Infrared and ultraviolet laser removal of crustose lichens on dolomite heritage stone

Laser removal of biodeteriogen layers warrants detailed studies due to the advantages it brings withrespect to mechanical elimination or the use of biocides. We have investigated elimination of biologicalcrusts on dolomite stones from heritage sites in central Spain. The samples were colonized by epilithiccrustose lichens of different species, such as Caloplaca sp. and Verrucaria nigrescens. A comparative studywas carried out by applying infrared (1064 nm) and ultraviolet (355 nm) nanosecond laser pulses andsequences pulses of the two wavelengths using a Q-switched Nd:YAG system. To detect anatomical andultrastructural damage to the lichens, and to assess possible morphological and chemical changes on theunderlying stone induced by laser irradiation, we used stereomicroscopy, scanning electron microscopywith backscattered electron imaging and Fourier transform Raman spectroscopy. The optimal conditionsfor removal of the colonization crust, while ensuring preservation of the lithic substrate, were obtainedfor dual infrared-ultraviolet sequential irradiation.