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Use of micro-Raman spectroscopy for the study of the atmospheric corrosion of copper alloys of cultural heritage

Tuesday, 24 January, 2006 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
Valérie Hayez
phd defence

Copper has played a crucial role in human development. The profusion of copper-based
artifacts led to the development of an important interest for these objects and their
conservation. In the field of art, copper based objects are preferred in the corroded state, not
only because of the aesthetically pleasing colors reached by the alloys, but most of all because
the presence of corrosion products becomes an evidence of time past and time passing, hereby
adding extra value to the object.

The variety of corrosion products that can form on copper alloys is enormous. It is important
to precisely identify the composition of the corrosion compounds present on art objects and to
follow their evolution. This knowledge provides valuable information for art historians about
the past environment of the object and its original composition. It also helps conservators to
decide whether an object's patina is stable in the current environment and which conservation
treatment could be used, if one is required.

Many different analysis techniques are used for the investigation of the composition of the
corrosion of copper alloys. However, few of them provide the needed information in a non
destructive way, consequently preserving the integrity of the object. Raman spectroscopy (RS)
has been used successfully in the field of art analysis, whereby the composition of different
materials was analyzed non destructively. The successful application of RS to other domains
of art consequently encouraged us, to start an extensive study of the possibilities offered by
this technique, for the investigation of corrosion products on objects of cultural importance.
The main goal was the positioning of RS in respect to existing methods and most of all, the
development of a general methodology which could be used to determine with RS the
composition of the corrosion products.

In order to reach this aim, a model system was set up. The study was started from reference
products consisting of various copper based minerals, expected to occur in the corrosion of
copper based alloys. Their Raman spectra were determined and added to a database, in order
to obtain a unique signature for each of the compounds. In a following step, the model was
extended to the study of different corroded samples. Finally, the established model was used
for the study of the corrosion and the spectra of some examples of naturally formed corrosion.
A searching algorithm based on principal component analysis was developed to ease the
identification procedure.

It was shown that RS constitutes an efficient tool for the identification of the main products
formed during natural corrosion in various environments or during artificial corrosion and that
the established model system was a good choice to solve the considered goal. The study of RS
allowed to draw interesting conclusions concerning the method but also the investigated
materials. The established methodology can be further extended to the study of many other
corrosion processes and metals, like e.g. iron or aluminium. The importance of studying the
corrosion, in other fields than the domain of art objects, has not to be proved. The results of
this work will consequently have a great technological value and contribute to the
development of these fields.