Spectroscopic characterization of an innovative biological treatment for corroded metal artefacts

Nowadays, organic coatings such as waxes, acrylic resins, and corrosion inhibitors are commonly used for the protection and corrosion inhibition of metal artefacts. However, research efforts still need to be emphasized on innovative treatments that aim at modifying existing corrosion products into more stable and less soluble compounds while maintaining the surface’s appearance. Within the framework of the Biological patinA for arcHaeological and Artistic Metal ArtefactS project, biological treatments based on such criteria are being evaluated for the preservation of metal artefacts. In particular, the capacity of a fungal strain, Beauveria bassiana, to precipitate copper oxalates is exploited for the stabilization of soluble patinas (copper hydroxysulfates) or the transformation of active corrosion products (copper hydroxychlorides). In fact, copper oxalates produce green compact patinas showing a high degree of insolubility and chemical stability even in acid atmospheres (pH 3). In the present work, cultures of B. bassiana were applied on copper-based coupons naturally aged in urban or marine environment. The results clearly showed that the original patina was gradually transformed into copper oxalates and that the conversion is completed on the surface areas where B. bassiana grew. Cross-sectioned samples were also analyzed to determine the distribution of the copper oxalates. Raman mapping was demonstrated here to be a valuable tool for precisely and nondestructively localizing corrosion products and for evaluating protective treatments on metal artefacts. This study also permitted the further improvement of