Medium-term field evaluation of several widely used cleaning-restoration techniques applied to algal biofilm formed on a granite-built historical monument

This study evaluated the medium-term performance of some of the most widely used methods of cleaning stone: dry brushing or washing with water or with an ethanol-water (1:1) mixture, benzalkonium chloride or commercial products based on quaternary ammonium salts with biocidal properties (Biotin R (R), Biotin T (R), Preventol RI80 (R), and New Des 50 (R)). The ability of the different methods to remove an algal biofilm formed on a granite-built historical monument was compared. The biofilm has been shown to affect the appearance, but not the integrity, of the building material. In the cleaning trials, the presence and vitality of the organisms colonizing the substrate were determined, immediately after treatment and after one, three, six and twelve months, by colour spectrophotometry and chlorophyll-a fluorescence (ChlaF) analysis. The quantity and physiological state of the biofilm-forming phototrophs in the test areas were also determined one year after the treatments. Although recolonization of the test areas was barely noticeable after all of the treatments, Preventol RI80 (R) proved to be the most effective, followed by benzalkonium chloride and the ethanol-water (1:1) mixture, which yielded similar results. New Des 50 (R) caused clearly visible colour changes on the granite wall, and the biocidal efficacy was poor. Biotin R (R) performed better than Biotin T (R), which yielded similar results to New Des 50 (R) regarding recolonization. Considering the overall results and the potential toxicity of the products tested, cleaning with water is recommended if and when required.