The bronze shields found at the Ayanis fortress (Van region, Turkey): manufacturing techniques and corrosion phenomena

The manufacturing techniques, chemical composition and corrosion products grown on archaeological bronze shields found during the excavation of the Ayanis fortress (Lake Van region, Eastern Anatolia, Turkey) were studied by means of the combined use of scanning electron microscopy combined with energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD) and optical microscopy (OM) techniques. The shields are dated back to 673-645 BC and in some cases are decorated with cuneiform inscriptions and ornaments representing various animals, such as lions and bulls. The micro-chemical and structural results show the presence of corrosion products constituted by copper carbonates (azurite and malachite), cuprite and dangerous chlorine-based compounds (nantokite, atacamite, clinoatacamite and paratacamite) due to the Cl-enriched soil of Ayanis. Furthermore, the results show that the bronze shields are coated with a Sn-enriched thin layer, likely due to an intentional tinning process carried out in ancient times to achieve a silver-like finish. Analytical data show an inter-granular corrosion phenomenon that heavily corroded the boundaries of the large equiassic grains inducing mechanical weakness. This latter phenomenon was induced by the repeated cycles of mechanical work and annealing thermal treatments carried out to restore the ductility and malleability of the alloy. Such processing caused the crystallisation and growth of the flattened grains, as well as impurities segregation phenomena along grain boundaries, thus enhancing the inter-granular long-term corrosion.