Removal of Verrucaria nigrescens from Carrara marble artefacts using Nd: YAG lasers: comparison among different pulse durations and wavelengths

The periodical removal of biodeteriogens is a fundamental need for the preservation of outdoor stone culturalheritage, which is stimulating significant efforts toward the development of low-impact conservation strategies. In thepresent work, the potential of laser removal of Verrucaria nigrescens Pers. from Carrara marble and the evaluation of theassociated biocide effect on the organism residues embedded in the surface texture and through the outer porosities of thestone substrate were investigated. The fundamental wavelength of Nd:YAG laser (1,064 nm), commonly used in stonecleaning, and its second harmonic (532 nm) were comparatively tested. The phenomenology of laser treatments carriedout in different irradiation conditions was characterized using optical, epifluorescence, and electron microscopes along withchlorophyll fluorescence with pulsed amplitude-modulated imaging. The results achieved show that 532 nm can providesignificant advantages with respect to 1,064 nm. The potential of the latter against the biodeteriogens appears rather limitedbecause of the low optical absorption, whereas the former can allow effective and practicable laser treatments, which disclosea significant application perspective.