Analytical methodologies for the investigation of soil-induced degradation of Cu-based archaeological artefacts

Archaeological bronze artefacts are covered with corrosion products, that is, the patinas, whose nature depends on different degradation processes occurring during long-term burial. As a consequence of the corrosion phenomenon, surfaces of archaeological metal objects are composed of a complex structure. By means of the combined use of different analytical techniques such as XPS, scanning electron microscopy combined with energy dispersive X-ray spectroscopy, optical microscopy and X-ray diffraction, some typical patinas grown on Roman and Punic archaeological bronze artefacts have been studied in details by selecting a suitable methodological approach to acquire micro-chemical, morphological and structural information. The XPS measurements of Cu 2p, Cu LMM and valence band spectra of archaeological objects and reference samples such as CuCl2, CuCl, Cu2Cl(OH)3, Cu2O and CuO allowed us to identify the different copper oxidation states and crystallographic phases present in a patina. Moreover, the XPS elemental concentration depth profiles have yielded information about the stratified outermost layers and the occurrence of the migration phenomenon from bulk to surface due to selective corrosion processes. These pieces of information compared with those achieved via scanning electron microscopy combined with energy dispersive X-ray spectroscopy, optical microscopy and X-ray diffraction reveal the complex chemistry and morphology of the patina, highlighting also the correlation between patina and burial context.