A Review of Copper Chlorides and Related Salts in Bronze Corrosion and as Painting Pigments

The copper chlorides are important both in corrosion processes of copper alloys and in their occurrence as pigments in wall paintings, manuscript illumination, and other paintings. It is now known that there are four isomers of copper trihydroxychloride, namely clinoatacamite, atacamite, paratacamite and botallackite. Clinoatacamite should replace most previously reported occurrences of paratacamite in the conservation literature. The nature of the reactions in 'bronze disease' is described, and the importance of copper(I) chloride in this process is emphasized. Calumetite and other potentially important chloride species such as connellite are discussed in relation to their occurrence as corrosion products on ancient bronze objects. Painting pigments which are assumed to have originally been made using one of the copper trihydroxychlorides may often be alteration products from original pigments, such as malachite. This is particularly true for botallackite but may apply to other isomers, such as atacamite.