Femtosecond laser ablation for the conservation of Galician granites

This paper investigates the femtosecond pulse laser ablation and cleaning of two Galician granites of high cultural significance used for many monuments and infrastructure in Galicia, Spain: Vilachan which is affected by biological colonisation, and an ashlar granite in Vigo affected by a gypsum-rich black crust. The ablation thresholds of the granites were established to be 1.3 ± 0.2 J∙cm⁻². The ablation efficiency was determined to be 0.31 ± 0.05 mm³∙(min∙W)⁻¹ for a fluence of 7.8 ± 1.0 J∙cm⁻² for Vilachan and 0.33 ± 0.05 mm³∙(min∙W)⁻¹ for a fluence of 8.3 ± 1.0 J∙cm⁻² for Vigo ashlar. The damage threshold of both types of granite, when damage may occur without ablation, was then established at 0.8 J∙cm⁻² for Vilachan and 0.5 J∙cm⁻² for Vigo ashlar, values above which both stones were found to discolour and lose their characteristic yellow colours. Following the determination of these parameters, femtosecond laser cleaning was undertaken on the biological colonisation covering Vilachan, and the gypsum-rich black crust covering Vigo ashlar. Complete removal of the Vilachan biological colonisation was achieved under the ablation threshold of the stone, at 0.5 J∙cm⁻², and no damage was induced to the underlying substrate. Acceptable removal of the black crust was reached, at 1.0 J∙cm⁻² with limited damage to the substrate. The effects of laser irradiation on both types of granite were investigated using optical and scanning electron microscopy, optical profilometry, colourimetry, and multispectral imaging.