Experimental results of UV laser cleaning on a silver Carlino coin

Silver artefacts and particularly ancient silver artefacts present the serious problem of turning black due to ambient contamination. The black colour is the consequence of a layer forming on the surface made of acanthite and jalpaite; both compounds are sulphur composts. Recently, a UV laser cleaning technique has demonstrated to be very promising in the processing of cultural artefacts. To operate on artefacts without contaminating the bulk, we first determined the laser fluence threshold of the bulk; second, we controlled the surface compounds, and finally we operated the laser irradiation. In this work, we utilised certified silver and silver/copper samples in order to perform preliminary studies. The threshold fluences we found were 780 and 510 mJ/cm2 for the pure Ag and for the Ag/Cu alloy, respectively. The jalpaite concentration on the Ag/Cu sample increased after a laser dose of 25 J/cm2, whereas for a higher laser dose its value lowered, pointing out that complex processes take part during the laser application. We successfully applied the technique on a Carlino coin made of silver coined in 1689 under King Carlo II. The analyses were performed by two different techniques that are able to find the percentage of elements and the chemical compounds: the energy dispersive X-ray fluorescence and the X-ray diffractometer spectrometer. We found that the sulphur concentration decreased upon the increase of the laser dose. Operating on the coin up to a dose of 280 J/cm2, the sulphur concentration deceased by up to 20%, while the coin looks clearer just after 50 J/cm2.