Excimer laser cleaning of encrustation on Pentelic marble: procedure and evaluation of the effects

This work focuses on the use and control of excimer lasers (KrF, lambda = 2 48 nm and XeCl, lambda = 308 nm) for the removal of encrustation (black cru sts, soil-dust and biological deposits) from Pentelic marble. A number of s urface analytical techniques, such as Fourier transform infrared spectrosco py (FTIR), scanning electron microscopy (SEM) interfaced with microprobe an alysis (energy dispersive X-ray analysis: EDX), X-ray diffraction (XRD), la ser-induced breakdown spectroscopy (LIBS) and optical microscopy (OM) were used to detect chemical composition and crust morphology, as well as to mon itor the effects induced by the laser treatment. SEM and OM providing struc tural information about the layers of encrustation, LIES and SEM-EDX provid ing information on the elemental composition of the ablated material, XRD a nd FTIR detecting the changes of minerals appear to be particularly effecti ve in assessing the quality of cleaning process, FTIR and XRD showed remova l of pollutants from encrustation and partial transformation of calcium sul fate dihydrate (gypsum) to hemihydrate and anhydrite, which have lower spec ific surface than gypsum. Irradiation conditions creating minimal damaging effects were defined before treatment through surface analysis, ablation ra te studies and optimization of laser parameters. On the basis of structural and analytical examinations, both lasers are shown to be appropriate for a chieving sufficient removal of unwanted selected layers without modifying t he surface morphology and surpassing by far the effectiveness afforded by t raditional cleaning techniques. (C) 1999 Elsevier Science B.V. All rights r eserved.