The computer-based Decision Support
System (DSS) MiCorr is a new
non-invasive diagnostic tool for metal objects. Three search engines have been developed: the
first one uses interlinked keywords, the second a decision chain to identify metal families and the third digital stratigraphies of corrosion forms. The latter, based on
Bertholon’s schematic description of metal corrosion[1],
is the most innovative tool of this application. Corrosion forms are described as structures of strata (metal, corroded metal, corrosion
layers…), each stratum having its specific characteristics (morphology,
microstructure, texture and other properties). A modelling program allows to build a corrosion structure from coded 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 forms/case studies of the database. Some of them 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 MiCorr user is able
to find case studies/corrosion forms of objects showing similar corrosion phenomena. In case the user is a conservation professional, this should help him.her to implement an appropriate conservation protocol.
The search engines allow the
user to question the database but if registered he.she has also the possibility to enrich
the database with new corrosion forms and/or case studies, thus becoming an active contributor. The
basic format of the corrosion forms can be further improved with any
additional analytical data and the operating conditions used.
In short, MiCorr aims to:
The didactic, self-learning and freely accessible and participatory MiCorr tool should enable the various scientific communities involved in the analysis and conservation of metals to better understand existing knowledge and contribute to unsolved research problems.
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 alteration 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 forms published in the literature (Corrosion
Science, Studies in Conservation, Journal of Cultural Heritage, etc.), 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.
The professional associations of corrosion experts (NACE - the Professional society for
corrosion engineers since 1943 and the European Corrosion Federation since
1955) have published numerous books on metal corrosion. Although the corrosion
mechanisms under particular environmental conditions (and the means
to prevent it) are known, there are no online diagnostic tools to link
observed alterations to the causes of corrosion. Websites such as http://corrosion-doctors.org/Contents.htm provide illustrations of corrosion forms. Unlike heritage metals which show extensive and complex
corrosion 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) existed in the past but is no longer available. It offered descriptions of corrosion forms encountered on heritage metals, but did not provide 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 focused 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,
such as those found in museum display cases 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.
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:
Fig. 1: Schematic representation of the research methodology.
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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.
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[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.
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