of this research is based on the study of samples taken from the core of the metal under investigation. Until the research of Bertholon on the visual description of corrosion products and their structure (Bertholon 20011,[6]), it was impossible to adequately describe corrosion phenomena in a standardized way and refer then to existing models. Some conservation training programmes such as at HE-Arc CR, are trying to implement this method to diagnose alterations observed on both studied and unstudied heritage metals. MiCorr is the logical continuation of these efforts that will allow a broader community to adopt this methodology and improve their diagnosis on heritage metals.


Method


The research methodology of MiCorr follows the steps of a conservation project. It starts with the visual observation of the altered object, continues with the diagnosis and ends with the proposal of conservation treatments. Figure 1 illustrates these 3 steps:


Figure 1

Fig. 1: Schematic representation of the research methodology.


Development team of the MiCorr application


For HEG-Arc

  • Prof. Cédric Gaspoz, Professor in information systems at HEG-Arc. His research activities concentrate on Decision Support Systems and business intelligence. Project leader of MIFAC-Metal Online during which the MiCorr application was designed. Partner of ODOP_Corr project that had the purpose of setting up the knowledge sharing required for the further development of MiCorr and disseminating the application within the professional communities involved in metal conservation. He coordinated the work of bachelor students who contributed to the project.
  • Antoine Rosselet, was a research assistant in information systems at HEG-Arc. His main activity within the project was to create the MiCorr application and implement the database which contains the corrosion models.
  • Alessio De Santo, got an MSc in information systems. As research assistant at HEG-Arc, he resumed the tasks of Antoine Rosselet and led the application to a first final version.
  • Bernard Letourmy, independant IT consultant and was contracted by HEG-Arc to reshape partly the structure of the MiCorr application and to make it more robust.

For HE-Arc CR

  • Prof. Christian Degrigny, lecturer-researcher at HE-Arc CR. C. Degrigny has worked on many corrosion forms encountered on heritage artefacts: iron, copper (Degrigny 2012[7]), lead (Degrigny 1999[8]) and silver based (Degrigny 2006[9]) as well as on modern metals such as aluminium alloys (Degrigny 1993[10]) conserved in aggressive environments. In each case, the objective of the diagnosis was the definition of adequate conservation treatments (Degrigny 2007[11]). As the coordinator of the MIFAC-Metal research project, he developed with M. Senn (Swiss Federal Laboratories for Materials Science and Technology - EMPA) the methodology to describe corrosion models on a first group of heritage metals representative of the Swiss collections. Samples that had been taken in the past were re-examined. They constituted the first corrosion models of the MiCorr database. He contributed as a partner to the MIFAC-Metal online project and was project leader of the ODOP_Corr project during which the robustness of MiCorr was increased.
  • Valentin Boissonnas, conservator of archaeological and ethnographic artefacts is lecturing at HE-Arc CR. He is responsible of the module on conservation of metal artefacts and has been at the origin of the MIFAC-Metal project. Conservation projects carried out by master students and supervised by V. Boissonnas enrich regularly the database with new corrosion models.
  • Prof. Régis Bertholon, Head of studies, continuing education and research at HE-Arc CR. As a conservator of archaeological metal artefacts, R. Bertholon has worked extensively on the problem of the location of the limit of the original surface on heavily corroded artefacts (Bertholon 1998 [12], Bertholon 2000 [13], Bertholon 2001 [14], Bertholon 2001 [15], Bertholon 2007 [16]), on the stabilisation of archaeological copper alloys and conservation treatment of archaeological metal artefacts. As mentioned above he developed a methodology to describe the corrosion forms of heavily corroded metals (Bertholon 2002 [17], Bertholon 2003 [18]).
  • Romain Jeanneret is a conservator of scientific, technical and horology objects. As a research assistant in the Research unit of HE-Arc CR, he contributed to the development of the application.


Coordination team of the MiCorr application


For HEG-Arc

  • Prof. Cédric Gaspoz.

For HE-Arc CR

  • Prof. Christian Degrigny.


Administration team


  • Prof. Christian Degrigny, main administrator
  • ...




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[2] Neff, D., Bellot-Gurlet, L., Dillmann, P., Reguer, S. and Legrand, L. (2006) Raman imaging of ancient rust scales on archaeological iron artefacts for long-term atmospheric corrosion mechanisms study, J. Raman Spectrosc, 37: 1228–1237.

[3] Dillmann, P. (2005) Corrosion des objets archéologiques ferreux, Les Techniques de l’Ingénieur, AF 6920, COR675 : 1-20.

[4] Robbiola, L., Blengino, J-M., Fiaud, C. (1998) Morphology and mechanisms of formation of natural patinas on archaeological Cu-Sn alloys, Corrosion Science, 40, 12, 2083-2111.

[5] Turgoose, S. (1985) The corrosion of lead and tin: before and after excavation, in Lead and tin: Studies in Conservation and Technology, ed. C.E. Miles and S.C. Pollard, UKIC, Occasional Papers, n°3, London, 15-26.

[6] Bertholon, R. (2001) Characterization and location of the original surface of corroded archaeological objects. Surface Engineering, 17 (3), 241-245.

[7] Degrigny, C. (2012) Methodology to study and analyse the microstructures and corrosion forms of ancient and historic metals: application to metallographic samples from Swiss collections, rapport final du projet MIFAC-Métal, rapport interne, HE-Arc CR, 188p. (available from the author).

[8] Degrigny, C. and Le Gall, R. (1999) Conservation of ancient lead artefacts corroded in organic acid environments: electrolytic stabilisation / consolidation, Studies in Conservation, 44, 157-169.

[9] Degrigny, C. and Witschard, D. (2006) La chasse des enfants de Saint Sigismond de l’Abbaye de Saint-Maurice: traitements électrochimiques des reliefs en argent en cours de restauration, in Medieval reliquary shrines and precious metalwork, in Proceedings of a conference at the Musée d’Art et d’Histoire, Geneva, 12-15 September 2001, Anheuser K. and Werner, C. (eds), Archetype, 9-16.

[10] Degrigny, C. (1993) La mise au point d’un traitement cathodique de stabilisation de vestiges aéronautiques immergés en alliages d’aluminium, actes du colloque Sauvegarder le XXème siècle : la conservation des matériaux modernes, Ottawa 1991, Pub. ICC, 373-380.

[11] Degrigny, C. (2007) Examination and conservation of historic and archaeological metal artefacts: a European overview, in Corrosion of metallic heritage artefacts, Dillmann, P. et al. (eds), EFC 48, Woodhead Publishers, 1-17.

[12] Bertholon, R., et al. (1998). Corrosion du rouleau de cuivre de Qumrân et localisation de la surface originelle. Metal 98 International Conference on Metals Conservation, Draguignan, James and James

[13] Bertholon, R. (2000). La limite de la surface d'origine des objets métalliques archéologiques. caractérisation, localisation et approche des mécanismes de conservation. UFR03 Art et Archéologie. Paris, Université Paris 1 Panthéon-Sorbonne: 419p.

[14] Bertholon, R. (2001). The location of the original surface, a review of the conservation literature. Metal 2001 Proceedings of the ICOM-CC Metals Working Group, Santiago, Chile 2-6 April 2001, Western Australian Museum.

[15] Bertholon, R. (2001). "Characterisation and Location of the Original Surface of Corroded Archaeological Objects." Surface Engineering 17(3): 241-245.

[16] Bertholon, R. (2007). Archaeological metal artefacts and conservation issues : long-term corrosion studies. Corrosion of metallic heritage artefacts. P. Dillmann, G. Béranger, P. Piccardo and H. Matthiesen. Cambridge, Woodhead Publishing Lmd: 31-40.

[17] Bertholon, R. (2002). "Proposition d'une méthode de description de la corrosion des objets métalliques archéologiques : schéma général." Cahier Technique de Conservation-Restauration des biens culturels 9: 56-65.

[18] Bertholon, R., et al. (2003). Comprehensive approaches of corroded conditions for archaeological iron artefacts : filling the gap between macroscopic observation and microanalysis for conservation diagnosis. International Conference on the Application of Raman Spectroscopy in Art and Archaeology, Ghent.