Induced biological colonization on model historical glasses and biocide treatment effects for its elimination

Biological colonization and later biodeterioration damage are well-known phenomena affecting cultural heritage. Glasses, as those from historical stained glass windows, are frequently biocolonized under certain conditions. There is still limited knowledge about biocolonization growth patterns on glass and safe removing procedures to act before irreversible biodegradation damage occurs. This paper presents the results of an experimental study in which biological colonization of three different model historical glasses, namely sodalime, lead crystal, and potash-lime silicate glasses, has been naturally induced. Glass samples were exposed for 13-15 months to natural conditions at the mountain region near Madrid (Spain) and monitored over time. After exposition, samples were first observed through fluorescence microscopy (FM), scanning electron microscopy (SEM), and UV-Vis-NIR spectrophotometry. Next, the Acticide (R) CF biocide was applied on glass samples exposed and were then again observed through the same techniques to evaluate its effect. Biocolonizers detected were algae, fungi, cyanobacteria and heterotrophic bacteria in different combinations and amounts and sometimes forming a biofilm. Soda-lime silicate glasses were the ones that showed the highest biocolonization growth, while lead crystal silicate glass appeared more biocolonized than the potash-lime one. The biofilm formed appeared intensely damaged and was practically no longer visible after the biocide treatment which proves its efficiency for removing glass biocolonization.