Integrated analytical methodologies for the study of the corrosion products naturally grown on Roman Ag-based artefacts

Ancient silver-based alloys objects are of a great interest from a historical, artistic and economical point of view because they were precious works of art or commonly used items as jewels, ornaments and amulets. Silverbased alloys were also largely used as monetary currency or like a form of saving, playing a relevant role for the development of economies. By means of the combined use of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy coupled with energy dispersive spectrometry (SEM+EDS) and optical microscopy (OM) we have studied the micro- and nano-chemical nature of the corrosion products grown on Ag-based artefacts found in different archaeological Italian sites. The results allow the identification of different corrosion products naturally grown during the longterm interaction between the silver-based alloy elements and degradation agents from the surrounding environment. The FTIR, XPS, SEM-EDS, XRD and OM results highlight the role of chlorine, that is always present in the patina as chlorargyrite (AgCl), and in few cases of bromine or iodine. Furthermore, our results reveal that chlorine often preferentially corroded the copper islands scattered in the alloy matrix giving rise to the formation of the dangerous copper-chlorine species such as atacamite and paratacamite. These latter compounds usually indicate the occurrence of the copper cyclic corrosion called by conservators "bronze disease". Finally, different corrosion patterns have been observed with the outermost layers generally enriched of S, Ca, Al, Si and Fe, revealing a strict relationship between some soil constituents and the silver-based alloy that reacted forming different compounds in some case unusual.