Degradation mechanisms occurring in precious metallic artefacts

摘要

Since ancient times, precious metallic artefacts have gained a remarkable interest from a historical, artistic and economical point of view. They were used as jewels or artistic items but also for currency, medium of exchange and form of saving, and in many cases were manufactured with a high level of technological competence. Several Roman and Phoenician-Punic gold, silver and copper-based, gold and silver-plated artefacts characterised by a wide compositional range and different metallurgical nature have been studied to identify the microchemical surface structure and the metallurgical features. Four case studies are reported. The investigation was carried out by means of the combined use of surface and microanalytical techniques, such as XPS, X-ray diffraction and SEM equipped with an energy dispersive X-ray spectrometer. The results revealed that the artefacts surface is characterised by a thin layer of insoluble corrosion products like AgCl, Ag _2S or AgCuS, CuS, Cu _2O, Pb or Cu-carbonates mixed to form a complex composite structure. The presence and the concentrations of these corrosion compounds is strongly related to the archaeological context. The overall experimental findings confirm that the nearly pure gold artefacts and the gold-based artefacts are more stable with respect to the silver ones where the ubiquitous presence of chlorides is often found. The microchemical and micromorphological characterization allows to identify the plating and manufacturing techniques. Several methods were used by the craftsmen, including fire gilding, a highly complex and sophisticated method based on the use of mercury. Finally, the proposed approach may help, when feasible, to preserve, restore and protect the artefacts without causing further damaged.