Abstract

Archaeological bronze statues discovered in both fresh and marine water often appear in an excellent conservation state, with little or no evident corrosion. The black colour and the nature of the patina formed seems to be related to the presence of superficial copper sulphides. Their formation could be connected to: 

• an intentional patination; 
• the occurrence of specific anaerobic conditions and the presence of bacteria. 

It is thus well known that microorganisms can survive and thrive in anaerobic conditions and in presence of copper alloys exploiting sulfur (cathodic half reaction) and the metallic matrix (anodic reaction) as alternate sources of carbon promoting the precipitation of copper (I) and (II) sulfides. In this perspective, this study aims to characterize the effect of anaerobic bacteria on replicas of bronzes with specific compositions (CuSn10, CuSn10Pb5, CuSn3Pb5Zn3) reproducing those of classical statuary buried in seawater. Samples were produced by casting, cut into small sizes, and suitably prepared for the ageing test. Seawater with bacteria (SWB) was directly collected from the Genoa harbour and used for the experimentation after deoxygenation. In addition, blank conditions (without living bacteria-SW) were set up after sterilization of the solution. Electrochemical tests such as potentiodynamic polarization (POL) and electrochemical Impedance Spectroscopy (EIS) were carried out using a standard three electrode cell with the sample as the working electrode (WE), an Ag/AgCl as the reference electrode (RE) and a platinum mesh as the counter electrode (CE). The tests in anaerobic conditions (SW and SWB) were repeated three times to have a reproducibility of the results. Post experiment analyses were carried out through scanning electron microscopy (SEM-EDS) and X-Ray Diffraction (XRD). By simulating the conditions of an anaerobic marine environment through electrochemical methods, it was possible to obtain patinas with a composition and appearance very close to those analysed on submerged objects. Furthermore, all carried out tests and analysis have shown how the environment enriched in microorganisms allows the production of a passive patina that slows down the anodic reaction. It is hypothesized that the presence of sulphate-reducing bacteria (SRBs) catalyses the precipitation of more protective compounds (e.g., CuS) that limit the kinetics of corrosion.