Developing New Cleaning Strategies of Cultural Heritage Stones: Are Synergistic Combinations of a Low-Toxic Solvent Ternary Mixtures Followed by Laser the Solution?

Abstract

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.

Anti-Graffiti Coatings on Stones for Historical Buildings in Turin (NW Italy)

“…The inorganic component of the paints investigated by XRF showed the presence of Ti, Si, Cl, Ca, S, and Al for both paints [30]. Other chemical elements were found: Fe, Co, P, and K in the violet spray paint and Cu in the green dabber paint [30].…”

“…These graffiti paints were previously used in [30]: the violet paint exhibits an alkyd organic base, while the green paint is an acrylic modified with styrene. The inorganic component of the paints investigated by XRF showed the presence of Ti, Si, Cl, Ca, S, and Al for both paints [30]. Other chemical elements were found: Fe, Co, P, and K in the violet spray paint and Cu in the green dabber paint [30].…”

“…The results obtained were compared with a chemical cleaning procedure applied on painted surfaces without anti-graffiti product. Specifically, a low-toxic solvent mixture was used, following Ricci et al [30], taking into account the sustainability regarding the environment and safeguarding of human health, as well as the lowest impact on the original stone materials.…”

Analytical Evaluation of Laser Cleaning Effectiveness in the Context of Contemporary Muralism

“…Graffiti vandalism on cultural monuments is causing significant problems both on surfaces' esthetical appearance and on historical material preservation. Recent studies have demonstrated the efficiency, safety, and feasibility of laser cleaning instruments, either used independently or in combination with chemical methods, even for on-site work [12,[16][17][18].…”

Formulation of Environmentally Safe Graffiti Remover Containing Esterified Plant Oils and Sugar Surfactant

“…Much effort is needed to develop low-cost, effective technologies for the production of alternative solvents that could replace the typically applied toxic organic solvents. Among many outstanding examples of current highly efficient graffiti removers, there are highly retentive chemical hydrogels [ 28 ], nanofluids, such as micelles or microemulsions [ 28 ] as well as low-toxic solvent ternary mixtures followed by Nd:YAG laser irradiation [ 29 ].…”