An integrated analytical characterization of corrosion products on ornamental objects from the necropolis of Colle Badetta-Tortoreto (Teramo, Italy)

Selected groups of copper-based artifacts found during archaeological excavations of the necropolis of Colle Badetta-Tortoreto (Teramo, 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 artifacts were 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 characterization was performed by using different surface and bulk techniques, such as Optical Microscopy (OM), Scanning Electron Microscopy coupled with energy dispersive X-ray micro-analysis (SEM-EDS), X-ray Diffraction (XRD). Results evidenced that the bronze artifacts were produced by casting an unusual high-tin bronze alloy in a ceramic bivalve mould or by using a low tin bronze alloy coated with a thin layer of tin for producing a silver-like appearance. The main alloying elements were corroded to form different stratified structures of different chemical species, such as tin oxides (romarkite and cassiterite), cuprous oxide and copper carbonates (azurite and malachite), dangerous chlorine-based compounds (nantokite and atacamite polymorphs). This evidenced the strict interaction with the soil components and the pH variation of the different areas of the necropolis, as well as the occurrence of post-burial degradation phenomena. These results confirm that SEM-EDS, XRD and OM can be successfully used for studying the corrosion products of archaeological copper-based artifacts in order to identify the corrosion agents and mechanisms and, thereafter, to propose the best reliable tailored strategies for the conservation of these ancient artworks.