Developing new cleaning strategies of cultural heritage stones: Are synergistic combinations of a low-toxic solvent ternary mixtures followed by laser the solution?

As the UN Agenda 2030 recognizes heritage protection in several goals, this research was focused on the improvement of the graffiti removal from stones. The cleaning of two graffiti paints with different composition (an alkyd- and an acrylic-based paints) from two stones (gneiss and travertine) was performed considering a synergistic approach based on the combination of a low-toxic solvent ternary mixture, followed by an Nd:YAG laser. The different concentrations of the low-toxic solvents were based on the triangular Teas graph, keeping similar Hansen solubility of the products commonly used in conservation of cultural heritage: methyl ethyl ketone (MEK) and n-butyl acetate. The n-butyl acetate was replaced by mixture A (51% ethyl alcohol/11% acetone/38% isooctane) and MEK by mixture B (7%/13%/80%). Regardless of the graffiti composition, the gneiss was satisfactorily cleaned with the mixture A, while for the travertine, the best results were achieved by the mixture B. Then, surfaces treated with the mixtures were irradiated with a QS Nd:YAG laser working at 532 nm. The cleaning evaluation was performed by stereomicroscopy, color spectrophotometry, roughness measurements, reflectance measurements, and scanning electron microscopy. As result, regardless on the stone and the graffiti paints, it was confirmed the enhancement of cleaning of the mixtures followed by the Nd:YAG laser. However, it is crucial to identify previously the concentrations of the chemical products that allow the best performance considering the graffiti-stone interaction, on the one hand, and the optimal laser's parameters, on the other hand.