Laser-Assisted Removal of Graffiti from Granite: Advantages of the Simultaneous Use of Two Wavelengths

Abstract

Currently, removal of graffiti from stone monuments is a particularly challenging task. Lasers, being highly controllable and precise tools with minimal chemical waste, offer a key solution in this respect and a significant amount of research has been dedicated to this subject. Studies related to the laser cleaning of carbonate stones (such as limestone and marble) reported the extraction of the graffiti layer, although minimal damage to the substrate can be also detected. Recently, research efforts have been focused on the cleaning of granite, which is a complex stone due to its grained and polymineralic texture. Tests involving different wavelengths indicated that the effectiveness of the cleaning procedure is highly dependent on two components: The composition of the binding medium of the graffiti and the fissure system of the granite. In that direction, the aim of this paper is to investigate and to compare the cleaning effectiveness of two wavelengths emitted from a nanosecond (ns) Q-Switched Nd:YAG laser system (IR at 1064 nm and UV at 355 nm), as well as their simultaneous application at different energy density ratios F IR /F UV. The effectiveness of this combined methodology has been shown in several other cases; i.e., for the removal of pollution crusts from carbonate stones (marble). For this study, three different in composition graffiti paints (blue, black, and silver) were applied on a fine-grained granite originating from the NW Iberian Peninsula. Prior to the irradiation tests, the damage thresholds of the granite, as well as the extraction thresholds of the graffiti, were determined. Then, several tests involving a variety of parameters (fluence value, number of pulses, etc.) were performed and the most satisfactory irradiation conditions from each individual wavelength as well as their combination were compared, based on graffiti extraction level and any damage induced on the granite forming minerals. The analytical techniques used for the evaluation were stereomicroscopy, color measurements in CIELAB and CIELCH color spaces, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) and confocal microscopy. The experiments indicated the superiority of the combined laser cleaning regarding blue and black graffiti extraction.

Influence of the Laser Wavelength on Harmful Effects on Granite Due to Biofilm Removal

“…In addition to the melting of biotite planes found in all the treated surfaces, the 532 nm-treated surfaces also showed a slight melting of the muscovite exfoliation planes. As reported in previous research [20,21,30], biotite is the first mineral to melt due to its high optical absorption along with its relatively low melting point of 650 • C while 1110 • C for plagioclase (albite), 1250 • C for potassium feldspar, and 1710 • C for quartz [51,51].…”

“…In order to ensure damage to the substrate, fluences higher than the damage threshold for this type of granite [20,30] were applied in only one scan. Therefore, 2 J•cm −2 and 5 J•cm −2 were irradiated for each wavelength.…”

“…As the damage threshold for this kind of granite is set in the 0.7-1.5 J•cm −2 range for an Nd:YAG laser [20,29], and because the laser effect depends on the patina or coating [11], fluences of 2 J•cm −2 and 5 J•cm −2 were used in order to exert damage on the stone. In previous studies, the damage threshold for the same granite as that used in this research was 1.1 J•cm −2 for 1064 nm and 0.7 J•cm −2 for 355 nm [29,30]. Since in this research, the objective was to evaluate the influence of the wavelength used on the by-effects in the different granite forming minerals due to an overcleaning, the fluences applied were higher than the damage thresholds of the granite.…”

Anti-Graffiti Coatings on Stones for Historical Buildings in Turin (NW Italy)

“…These Ti nanoparticle agglomerations are white or colorless powders, with a very low solubility in water and preferably inert to the action of acids and alkalis [56]. The use of Ti as a marker element to detect the presence of graffiti remains has already been applied in previous works [16,20]. However, in the cited references, these remains were notably lower (amount and extension) than those registered in this research because the cleaning procedures were different: laser and mechanical methods.…”

“…However, in the cited references, these remains were notably lower (amount and extension) than those registered in this research because the cleaning procedures were different: laser and mechanical methods. A laser [20] heats the material, which is consequently evaporated or sublimated, and mechanical procedures induce the removal of graffiti on the stone by abrading the surfaces. However, the chemical methods used in this research to clean graffiti, regardless of anti-graffiti coatings, are based on the application of solutions which react with the organic paint phases (pigment and binder) and dissolve them, leaving the inert compounds (extenders) susceptible to being extracted by the brush.…”