Trace elements analysis of 16th-century silver coins minted in Nantes - coupled HE-PIXE and p-XRF measurements

Previous studies have identified the origin of silver extracted from South American mines -Mexico and Potosí- and its circulation through trade between Spain and France during the second half of the 16th century. In this context, a selection of silver coins minted in Nantes was analysed to determine their chemical composition and to quantify trace-element markers such as gold, mercury, and indium. The objective of our work was to compare our measurements with those reported in the scientific literature, while providing a refined and local perspective. Particular attention has been given to the historical and political framework, as both factors significantly influenced the supply of raw materials. This article presents ongoing research conducted at the Arc'Antique laboratory and the ARRONAX cyclotron. Handheld XRF detects gold and mercury through its L X-ray lines, providing surface analyses down to 30 µm, with quantification limits in the range several hundred ppm. High Energy PIXE was employed to probe the coins at greater depth and to detect heavy elements via high-energy K X-ray lines generated with 68 MeV alpha particles (up to 500 µm) and proton beams (exceeding 1 mm). Weighted average values were calculated for coins groups by year and by chronological periods, in order to facilitate comparison with previously analysed specimens. Average gold and indium concentrations in Potosí silver were determined to be 23 ± 5 ppm and 6.9 ± 2.6 ppm, respectively. Gold is significantly more abundant in Mexican and European silver, with an average concentration of 1935 ± 57 ppm, whereas indium appears only at very low levels (around 0.1 ppm). The measured gold concentrations enabled the identification of two distinct groups of coins, thereby confirming the introduction of Potosí silver into Nantes after 1575. The weighted standard deviation of the gold concentrations measured in coins minted in 1598 reflects difficulties in silver supply during this period, which corresponds to the siege of Nantes. Regarding indium, the HE-PIXE set up was optimised to improve the detection limit to a few ppm. Although we detected a difference between two Ag–Cu targets containing 37 ppm and 65 ppm of indium, this sensitivity remained insufficient to detect indium in the coins. Moreover, in cases where mercury was detected with XRF, HE-PIXE analysis revealed no mercury at depth. This suggests that the mercury—introduced in Potosí in 1572 for ore refining—may be present only as a surface contaminant, or may have migrated during the coin-smelting process.