Green biocides for the conservation of hypogeal mural paintings raised from Western and Eastern traditions: Evaluation of interference on pigments and substrata and multifactor parameters affecting their activity

Hypogeal environments due to their limited air circulation, low constant temperatures and high humidity could be threatened by microbial spreading. In cultural heritage preservation, natural bioactive molecules have been proposed as a possible alternative to traditional biocides. Although several papers report the in vitro effect of green biocides, scant information is available about their efficacy in direct application and interaction with historical materials. Then, we investigated five green biocidal formulations never applied before to control the biodeterioration of subterranean mural painting raising from Italian and South Korean traditions. We tested three emulsified essential oil-based (oregano and clove bud) water suspensions, and usnic acid-based water and 2-propanol/dimethyl sulfoxide (7:3) solutions for establishing: (i) possible interference with the reconstructed ancient wall paintings materials (colour and water adsorption changes), (ii) efficacy on recurrent biodeteriogens, trough optical/electronic microscopy, bioluminometric and culture-based methods. Results evidenced the different impact of such biocides on wall paintings' materials and pigments (e.g., higher on red and black Etruscan tiles). They also highlight the importance of reconstruction experiments for customizing treatments, avoiding interference with historical materials (e.g., changes in colour) and biodeteriogens' spreading. The active principles concentration and stone porosity strongly affect the biocidal efficacy, but the solvent and emulsifier used also had a relevant role.