Characterization of Chemical Nature of Surface Corrosion Layers in Archaeological Bronzes

The archaeological metallic artefacts have suffered corrosion in long-term period and their surface covers with various corrosion products. The nature of corrosion compounds and layers occurred in metallic artefacts is strongly related to burial environment conditions and type and characteristics of the metal/alloy. In this paper، the corrosion layers and morphology has studied in some bronze artefacts from Sangtarashan ancient site، Luristan (1st millennium BC). The aim of this research is identification of corrosion layers composition and corrosion morphology by stratigraphy and chemical analysis of corrosion products. For this purpose، bronze samples were selected and analyzed by SEM-EDS، XRD and OM (Optical Microscopy) methods. The results show decuprification and internal oxidation with formation of a tin-rich layer and basic copper carbonate isomers at the surface of samples. This morphology has a three layered structure. This morphology and mechanism of corrosion in ancient bronzes shows the uniform surface or noble patina that observes in buried bronzes and have some similarities to corrosion type I that has been explained in some literature.