Combined use of Raman spectroscopy, cluster analysis, and SEM-EDS for the characterization of Roman bronze artifacts...

This study focuses on chemical and microstructural results on four bronze artifacts, dating back to the 3^rd-4^th centuries AD. These artifacts were unearthed in the archaeological site of the Roman Cistern at Spoletino (Civitella D'Agliano, VT, Central Italy). They were studied using a Reflected Light Microscope, micro-Raman spectroscopy, SEM-EDS analysis, and cluster analysis, providing reliable and complementary data on the samples. SEM-EDS results indicated the use of different Cu alloys at the Roman site. Lead was added to improve the fluidity of the molten bronze in the locked item, while scrap metal containing Zn and leaded bronze were mixed to produce gunmetal and furniture accessories. A binary Cu-Sn alloy was employed for decorative and cosmetic objects. The corrosion products, identified by micro-Raman spectroscopy, are cuprite (Cu₂O), malachite (Cu₂(OH)₂CO₃), hydrocerussite (Pb₃(CO₃)₂(OH)₂), and libethenite (Cu₂(OH)PO₄), reflecting interactions with burial environments. Cluster analysis simplified the interpretation of complex Raman maps, especially in samples with multifaceted corrosion processes that involved overlapping phases. The data confirmed the absence of chloride ions on the patinas and extensive decuprification processes on the external surface, with the diffusion of environmental elements, such as phosphates, towards the bulk metal. A strong correlation was observed between fractured paths and corrosion patterns along artifact edges.