Mechanism of corrosion due to unalloyed copper inclusion in ancient bronzes

Abstract

Ancient bronze objects buried in the soil for a long time corrode because of various environmental conditions. This can also cause different corrosion morphologies. One of the features of ancient bronzes is the presence of unalloyed copper with copper inclusions (UCI); when in the alloying process, the copper–tin alloy did not form properly. Because of the phenomenon of preferential corrosion, the tin content decreases in an environment with low oxygen and high chloride and sulphide content. The results are various corrosion products. A decorative plaque belonging to the National Museum of Iran has been investigated by X-ray diffraction, SEM-EDX and reflected polarised light optical microscopy. The investigations showed that the UCI effect appeared as type B, i.e. with copper sulphide in the metallic core. The presence of UCI (type B) in ancient bronzes proves that the objects corroded after deposition in the soil.