The role of patina on archaeological copper alloy coins in the outbreak and progression of bronze disease

Archaeological copper alloy objects form unique corrosion structures according to their manufacture and burial environment. Categorisation has identified protective Type I structures and chloride-containing Type II structures which may lead to bronze disease. Using visual examination to determine which of these exist on objects dictates conservation decisions; unfortunately, there is limited evidence linking surface features directly to corrosion risk. In this study, the chloride content of 39 archaeological copper alloy coins was determined non-destructively using prompt gamma-ray activation analysis. Their surfaces were examined through Raman spectroscopy, reflective transformation imaging and visually using descriptors reflecting Type I, Type II and ‘bronze disease’ surfaces. When subjected to high relative humidity, none of the coins exhibited bronze disease despite containing chlorides and showing characteristics of Type II structures. The results highlight the difficulty of determining risk and taking informed decisions for the management of archaeological copper alloy objects