Sustainable solutions for the marble preservation of Santa Maria del Fiore Cathedral: a protocol to combating biodeterioration with low-impact biocides

Weathering processes seriously affect the durability of outdoor stone monuments. They are subject to numerous degradation processes, particularly biological attack which causes not only aesthetic but also structural damage since metabolically active microorganisms interact with the inorganic stone material through mechanical and chemical mechanisms. Therefore, targeted interventions to control the growth of biodeteriogens are necessary. Currently, biocidal treatments used in outdoor environments are mainly chemical and involve the use of highly toxic and hardly biodegradable biocidal molecules. The widespread use of toxic substances, however, poses risks both to operators and the environment. The Cathedral of Santa Maria del Fiore in Florence (Italy) is the most emblematic monument and symbol of the city. It is an impressive building located in Piazza del Duomo, the beating heart of the city, standing tall over the city, and its conservation is a main issue of worldwide concern. The stone materials used for the external cladding of the Cathedral are made of white marble, red marly limestone, ammonitic limestone, and serpentinite. In urban environments, a very common deterioration phenomenon is the dark discoloration or blackening of marble. In a previous work, we studied the darkening of the external marble of the Cathedral and demonstrated it was due to the growth of complex microbial communities including black fungi and dark cyanobacteria (1). Moreover, we tested and demonstrated the effectiveness of essential oils as innovative low-impact biocides on selected areas of marble of the Cathedral affected by blackening (2). This research reports the initial results obtained in the laboratory by following a protocol, developed by the authors, which includes cleaning steps of biodeteriorated marble surfaces and treatment of biological alterations by essential oils, mimicking the cleaning procedures of restorers. Before in situ application on the external marble of the Cathedral, preliminary, non-invasive tests were carried out to evaluate the interference of the essential oils with marble after removing biological patinas (microscopic, colorimetric, and water absorption assays on marble specimens) through short- and mid-term evaluation. The acquired knowledge will be used to plan in situ application of these innovative methods to control microbial colonization of Florence Cathedral marble.