• Alkyd spray paints
• Artificial aging
• Chemical cleaning
• Combined cleaning
• Effectiveness
• Harmfulness.
• Laser cleaning
• Multi-analytical characterization

This work reports the results from the comparative evaluation of the global cleaning effectiveness of five graffiti colours (black, yellow, green, red and blue) applied on marble samples. Firstly, the chemical characterization of all the spray paints was performed. The binder was investigated with Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR), analytical pyrolysis coupled with gas chromatography/mass spectrometry (Py-GC/MS) and gas chromatography-mass spectrometry (GC/MS); fillers and additives were characterized by X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) and, finally, pigments and dyes were determined by Raman spectroscopy. All paint colours are composed by alkyd binder in which rutile as filler and a lot of different (inorganic and synthetic organic) pigments/dye are dispersed. In addition, a chemometric multivariate analysis was performed exploiting Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCG) was also applied to FTIR-ATR and FTIR-refl spectra of alkyd, (pure or modified) acrylic spray paints and felt-tip pens, to highlight the differences and similarities between various paint brands.
Taking into account the different parameters that may affect the cleaning efficiency, three types of samples were compared in this study: unaged marble samples, aged-marble samples and photochemical aged-paint samples. Three different cleaning approaches were compared in terms of their efficacy and invasiveness in the removal of the paint from the substrate: chemical cleaning, laser cleaning and laser followed by chemical cleaning. In particular, the latter method was expected to be more advantageous based on the preliminary, physical removal of the superficial paint layer, so as to minimize the risk of further colour diffusion into the substrate caused by the solvent-based cleaning performed in the second step.
Two commercial products were evaluated in the chemical approach: a commercial cleaner and a pure solvent (methylethylketone) supported on a commercial agar gel. Additionally, laser ablation by means of a Q-switched Nd:YAG at wavelength 1064nm and 532nm with different fluence values was also evaluated. A multi-analytical approach was also adopted to assess the cleaning effectiveness and harmfulness to the stone. The cleaning efficiency was evaluated in terms of presence of paint residues on the surface, paint diffusion in the marble and damage to the substrate through FTIR, XRF, laser scanner microprofilometry, optical microscope observation in cross section and colorimetric results. The most difficult paint to remove was the red one (alkyd-amino resin). Conoscopic measurements showed the negligible harmfulness of the laser ablation on unaged marble.
The combined approach laser followed by chemical cleaning proved the most efficient method in all cases. However, all the cleaning approaches resulted less effective on aged materials (marble or paint) than on unaged ones, due to paint penetration (in the aged marble) and, probably, to the increased polarity (in the aged-paints).