Effectiveness and harmfulness evaluation of graffiti cleaning by mechanical, chemical and laser procedures on granite

Abstract This paper reports the results from the comparative evaluation of the global cleaning effectiveness of four graffiti colours (red, blue, black and silver) applied on two granites with different textures and compositions. A conventional mechanical method and several chemical products were evaluated. Additionally, it was evaluated the cleaning based on laser ablation by means of a nanosecond pulsed Nd:YVO 4 at a wavelength of 355 nm. Optical and electronic microscopy, Fourier transformed infrared spectroscopy and spectrophotometry were applied in order to assess the cleaning effectiveness and also the harmful effects on the stones. Taking into account the graffiti extraction degree, it was found that blue, red and black colours were successfully extracted by all the evaluated methods. However, the extraction of silver colour, whose chemical composition differs from that of the other three colours, was more difficult and it was only achieved by the mechanical procedure and by several chemical products; Nd:YVO 4 laser at 355 nm was an ineffective procedure in removing this paint. All the cleaning methods produced damage on the granites: a roughness increase after the mechanical cleaning, chemical contamination related to some chemical procedures and melting of biotite crystals after laser cleaning. Moreover, the ability of different paints to penetrate into the rock affected the global cleaning effectiveness evaluation.