Silver Celtic Coins from Northern Italy (4th –1st c. B.C.): Compositional Analysis and Metrology

The so called “Celtic coinage of the Po valley” gathers together different series of silver coins, mainly drachmas but fractions (diobols or hemidrachms) as well, produced by the pre-Roman tribes settled in northern Italy between the 4 th c. and the 1 st c. B.C. All the drachmas reproduce the iconography of the Massilian heavy drachm, which bears the head of Artemis on the obverse and a walking lion with the ethnic ΜΑΣΣΑ[ΛΙΗΤΩΝ] on the reverse. In literature, the different artistic styles and the discovery places of single findings have been used for the attribution. Nevertheless, many unsolved problems, such as chronology, attributions, relationships between series and emissions through time still remain. In order to provide new data for metrological studies, which require average weights of the emissions and the knowledge of the bullion content, a wide research project has been developed. Our research group at the Physics Department (University of Torino, Italy) has carried out neutron diffraction measurements at the ISIS facility (Rutherford Appleton Laboratory, Didcot, UK) on a selection of specimens chosen from the main typologies, to analyze structural properties and composition of these coins. The first results suggest a new organization of the entire north-Italian Celtic coinage, based on the scientific data obtained. The most ancient coins of the series are made of almost pure silver, while coins attributed to Insubres and Boico-Cenomani peoples, which belong to a second stage and are contemporary to each other (3 rd c. B.C.), show a lower average silver content. Later coins (200-125 B.C.), attributed to the Celtic tribe of the Libui, show an even lower silver content. Simultaneously, the average weight of each typology, and for every known hoards, has been calculated, allowing us to trace a gradual weight reduction as well. The compositional data and the average weight have then been correlated in order to compare the bullion content in the Celtic coins and that in the contemporary Roman Republican coins (Victoriati), in order to establish the metrological relationship between the two currencies.