Combined use of FE-SEM+EDS, TOF-SIMS, ?-FTIR, XRD and OM for the study of ancient gilded artefacts

Gold coated commonly used copper-based objects such as studs, brooches, clasps and a small pendants, dating back to 16th-17th centuries CE, were studied by means of the combined use of FE-SEM/EDS, TOF-SIMS, ?- FTIR, XRD and optical microscopy (OM). In particular, the surface and subsurface chemical features of the gilded objects were investigated, revealing some aspects of the manufacturing methods as well as disclosing the mechanism of degradation occurring during the long-term burial in the soil and the agents involved in this process. Our investigation has made possible to identify the chemistry of the decorative Au layer deposited by using an amalgam, i.e. the fire-gilding, its structure and the corrosion products naturally formed from the interaction with the surrounding environment. The main degradation agents are chlorine, sulphur and phosphorous species that gave rise to the formation of different corrosion products. Indeed the corrosion phenomena are boosted by the metal galvanic coupling which makes gilded-metal art works unstable from a chemical-physical point of view. In perspective of preservation, our results suggest that it is important to prevent the interaction between moisture and nantokite (CuCl), located at the interface between the gilding layer and the substrate, in order to avoid further severe degradation phenomena such as "bronze disease" which transforms the copper substrate into a greenish powder.