Shedding light on the microstructure and chemical composition of rare early medieval coins from Italy (Berengario I) by combining pXRF and SEM-EDX analysis

The application of analytical techniques for archaeometric investigation poses a major problem when dealing with objects of high values such as coins. Due to the unique character and rarity of some coins, any kind of analysis should be better non-destructive, which means that neither original sample material can be taken nor any kind of modification is allowed. Moreover, ancient Ag-Cu coins are usually affected by silver surface enrichment which extension in depth can reach several hundreds of microns, far beyond the penetration depth of surface techniques such as pXRF. In order to gain insight into the composition and microstructure of ancient coins, to establish their "real fineness" and to determine the reliability of data obtained with pXRF, the present paper presents, for the first time in literature, the results of the compositional and microstructural features of four medieval denari made of silver-copper alloy obtained by pXRF, SEM-EDX, and FTIR analyses. Three of them are some rare denari in the name of Berengario I minted in Pavia (898-900/902-915? AD) while the fourth coin is one denarius in the name of Henry IV-V minted in Lucca (1100-1130 AD). Results provided by SEM-EDX and FTIR analyses clearly demonstrate that archeological samples could be deeply heterogeneous due to both production techniques and corrosion phenomena. This poses a big question on the representativity of analytical data obtained only with non-destructive superficial methods or with those methods where only a few spot analyses were performed. However, the pXRF results provided a data set that, even if not particularly accurate, can clearly evidence, like for Berengario I coins, some completely unexpected heterogeneities, that represent without a doubt, a big interesting archaeometric anomaly. Despite the limited number of examples, it was possible to provide important data on two monetary types of difficult chronological collocation and attribution.