Femtosecond laser texturing as a tool to increase the hydrophobicity of ornamental stone: The influence of lithology and texture

In the field of stone conservation, a very common direct intervention seeks to increase the water repellence of objects. By limiting the interaction between the solid and the water, the possibility of alteration due to the action of the water (liquid or solid) as well as that related to the colonisation of surfaces by organisms is reduced. To decrease wettability, chemicals are often applied to create a surface layer; however, laser texturing, currently used in different industrial and technological fields, could be considered as an alternative procedure, thus eliminating exposure to chemicals with varying degrees of toxicity. In this paper, modification of the wetting properties of stone surfaces upon texturing induced by a femtosecond laser structuring process was investigated. Four ornamental stones with different mineralogic and chemical compositions (slate, quartzite, granite and marble) and other surface characteristics were laser processed by applying different texturing patterns (parallel grooves and matrices of craters) and different irradiation parameters. After texturing, variations on the wettability characteristics of the different rock surfaces were evaluated by means of static contact angle measurements and modifications on the topography were studied via confocal microscopy. Furthermore, potential harmful effects on the ornamental stones were evaluated through colour spectrophotometry, petrographic microscopy and SEM-EDX. The results indicate two different behaviours in the ornamental stones analysed that were statistically confirmed: on the one hand, the group formed by slate, quartzite and granite, in which the laser texturing caused a decrease in the contact angle and, on the other hand, marble in which laser texturing was able to increase the contact angle above 90°, giving the stone surface a hydrophobic character. The topographic analyses together with microscopy observations indicate that the grain size, mineral composition, type of fissures and initial surface quality of the stones were key in the different wetting behaviour of the laser textured stones.