Synchrotron X-ray tomography and spectroscopy in numismatics: disclosing counterfeit practices in medieval silver coins

This study explores the use of advanced synchrotron radiation (SR)-based imaging and spectroscopic techniques to investigate the distribution and nature of mercury (Hg) in numismatic artifacts, with a particular focus on its role in historical counterfeiting practices. A selection of medieval silver coins exhibiting anomalous surface Hg signals, initially identified through portable energy-dispersive X-ray fluorescence (ED-XRF) and laser-induced breakdown spectroscopy, were further examined using high-resolution synchrotron X-ray micro-computed tomography (SR-µCT), micro-X-ray spectroscopy (SR-µXRF) and micro-X-ray diffraction (SR-µXRD) at the PSICHÉ beamline of the SOLEIL Synchrotron. Comparative analyses were conducted on coins produced by an unofficial mint at the Castle of Godano (La Spezia, Italy) and on verified genuine specimens. The results provide evidence for the use of Hg-Ag amalgam, indicative of fraudulent minting practices. This research highlights the methodological strength of integrating multiple non-destructive techniques. Portable ED-XRF demonstrates significant utility as a preliminary screening tool for detecting Hg in medieval and Renaissance coins, supporting the identification of potential forgeries. The combined use of SR-µCT, SR-µXRF and SR-µXRD offers a comprehensive framework for probing internal microstructures and elemental compositions of metal archaeological objects, advancing our understanding of both ancient coinage and fraudulent processes.