Micro-structural and micro-chemical composition of bronze artefacts from Tharros (Western Sardinia, Italy)

The present work reports the results of the SEM-energy dispersive spectrometry, X-ray diffraction (XRD), XPS and optical microscopy (OM) investigations performed to identify the corrosion products nature, i.e. the patina, grown on bronze common use artefacts found during excavations carried out at Tharros (on the west coast of Sardinia, Italy). The micro-chemical investigations of the patina revealed the structure and the chemical composition of the stratified corrosion layers, where tin depletion phenomenon has been observed within some cases of a surface enrichment of lead combined with elements coming from the soil such as Cl and P and mixed with other burial elements such as Ca, Si, Fe, Si and Al. The presence of soil elements was also confirmed by XPS investigations that evidence from the lineshape analysis and Binding Energy (BE) values of Cu 2p, Sn 3d and Pb 4f photoemission signals that these elements are mainly present in their maximum oxidation states on the surface. The surface information is confirmed by OM and XRD results that evidence also the presence of copper trihydroxychlorides. In the inner layers, OM, SEM-energy dispersive spectrometry and XRD results disclose the presence of Cu (I) compounds such as cuprite (Cu 2O) and nantokite (CuCl), the latter being the specie that induces the cyclic and self-sustaining degradation process commonly defined 'bronze disease'. The micro-chemical and micro-structural informations demonstrate the strict interaction between soil components and corrosion products that form different compounds as a function of the local characteristics of the soil and of the chemical composition and metallurgical features of the Cu-based alloys.