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

Chiara Ricci,Francesca Gambino,Anna Piccirillo,Arianna Scarcella,José Santiago Pozo-Antonio,Francesca Zenucchini,Marco Nervo

doi:10.3390/coatings10050466

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.

Highlights

The UN Agenda 2030 recognizes through its Sustainable Development Goal 11 the protection of the culture and heritage as a requirement to achieve a sustainable future for humanity [1]
Compared to the chemical cleaning procedures, by using the mixture A for gneiss (Figure 4, GVSa and GGSa) and the mixture B for travertine (Figure 4, TVSb and TGSb), the treatments combining these solvent mixtures and the laser Nd:YAG working at 532 nm achieved a notable improvement in terms of effectiveness in the removal of graffiti paints
Since the UN Agenda 2030 recognizes, through its Sustainable Development Goals 11, 4, 8, and 12, the protection of the cultural heritage as a requirement for sustainable future scenarios and strategies focused on the conservation of heritage elements have to be improved and fostered

Summary

Introduction

The UN Agenda 2030 recognizes through its Sustainable Development Goal 11 the protection of the culture and heritage as a requirement to achieve a sustainable future for humanity [1]. In its target 11.4, it is stated that the protection and safeguard of the world’s cultural and natural heritage is fundamental to make cities and human settlements inclusive, safe, resilient and sustainable. Graffiti represent one of the most threatening deterioration forms affecting cultural heritage materials. This form of vandalism affects more than three million of the protected monuments in Europe and millions of euros have been spent in the connected cleaning campaigns [3,4]

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Conclusion