Graffiti removal from historical buildings

Graffiti, as an act of vandalism, is a major danger to stone Cultural Heritage and a risk for the preservation of historical or modern buildings and monuments. Graffiti paints may damage architectural materials, accelerating their decay and leading to important loss in value and significance. Spray paints and felt-tip markers are the most widespread materials used for graphic vandalism. The preventive use of antigraffiti coatings that hinder the diffusion of the paints into the substrate may ease the recovery intervention but, on the other hand, many surfaces of historical value still remain vulnerable not having such protection at all, whose protection just became less effective or whose peculiar characteristics exclude such treatments. In this case, the interventions to remove graffiti most frequently rely on chemical products or mechanical tools. However, the most appropriate cleaning procedure should be chosen according to the chemical composition of vandalic paints and to the chemical-morphological characteristics of the substrate, in order to selectively remove the paints without damaging the support. The outcomes of recent laboratory research performed on painted marble are presented here, based on laser and low-toxicity chemical cleaning performed both as standalone methods and in combination. The advantage of the combined approach is based on the preliminary thinning of the superficial paint layer by pulsed laser radiation (physical approach) so as to optimize the action of the chemical cleaning performed in the second step. In addition, the risk of further colorant diffusion into the internal stone structure caused by the solvent-based cleaning is minimized. After the preliminary chemical characterization of the chosen paints, the different approaches were evaluated on both sound and aged marble through scientific examination of surfaces and cross-sections. Spectroscopic analyses and observation with optical microscopy, as well as with laser-scanner microprofilometry, were performed to check the presence of visible or hidden residues, possible damage of the marble structure, penetration depth and extraction efficiency of the paint.