Abstract
Despite the massive presence of biofilms causing aesthetic alteration to the façade of the Monza Cathedral, our team in a previous work proved that the biocolonization was not a primary damaging factor if compared to chemical-physical deterioration due to the impact of air pollution. Nonetheless, the conservators tried to remove the sessile dwelling microorganisms to reduce discolouration. In this research, two nearby sculpted leaves made of Candoglia marble were selected to study the effects of a chemical treatment combining the biocides benzalkonium chloride, hydrogen peroxide and Algophase® and mechanical cleaning procedures. One leaf was cleaned with the biocides and mechanically, and the other was left untreated as control. The impact of the treatment was investigated after 1 month from the cleaning by digital microscopy, environmental scanning electron microscopy, confocal microscopy and molecular methods to determine the composition and the functional profiles of the bacterial communities. Despite the acceptable aesthetic results obtained, the overall cleaning treatment was only partially effective in removing the biofilm from the colonized surfaces and, therefore, not adequately suitable for the specific substrate. Furthermore, the cleaning process selected microorganisms potentially more resistant to biocides so that the efficacy of future re-treatment by antimicrobial agents could be negatively affected.
Highlights
The investigated stone is Candoglia marble from the façade of the Monza Cathedral, a NorthernItalian architectural masterpiece
A biocidal treatment used on the monuments at the Angkor site in the 1990s led to a subsequent worsening recolonization that was characterized by intensive blackening of the treated stone surfaces caused by black-pigmented cyanobacteria [19]
The morphological features of fragments and polished cross-sections of the substrates before and after the treatment were investigated by environmental scanning electron microscopy (ESEM) and energy-dispersive X-ray (EDX) analyses, using a Zeiss EVO 50 EP ESEM (Jena, Germany), equipped with an Oxford INCA 200 - Pentafet LZ4 spectrometer (Jena, Germany)
Summary
The investigated stone is Candoglia marble from the façade of the Monza Cathedral, a Northern. According to the general requirements for the treatment of stone heritage surfaces, biocides must neither harm the substrate from the chemical point of view nor induce significant changes of its appearance. A biocidal treatment used on the monuments at the Angkor site in the 1990s led to a subsequent worsening recolonization that was characterized by intensive blackening of the treated stone surfaces caused by black-pigmented cyanobacteria [19]. Here a pilot area on the Monza Cathedral façade (Figure 1a) was selected with the aim of studying the effects of a treatment based on the combination of the three biocides benzalkonium chloride, hydrogen peroxide and Algophase® and mechanical cleaning. Algophase® (Phase, Italy, 3% dilution in isopropyl alcohol; active ingredient: 2,3,5,6-tetrachloro-4methylsulfonyl-pyridine) by brush This cleaning treatment was designed and performed by Coatings. Details of the sampling areas untreated and areas treated areas (b–d) respectively
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