Effectiveness and harmfulness of graffiti cleaning procedures and evaluation of anti-graffiti products on cultural heritage stones

Resumen

Culture and cultural heritage are recognized as key elements for future sustainable development and urbanization by the UN Agenda 2030 (Goal 11.4) and by ICOMOS (International Council on Monuments and Sites). In 2011, through the so-called “Paris Declaration”, ICOMOS addressed a series of recommendations to all stakeholders involved in heritage conservation, development and tourism. In particular, a focus was made on built heritage, recalling the importance of the conservation of original materials. The architectural heritage and monuments of cities all over the world are constantly threatened by several natural factors, along with anthropic actions, such as the phenomenon of graffiti writing. In fact, graffiti are not only a direct cause of damage when affecting valuable surfaces of buildings of historical and cultural interest, but inappropriate methods for their removal are often harmful and may induce further alterations. Moreover, those cleaning procedures often require cities and public authorities to spend huge amounts of money, in addition to raising problems in terms of environmental sustainability and health risk. The search of effective and convenient solutions explains the numerous works and studies funded and carried out over the past years about this topic. In the present PhD dissertation, the themes of the cleaning and protection of ornamental stones against graffiti were addressed. In particular, aiming to establish a link with a real urban context, the research was focused on four representative lithotypes used in the city of Turin (NW Italy), different for mineralogy, texture and surface finish: Baveno granite, Vico diorite, Luserna stone and Roman travertine. Both laser and chemical cleaning procedures were tested on stone mock-ups for the removal of both alkyd-based and acrylic-based graffiti paints. The effectiveness and harmfulness of each method were compared and some aspects related to sustainability and health effects were also considered. In particular, it resulted that a synergistic use of the two procedures allows to increase the effectiveness of each of the two single cleaning methods, as well as to overcome some of the main drawbacks encountered. This was proved especially for the unpolished stone surfaces, such as the rough gneiss (Luserna stone) or the porous Roman travertine. With respect to protection, the effectiveness of both a sacrificial (a ready-to-use aqueous dispersion of microcrystalline wax) and a permanent anti-graffiti product (a fluorinated polyurethane in a water emulsion) was assessed. The cleaning results achieved on the protected surfaces were compared with those obtained on the corresponding unprotected stones, cleaned by using low-toxic ternary solvent mixtures. On the whole, the performance of these anti-graffiti products showed to depend on many factors, such as the paint and the protective coating composition, as well as, to a lesser extent, to the stone features (e.g. mineralogy, surface finish and porosity). Also, the two anti-graffiti tested proved to raise some drawbacks, related to the visual alteration induced of the stone surfaces, their durability and the possible unwanted interaction with some graffiti paints. For a proper assessment of the usefulness of both a combined methodology for the removal of graffiti and the protection of ornamental stones with anti-graffiti products, a SWOT analysis (acronym for “Strengths, Weaknesses, Opportunities and Threats”) was carried out. The present work underlines the importance of an attentive analysis of the urban context, along with a characterization of the lithotype, the graffiti paints and the state of preservation of the surfaces, in order to select the most appropriate cleaning procedure and/or the protective coating for each specific case. Nevertheless, no definitive solution against the phenomenon of graffiti affecting the historical towns, monuments and buildings will be possible without the involvement of local communities, young people, graffiti writers and street artists.