Fibre Laser Cleaning of Painted Granite Surfaces: Influence of Mineralogy and Texture on Cleaning Effectiveness and Surface Modification

Abstract

[Abstract]: This study investigates spray-paint removal from Rosa Porriño and Vilachán granites, widely used in architectural heritage, using an industrial ytterbium-doped pulsed fibre laser (1064 nm), focusing on the influence of substrate characteristics on cleaning effectiveness and laser-induced surface modifications. A multiscale approach combining petrography, X-ray diffraction, optical microscopy, SEM-EDS and interferometric surface topography correlated cleaning effectiveness with substrate mineralogy, microstructure and surface texture. Near-complete paint removal was achieved for both granites, although higher fluences were required for Vilachán, demonstrating amaterial-dependent cleaning response. Paint residues preferentially persisted within polycrystalline quartz aggregates and surface discontinuities, indicating an influence of substrate microstructure on paint retention. Laser irradiation produced a heterogeneous mineral-scale response: quartz exhibited conchoidal fracture features consistent with brittle fracture, K-feldspar showed increased surface roughness, biotite showed superficial melting and groove-like texturing, while muscovite showed treatment-induced texturisation. These responses were accompanied by measurable changes in surface topography and roughness. The paint ablation threshold exceeded that of the granitic substrate, indicating that laser-substrate interaction may occur before complete paint removal and that a simple fluence window for selective cleaning without substrate modification cannot be assumed. Overall, cleaning effectiveness and selectivity depend on processing conditions, mineralogical composition, microstructural characteristics and surface texture. By integrating cleaning effectiveness with morphological responses and quantified topographical changes at the individual mineral-phase scale in two granites, this comparative, multiscale assessment relates the conditions required for paint removal to the induced surface modifications, providing a material-specific understanding of the balance between paint removal and substrate modification.