Corrosion characterization of late Roman coins from the tomares hoard using PIXE and FIB-SEM-EDS techniques

The Tomares Hoard, one of the most significant Late Roman numismatic finds in the Iberian Peninsula, comprises more than 55,000 coins dated to the 3rd–4th centuries CE. This study focuses on the characterization of corrosion layers and alloy compositions of selected specimens using a multi-analytical approach that combines Particle Induced X-ray Emission (PIXE), Rutherford Backscattering Spectrometry (RBS) and Focused Ion Beam Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (FIB-SEM-EDS). PIXE and RBS spectra obtained with α-particles (2, 3, 5 and 6.4 MeV) and 3 MeV protons were compared across polished and non-polished areas, revealing silver enrichment in the outer layers and contamination from soil-derived elements (Si, Al, P, S, Ca, Fe, Cl). FIB-SEM-EDS cross-sections confirmed the presence of multi-layered corrosion products, consistent with copper oxides, copper carbonates, and tin- and lead-rich oxidized phases, together with localized signals suggestive of cuprous chloride-related instabilities. These results shed light on the metallurgical practices and post-depositional processes affecting the hoard, while underlining conservation challenges posed by possible chloride-induced active corrosion.