Remarkable corrosion resumption of archaeological bronzes, induced by the oxidation of ternary Cu-Sn-S phases in atmosphere, after long-term burial with sulfides

A drastic alteration resumption observed on archaeological bronze objects was investigated, characterized by crystals growing under the surface of the objects, inducing cracks, desquamation and detachment of patina scales. Analyses shown that artefacts were covered by a thick layer of covellite CuS, which proved to be air-stable. On the contrary, Cu-Sn-S compounds (mohite and kuramite), replacing the alloy in the bulk, showed an extreme reactivity towards air and a rapid transformation into hydrated copper sulfates. This phenomenon was unknown in the restoration field and an overall alteration process, from abandonment until exhibition, implying occurrence mechanisms of alteration products, is proposed.