Neutron diffraction measurements: technological and compositional features of Celtic coins from northern Italy

Neutron diffraction is a powerful tool for the analysis of metals. This technique, being non destructive and non invasive (apart from a radioactive activation that expires in some days), is particularly suited to study bulk properties of dense materials in objects that cannot be sampled. Moreover, neutron diffraction analyses could provide a wide range of structural and compositional information, useful for numismatic studies concerning, in particular, less known coinages such as those of the Celtic tribes. The silver coinage of the ancient Celtic peoples settled in northern Italy is a topic with many unsolved problems, such as chronology, attributions, relationships between series and emissions through time (from IV to I century B.C.). Furthermore, nothing is known about the production processes and the mint organization for the different operations (blanks molding, eventual blanching, cooling, striking), since mint workshops have never been found and no literarysources are available. Our research group at the Physics Department (University of Torino, Italy) has carried out neutron diffraction measurements at the ISIS facility (Rutherford Appleton Laboratory) to analyze a selection of these coins, which have almost never been characterized with scientific techniques. The instrument used is the TOF diffractometer of the Italian Neutron Experimental Station (INES), equipped with 144 3He squashed detectors, grouped in 9 banks, covering a range of about 160 degrees on the horizontal scattering plane. This experimental setup allows to measure a wide d-spacing range, and is then suitable for most metals. The analysis of diffraction patterns has been done with GSAS software, considering the data collected in 8 banks, since the Bragg peaks of silver and copper phases fall in the interval covered (d-spacing between 0.10 and 4.29 Å). In the analyzed coins two main phases were detected, accordingly to the biphasic diagram for silver-copper alloys, while minor phases (> 0.5 wt.%), such as cuprite, are due to alteration phenomena. Our measurements allowed us to prove the existence of a silver debasement throughout the years, and they permit now to draw some preliminary conclusions about these emissions. A close relation between the first Celtic emissions and heavy drachms of Marseille has now been proved, while a relationship between later emissions of drachmas and Roman contemporary coins is proposed. Other results, concerning the average dimension of alpha and beta crystallographic phases, the texture index and the strains, all parameters representing fingerprints of the working history of the alloy, brought some useful data for the comprehension of the working processes of ancient Celtic metallurgists. These last results have been compared with other neutron diffraction data concerning other silver coins, recently published in literature, and with some metallographic sections.