Surface studies of patinas naturally grown on ornamental high-tin bronze artefacts from the pre-Roman necropolis of ancient Abruzzo (Central Italy)

High-tin copper-based artefacts, recently discovered during archaeological excavations of the Italic necropolises of ancient Abruzzo (Central Italy), were investigated in order to determine the chemical composition and metallurgical features of the alloys and the micro-chemical and micro-structural nature of the corrosion products grown during the long-term burial. The investigated Cu-based artefacts, dated back to 6th and 5th centuries BC, were defensive weapons, in particular bronze belts and disc armors with embossed decoration and ornamental objects such as rivets, buckles and small rings that were applied to large belts to form complex patterns with the addition of small and large shaped pieces of amber. The analytical characterisation was performed by means of the combined use of scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), optical microscopy (OM) and X-ray diffraction (XRD) analysis. The results put in evidence that the bronze artefacts were manufactured by casting an unusual high-tin bronze alloy in a ceramic bivalve mould thus producing artefacts with a silver-like appearance. In some cases the manufacturing process included also tailored cycles of thermal treatments and hot mechanical working under reducing conditions and well defined thermal parameters in order to shape the high-tin alloys in the form of sheets. The ancient metallurgists have also used low tin bronze alloys coated with a thin layer of tin for producing a highly reflective silver-like appearance. From a corrosion point of view, the main alloying elements have formed degradation products in the form of stratified structures containing tin oxides (romarkite and cassiterite), cuprous oxide, copper carbonates (azurite and malachite) and dangerous chlorine-based compounds (nantokite and atacamite polymorphs). These findings show that there is a strict interaction with the soil components and the pH variation of the different areas of the necropolis, suggesting that post-burial degradation phenomena occur. It can be concluded that SEM-EDS, XRD and OM can be successfully combined for studying the corrosion products of archaeological copper-based artefacts, thus helping to choose the best strategies for the long-lasting conservation of these ancient artworks.