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 FIR/FUV. 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.
Highlights
Graffiti is one of the most immediate threats encountered on heritage monuments worldwide and its cleaning is considered by the entities responsible for the heritage protection as one of the mostCoatings 2018, 8, 124; doi:10.3390/coatings8040124 www.mdpi.com/journal/coatingsCoatings 2018, 8, 124 demanding conservation issues [1]
The damage thresholds of the granite subjected to both wavelengths were determined by means of stereomicroscopy, roughness measurements and scanning electron microscopy (SEM)
These ablation thresholds on granite were determined based on the induced damage to the biotite grains, For the blue and black graffiti cleaning experiments, the most satisfactory removal was obtained as this is the most susceptibleuse granitic component to the laser beam with the simultaneous of UV and IR wavelengths at fluence ratio FIR
Summary
Graffiti is one of the most immediate threats encountered on heritage monuments worldwide and its cleaning is considered by the entities responsible for the heritage protection as one of the mostCoatings 2018, 8, 124; doi:10.3390/coatings8040124 www.mdpi.com/journal/coatingsCoatings 2018, 8, 124 demanding conservation issues [1]. The necessity of methods to completely extract the graffiti paints safeguarding the original surfaces was discussed in scientific projects focused on the optimization of cleaning methodologies, as well as on the development of protective anti-graffiti coatings [1,2,3,4,5,6]. Conventional cleaning methods, involving the use of chemical products and mechanical tools, do not completely remove the paint and at the same time they can be rather aggressive to the stone surface [2]. Recent studies have been focused on the determination of optimum laser cleaning parameters and procedures for graffiti paint removal [15,16,17,18]
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