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

Archaeological and Anthropological Sciences (2023) 15:141 https://doi.org/10.1007/s12520-023-01834-0 RESEARCH 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 N. K. Kladouri1 · S. Skaltsa2 · Th. Gerodimos3 · K. Pezouvani2 · A. G. Karydas1 Received: 30 March 2023 / Accepted: 23 July 2023 / Published online: 30 August 2023 © The Author(s) 2023 Abstract 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. Keywords Coins · Bronze · k-means clustering · Silver alloys · μ-XRF · Rhodes Introduction Aim of the study The present study reports on μ-XRF analysis of 111 copper-based and 11 silver alloy coins from the ancient city of Rhodes, Greece. It is the first time that elemental analysis on both silver and bronze coins issued by the Rhodian mint from the mid-4th c. BCE to the 2nd c. CE has been * N. K. Kladouri 1 Institute of Nuclear and Particle Physics, NCSR “Demokritos”, Agia Paraskevi, Greece 2 Rhodes Centennial Project, The SAXO Institute, University of Copenhagen, Copenhagen, Denmark 3 Department of Materials Science and Engineering, University of Ioannina, Ioannina, Greece performed. The objectives of the μ-XRF analyses were to identify the raw materials used for coin production, to characterize their typology in terms of their elemental composition, and to determine and quantify certain minor and trace elements that could allow for a more detailed classification. As all coins included in the analysis come from rescue excavations in Rhodes, it was only allowed to perform analysis of a non-destructive nature. In this respect, μ-XRF allows us to gain some insights on the elemental composition of coins. Usually destructive techniques that offer evidence for bulk composition are preferred for coin analysis in an attempt to minimize the risk from surface enrichment, especially in silver coins (Ager 2016). Impetus for the study was given by the varying degree of preservation of coins that belong to the same issuing type and come from the same archaeological context. Given the absence of studies on the elemental composition of Rhodian coins, except for one conducted nearly three decades ago on 12 silver coins (J.-N. Barrandon and Bresson 1997), the present analysis aims to stimulate research in this field. Metals 13 Vol.:(0123456789) 141 Page 2 of 18 reached Rhodes via trade, gifts or booty, as the island of Rhodes lacks ores and metal resources. Although the provenance of metals lies outside the scope of the present study, the analysis helps us to detect patterns in the composition of coins. This raises some archaeological questions especially with regard to the evolution of Rhodian bronze coinage as well as to the changing practices in metal resources in the Rhodian mint. In other words, this study aims to further underline the importance of applying scientific methods in archaeological material, as the results can give new directions to historical research. In Hellenistic times, Rhodes emerged as a bustling and thriving cosmopolitan center, since the Rhodian fleet had a leading role in transit trade across the Mediterranean. Although at this stage of research solid answers cannot be provided, it is nevertheless worth bringing together archaeological and archaeometric evidence by posing the question to what extent, if any, differences and similarities in the composition of Rhodian bronze coinage over time may reflect shifting or recurrent trading networks. Contextualizing the coins The coins under examination are issues of the Rhodian mint from the second half of the 4th c. BCE down to the 2nd c. CE, except for two Roman coins of imperial mints which were Fig. 1  Plan of the city of Rhodes indicating the location of the excavations included for study in the “Rhodes Centennial Project” (after Filimonos and Patsiada 2018, Fig. 3) 13 Archaeological and Anthropological Sciences (2023) 15:141 nevertheless included for analysis (nos. R113 and R116). The coins selected for analysis came to light during rescue excavations in the ancient city and necropolis of Rhodes. They originate from five building complexes (Fig. 1, Table 1), which form the core of the research undertaken in the framework of the “Rhodes Centennial Project” (the geographical coordinates of the excavated plots are listed in the Supplementary Material). In other words, the present study relies exclusively on material retrieved from rescue excavations and included for study in the framework of the “Rhodes Centennial Project.” The small selection of silver coins (11 in total) in our sample reflects the dearth of precious-metal coins retrieved from excavations, unlike base metal coins (i.e., bronze coins), which constitute omnipresent finds in any excavation; the accidental loss of bronze coins would not have been severely felt by their owner, due to their low intrinsic and metal value (Callataÿ, 2006: 181–183). Despite the paucity of silver coins in our material, we nevertheless included these coins for analysis in order to get some first insights into their elemental composition. The silver coins selected for analysis constitute a chronologically close group; nine out of 11 coins date after ca. 190 BCE, a period during which the Rhodians introduced significant changes to their coinage: the weight of the drachm was slightly increased from ca. 2.70 gr to ca. 3.00 gr, and an incuse square (plinthos) was (...truncated)