New insights on the corrosion of ancient bronzes using X-ray powder diffraction: The importance of paratacamite,sampleite, and connellite

Research employing X-ray powder diffraction shows that clinoatacamite as the phase which should be predominant in the chloride-induced corrosion of ancient bronzes is not necessarily the dominant copper trihydroxychloride species at all, since mixtures are common, and other phases such as paratacamite or the dubious mineral phase anarakite have relevance to the corrosion products or patina of bronze antiquities. The potential reasons for this are discussed and related to the compositional parameters of ancient bronzes as well as the burial environment. Paratacamite is still identified, even in the absence of zinc, which is supposed to be the major stabilizing influence on the paratacamite lattice. X-ray power diffraction has been employed in the examination of a range of ancient bronzes from Chinese, Egyptian, Ecuadorian, Lydian, Albanian, Palestinian, and Greek contexts. This study has revealed that, far from being exotic novelties, the occurrence of both sampleite and connellite as important corrosion products of ancient bronzes has been underestimated in the published literature. Many occurrences of sampleite are related to the decomposition of contiguous human remains rather than to arid environmental factors which is the explanation for the occurrence in a geological environment, but in a burial environment other factors come into consideration.