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

Abstract

This work focuses on the use and control of excimer lasers (KrF, λ=248 nm and XeCl, λ=308 nm) for the removal of encrustation (black crusts, soil-dust and biological deposits) from Pentelic marble. A number of surface analytical techniques, such as Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) interfaced with microprobe analysis (energy dispersive X-ray analysis: EDX), X-ray diffraction (XRD), laser-induced breakdown spectroscopy (LIBS) and optical microscopy (OM) were used to detect chemical composition and crust morphology, as well as to monitor the effects induced by the laser treatment. SEM and OM providing structural information about the layers of encrustation, LIBS and SEM–EDX providing information on the elemental composition of the ablated material, XRD and FTIR detecting the changes of minerals appear to be particularly effective in assessing the quality of cleaning process. FTIR and XRD showed removal of pollutants from encrustation and partial transformation of calcium sulfate dihydrate (gypsum) to hemihydrate and anhydrite, which have lower specific surface than gypsum. Irradiation conditions creating minimal damaging effects were defined before treatment through surface analysis, ablation rate studies and optimization of laser parameters. On the basis of structural and analytical examinations, both lasers are shown to be appropriate for achieving sufficient removal of unwanted selected layers without modifying the surface morphology and surpassing by far the effectiveness afforded by traditional cleaning techniques.