Neutron diffraction characterization of pre-Roman coinage from northern Italy: silver debasement and relationships within different emissions

The pre-Roman coinage from northern Italy gathers together different series of silver coins, mainly drachmas but fractions as well, produced by different tribes settled in northern Italy between the 4th and the 1st century B.C. The drachmas are all characterized by the imitation of the iconography of Massalia’s heavy drachm, bearing a lion with the ethnic ΜΑΣΣΑ on reverse. The different styles in the depiction of the lion have been used by numismatists to group the drachmas in different typologies. The smaller fractions bear, instead, a new depiction showing a spotted panther. This coinage is characterized by many issues such as chronology, attributions and relationships within the several emissions and nominals [1-2], which only recently have started to be solved. While stratigraphic excavations provided data for an absolute dating of some emissions and for the attribution to specific tribes, the archaeometric approach presented here is bringing new light on other aspects, such as the composition and the technological aspects of the mint production in pre-Roman times. To this aim, we performed time-of-flight neutron diffraction measurements with the INES diffractometer [3] at the ISIS facility (Rutherford Appleton Laboratory, UK). This technique for non-destructive bulk analyses was essential to overcome the problem of the silver surface enrichment, often occurring in ancient coins. A selection of specimens coming from different hoards and collections from museums, representative of different emissions, has been analyzed. The analysis of diffraction patterns has been carried out with the GSAS software to determine phase weight fractions through Rietveld refinement. Then, thanks to the relationship between d-spacing and Ag, Cu content in the α and β phase (respectively, copper-rich and silver- rich phase) the Ag/Cu ratio, important for numismatic studies, has been determined. The results show a clear silver debasement occurring between the first and latter drachmas emissions, due to inflation processes which can be related with the increasing economic power of the Roman republic in the Po valley. The silver loss can be also used to establish a relative chronology between the different emissions, in agreement with the few dating data available from archaeological research. Moreover, we are now able to assess the ratio between drachmas and the minor fractions, which was still unclear. Finally, these first results are providing new fundamental elements for the study of metrological relationships with the contemporary Roman republican currency, in a period of strong Roman influence followed by the military conquest of northern Italy. This project has been supported by the Cooperation Agreement no. 06/20018 between CNR and STFC. References [1] Pautasso, A., 1966. Le monete preromane dell’Italia settentrionale, Sibrium, 7: 1-162. [2] Arslan, E., 1995. La monetazione celtica cisalpina. Un nuovo quadro generale, Sibrium, 22: 179-215. [3] Grazzi, F., Celli, M., Siano, S., Zoppi, M., 2007. Preliminary results of the Italian neutron experimental station INES at ISIS: Archaeometric applications, Il Nuovo Cimento C, 30: 59-65.