Roman and Early Medieval bronze artifacts from the Middle Tiber Valley. Technological and conservation insight through optical microscopy, Raman spectroscopy, SEM-EDS, and electrochemical analysis

This study focuses on microstructure, chemistry, and patina characterisation of eleven archaeological bronze artefacts from two contexts of the Middle Tiber Valley (Viterbo, Central Italy). The samples were unearthed in the cistern at Spoletino (1st -4th century AD) and in the late Roman-early Medieval necropolis of Castel Sozzio (5th-7th century AD). They were analysed using optical microscopy (OM), micro-Raman spectroscopy (μ-Raman), Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM/EDS), and electrochemical investigations. OM explored corrosion products on the surfaces, highlighting mineralogical and structural heterogeneity of the patinas. SEM-EDS analysis showed the presence of binary (Cu-Sn) and ternary (Cu-Pb-Sn) alloys with minor and trace elements. The binary alloys were used for good hardness artefacts, and the ternary ones for those that required easy metal working. Selective enrichment and depletion of the alloying metals produced broad chemical variations and structural heterogeneity in the patinas. Micro-Raman spectroscopy revealed different corrosion products such as cuprite (Cu2O), lazurite (Na7Ca(Al6Si6O24)(SO4)(S3)·H2O), malachite (CuCO3Cu(OH)2), and phosgenite (Pb2Cl2CO3). The presence of lazurite in two samples was linked to sulphurrich burial environments, while phosgenite was linked to the interactions with Cl and a CO2-rich burial environment. Metallurgical practices, such as slow cooling rates and lead segregation during casting, promoted mineralogical and structural heterogeneity in the patinas due to interactions with the burial environment, where decomposition of organic matter occurred. Electrochemical data permitted the estimation of the corrosion rates for all artefacts at remarkably low levels (0–0.1 mm/year), reflecting stable burial conditions and the protective nature of the patinas. These results suggest that the burial environments in the Middle Tiber Valley were non-aggressive, facilitating the development of layered patinas influenced primarily by oxygen and carbon dioxide from humus-rich soils, with a lesser contribution from salts.