Silver patina investigation on ancient coins by means of PIXE-alpha and XRF complementary methods

A new method suitable to study ancient silvered coins is presented and discussed. It is based on the complementary use of the non destructive PIXE and XRF techniques; these present different penetration depth and their use enables to analyse different layers of the metallic alloy composing the coins. Measurements have been performed by using the portable non destructive spectrometers operating at the LNS/INFN (Laboratori Nazionali del Sud, Istituto Nazionale di Fisica Nucleare) laboratories of Catania. Three roman silvered coins have been analysed. PIXE and XRF results allowed to establish if a silver enriched patina is present on the coin surface; in such cases it has been possible to determine the surface composition of the alloy.