Morphology and mechanisms of formation of natural patinas on archaeological Cu–Sn alloys

Abstract

Natural patinas on archaeological bronzes (Cu–Sn alloys) have been classified and characterized in order to get a deeper insight into their formation mechanisms. From examinations of cross-sections on archaeological artefacts, two classes of corrosion structures were defined (Type I and Type II), using both optical and electron microscopies, EDSX, XRD, IRS and a statistical treatment of data (Principal Components Analysis). A Type I structure (even surface) is defined as a two-layer passivating deposit due to an internal oxidation with a decuprification process (i.e. selective dissolution of copper). A Type II structure (coarse surface) corresponds to more severe attacks, such as pitting but also general uneven corrosion; it is modelled by a three-layer structure, characterized by the presence of cuprous oxide and by an increase in the chloride content at the internal layer/alloy interface related with selective dissolution of copper. A phenomenological model to explain the formation of bronze patinas is developed on the basis of a decuprification phenomenon.