Silver coins analyses by X-ray fluorescence methods

The investigation on the differences occurring in the manufacture of silver coins allows to get information on their elemental composition and represents a powerful support to the methodology to identify the producing technologies, workshops being also instrumental to distinguish between original and counterfeit ones. Aim of the present work is to study recent and old silver coins through non-destructive X-Ray Fluorescence (XRF) analysis. The XRF was applied to extend the analysis to the deepest layers of the coins; for surface layers an X-ray tube or an electron beam were employed to induce the atom fluorescence to obtain information on the surface elemental composition. Moreover, a detailed study has been performed to evaluate the influence of the surface curvature on the measurement, by deducing a proper corrective factor to keep into account in the data analysis. The elemental atomic composition was measured for each coin, mainly by means of the X-ray tube excitation for the bulk and the electron Scanning Electron Microscope (SEM) microbeam probe for the surface patina analysis. Ionization was induced by an X-ray tube using an Ag anode for the bulk and by an electron microprobe for the surface composition. X-ray detection was performed by using a semiconductor Si device cooled by a Peltier system. The Ag L-lines X-ray yield is affected by coin surface morphology and geometry. The comparison between coin spectra and standard samples, shows that the Ag quantitative analysis is influenced by error of the atomic concentration lower that 10%.