Microscopic X-ray fluorescence analyses (μ-XRF) of copper-based and silver alloy coins minted in Rhodes, Greece, from the fourth century BCE to the second century CE

The present study provides new data from the analysis of 111 copper-based and 11 silver-alloy coins from the ancient city of Rhodes, Greece. This is the first time that an elemental analysis has been carried out on copper-based coins issued by the Rhodian mint from the mid-4th c. BCE to the 2nd c. CE. Based on the μ-XRF results, three different groups of copper-based alloys are formed, with tin and lead as their major alloying components. The elemental composition shows that most of the coins are binary, leaded and ternary bronzes, while two samples—confirmed as imports—belong to a different technological tradition. All impurities present, such as iron, nickel, cobalt, antimony, or lead, are typical of copper ores, while the diminutive amounts of tin, lead and/or arsenic in some coins could indicate the use of alloys made from copper scrap. Based on a complex and extensive set of compositional data and using silhouette analysis for k-means clustering, we identified the existence of four distinct groups clustered on the basis of similar trace element values. The groups give indication to the raw materials used for minting and provided valuable insights into the technological and origin-related aspects of Rhodian coin production. In the silver coins, four groups of different silver grades can be identified on the basis of the most important alloying elements. Overall, the differences in the composition of the silver coins become clear, but a more extensive sample is needed to better understand the evolution of Rhodian silver coinage.