Surface studies of patinas and metallurgical features of uncommon high-tin bronze artefacts from the Italic necropolises of ancient Abruzzo (Central Italy)

Archaeological high-tin copper-based artefacts, dated back to VIII and IV centuries BC and recently discovered in the Italic necropolises of ancient Abruzzo (Central Italy), were investigated in order to determine the microchemical and micro-structural nature of the corrosion products and the chemical composition and metallurgical features of the alloys. The Cu-based artefacts were defensive weapons mainly as bronze belts and pectoral disc armours with engraved decorations. A rather detailed picture of the chemical composition, structural and morphological characteristics of such Cu-based artefacts was provided by combining different techniques as scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), optical microscopy (OM) and X-ray diffraction (XRD) analysis. The results, as a whole, evidence that some artefacts were manufactured by casting and shaping uncommon high-tin bronze alloys thus conferring an attractive silver-like appearance. The manufacturing process was based on tailored multiple hot hammering, carefully carried out at about 700 degrees C to shape the high-tin alloys in the form of sheets avoiding breaking. The mechanical work was followed by an annealing treatment, polishing and final decorative finishing. Furthermore, our investigation on the corrosion process, suggest that it is a mixture of rather complex phenomena leading to the degradation of the main alloying elements which give rise to mineral alteration products in the form of complex structures. These latter contain SnO2 (cassiterite), cuprous oxide, copper carbonates (azurite and malachite) and, in particular, chlorine-based compounds like nantokite and atacamite and its polymorphs, which have heavy detrimental effects, their presence being considered a symptom of the destructive bronze disease degradation phenomenon. Moreover, it was found an unusual presence of re-deposited Cu inclusions to be likely related to long-term corrosion processes and to a low oxygen availability in the burial environment. All these findings show, as a whole, that there is a strict relationship between alloy elements, metallurgical features, burial soil conditions and chemical and structural features of the corrosion products. This paper highlights also as the combined use of different techniques as SEM-EDS, ATR-FTIR, XPS, XRD and OM can be one of the possible successfully way to study the corrosion products of archaeological copper-based artefacts, thus helping to choose the more appropriate strategies for the long-lasting conservation of archaeological bronze artworks, in most cases so valuable for the cultural heritage.