Simulation of corrosion processes of buried archaeological bronze artefacts

Research activities regarding the conservation of Cultural Heritage have significantly increased in recent years in every European and Mediterranean country, as artefacts are important remains of our civilisation and valuable signs of how human being evolved in terms of art and history. One major concern of curators and archaeologists is the preservation of archaeological objects taken out of excavations and kept in museums. In the case of bronzes and other copper based alloys, the degradation phenomena occurring during burial or exposure to a museum atmosphere usually initiate by the formation of a cuprite layer, which darkens the surface, and then, depending on the environment surrounding the sample, various corrosion products of copper II can develop, giving either a brown-black colour or a green one to the surface. This process damages the alloys, as the copper required to form the corrosion products comes from the metal core, which can then considerably decrease, indeed disappear, following the corrosion rate. It is therefore crucial to find methods of protection and conservation of these metallic artefacts that prevent them from getting damaged. In this paper, we present analyses carried out on Italian copper-based archaeological value artefacts selected as a function of the archaeological context and of the chemical composition and structure; and on aged copper-based reference alloys of similar composition to that of the ancient alloys, in order to characterise their corrosion products and modelize the mechanism of the degradation process. The archaeological artefacts came from two different sites in Sardinia and the riverbanks of the Tiber. They were analysed by means of optical and scanning electron microscopy, X-ray diffraction and fluorescence performed with portable instrument and glow discharge optical emission spectroscopy. The corrosion layer was also characterised by electrochemical impedance spectroscopy. The reference alloys were produced with the similar composition determined after the analyses of a large selection of artefacts. These analyses gave us either ternary or quaternary alloys of copper, tin, zinc and lead. These alloys were then buried in soils coming from the Sardinian and Roman sites, in presence of water. Ageing tests were run on these samples in a climatic chamber, where the samples were put for 1, 4 and 7 months. The aged samples were characterised by means of the same techniques than those used for the artefacts. The first results obtained on the artefacts show the formation of malachite on samples coming from sites exposed to a marine and chlorinated atmosphere and a partial mineralisation of the alloys due to their interaction with the soils. Cuprite is also detected as a major component of the corrosion layer. For the one-month degraded reference alloys, wide dark red and red spots of cuprite have formed a first thin corrosion layer. Green traces of malachite are also noticeable on samples buried in soils containing higher water content than the field capacity.