Elemental characterization of surface and bulk of copper-based coins from the Byzantine-period by means of spectroscopic techniques

Thirty-three Byzantine copper-based coins (so-called anonymous follis) from 970 to 1085 CE were investigated in order to determine the elemental composition both of the bulk and of the surface. The ancient coins analysed - belonging to the Thomas Bertelè Collection, now located in the Bottacin Museum of Padova (Italy) - were studied by way of physical, chemical and chemometric testing techniques. This study aims to obtain information on socio-economic aspects of the Byzantine Empire using chemical and physical data of currency as indirect indicators. First, the weight, diameter and thickness of each piece were measured, then a micro-Energy Dispersive X-ray Fluorescence (μ-EDXRF) technique was applied on a cleaned and polished area of the surface of the coin to evaluate the presence of minor elements in a non-destructive fashion. A micro-destructive sampling method for both surface and bulk samples, based on mechanically drilling the edge, was finally employed, and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) was used for the quantitative determination of Cu (the major alloying element), Pb, As, Fe, Zn, Ag, Ni, Sn and Mn (minor elements) in the powder which resulted from drilling each coin, previously dissolved via aqua regia dissolution technique overnight at room temperature. The average copper concentration of the folles was 95.9 ± 3.1% for the bulk and 91.7 ± 3.5% for the surface. The Pb-Cu ratio appears to be an index which can be used to discriminate between different coins, in particular for Class B (from 1030 to 1042 CE), as confirmed by exploratory chemometric data analysis. According to our results, all the anonymous Byzantine folles under examination were minted in much the same way and using the same mineral raw materials. Using the chemical data measured on the bulk, we can grasp that the monetary choice of the period under study excluded silver, gold and bronze (concentrations of Pb is low, while Sn resulted undetectable or unquantifiable) to turn, substantially, to a copper-only coinage (deducted the impurities given by the gangue of the starting minerals). Moreover, along the observed period of 120 years, the mass of the coins was significantly reduced (with an overall loss of material of approximately 60%)