Plasma Treatment of Artefacts

Abstract

Corrosion products on ferrous artefacts generally consist of iron oxides and hydroxides. After excavation of the artefact, chlorinated products included between the object and its surface become very active owing to the oxygen and water vapour in ambient air. Therefore, the elimination of these chlorinated products is very important in order to preserve the excavated object. Chemical treatments are usually applied to these objects, such as alkaline sulphite baths, but this method takes a long time (a few months), and is also quite high maintenance. In the 1980s, it was shown by Veprek et al, at Zürich University, that plasma treatment of these artefacts could be of interest, mainly by using hydrogen plasma, which allows reduction of chlorinated products as well as oxides. Another reason for interest in this new method was that the treatment is much shorter: a few tens of hours compared to a few months for conventional chemical treatments. In this paper, a new type of large plasma reactor (approximately 200 L) for treatment of artefacts is presented, which is used at the Archaeological Museum of Val d'Oise. The reactor is described in terms of the high frequency power and plasma creation. The plasma itself is also analysed using optical analysis of the active species with and without an artefact inside the vacuum chamber. The operating conditions are also described. Results of plasma treatments on artefacts are presented with details of the treatment time and the restoration of the object after the plasma treatment. The potential extension of the use of this kind of plasma reactor is investigated for industrial applications such as fine cleaning or surface oxide reduction on metallic objects.