Testing of a new Yb:YAG fiber laser system for the removal of graphic vandalism from marble

Graphic vandalism causes detrimental effects on architectural stone surfaces. Among the cleaning methods developed in the last decades, laser cleaning showed considerable effectiveness but the available equipment still lacks full adaptability for the use on site, while the cleaning process is time-consuming for the site work requirements. In this paper, the feasibility of a fiber laser operating at 1064 nm (Yb:YAG) at high repetition rate (kHz) for the removal of vandal graffiti from marble is investigated, as it is potentially suitable for cleaning of large surfaces. Indeed, this device exploits a scan system covering a wider area than that of solid state lasers, while maintaining excellent portability. Evaluation of the cleaning procedure on a selection of spray paints and felt-tip permanent markers applied on marble mock-ups is carried out with a multimodal approach (optical microscopy combined with surface pattern reconstruction, colour monitoring, Fourier Transform Infrared spectroscopy in external reflection and ATR mode, Thin Layer Chromatography, Raman spectroscopy, thermal monitoring, scanning electron microscopy). Ablation and thermal effects are observed varying the laser setting and the type of paint layer, with repetition rate showing a strong impact on the type of interaction. Overall, best cleaning procedures are achieved for the thinning of the black marker and the removal of the black spray paint. Some residues on the best results were observed, suggesting that the laser needs to be integrated with a second step of chemical cleaning. This study, focused on graffiti removal, shows that fiber laser can be a useful tool for cleaning of large monumental surfaces.