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Fluorescence spectroscopy of algae commonly found on stone

Algal growth on stones, together with the deposition of other soiling layers, cause major conservation problems for buildings and monuments, as it not only covers the surface with a green layer, but also accelerates stone decay. In recent years laser ablation techniques have been used to clean masonry as they are potentially less destructive than chemical or physical techniques due to the high selectivity in removing the coating covering the stone and the absence of secondary products as with conventional techniques such as use of chemicals or of sandblasting. Whilst laser ablation cleaning is finding favor in removal of surface layers from stones there has been little or no reported work relating to the effect of the laser radiation on the algae found on stones. In order to optimize any cleaning or preservation technique for algae covered stones it is necessary to have a detailed knowledge of such effects. In this paper we report some initial results from the analysis of several algae commonly found on masonry before and after irradiation at different wavelengths using two lasers, a nitrogen laser and a Nd:YAG laser.