Nd:YAG and Er:YAG laser cleaning to remove Circinaria hoffmanniana (Lichenes, Ascomycota) from schist located in the Côa Valley Archaeological Park

Lichens are part of the dominant colonizer crusts of rock-art on schist in the Côa Valley Archaeological Park (NE Portugal). Among the several species observed at the site, Circinaria hoffmanniana is one of the most extensive on the rock surfaces. Given the historic value of the park's open-air prehistoric rock-art, which has been designated UNESCO World Heritage, an effective, precise, selective and controllable cleaning method for the engravings is required, in order to extract the lichen while avoiding any grain extraction and/or physical changes in the surface. Based on these requirements, 2 different lasers commonly used in the conservation and restoration of cultural heritage were applied in one scan, with the aim of obtaining a satisfactory level of cleaning as quickly as possible. The 2 lasers were: a Q-switched Nd:YAG (pulse duration of 6 ns) and a free-running Er:YAG (pulse duration of 250 μs), working at IR radiation of 1064 nm and 2940 nm respectively. The cleaning was evaluated by assessing the extraction rate and the damage induced on the schist using a multianalytical methodology based on stereomicroscopy, colour measurements in CIELAB and CIELCH colour spaces, Fourier transform infrared spectroscopy, Raman spectroscopy and scanning electron microscopy. Further studies should be performed to evaluate the tertiary bioreceptivity of the treated surface.