## 4 Discussion  
We were surprised that, until now, the majority of biodiversity and biodeterioration studies have been performed on stained glass works of art coming from Germany. On the other hand, studies on glazed tiles from Portugal are more prevalent in the literature. Biodiversity data reported from the 21 st century literature, indicates that fungi and bacteria are the main colonizer of stained glass windows (Figures 2 and 4). While glazed tiles are also colonized by macroalgae and cyanobacteria. Nevertheless, the authors found that there are several works that identify microorganisms on stained glass and there are less studies published concerning glazed tiles. Still, it is important to study bacteria on glazed tiles since only two studies (Table S2) have been conducted in the 21 st century literature. A further research should be performed on green biofilms on stained glass windows, in order to find out if there are phototrophic microorganisms on this glass cultural heritage. versity makes it impossible to determine which microorganism was responsible for the biodeteriogenic action. When laboratory based studies are made the use of pure cultures allows to understand if a specific microorganism is a biodeteriogen. However, multi-species mimic better the real conditions in which biodeterioration occurs, since colonization usually occurs in a community of organisms. Laboratory experiments have the advantage of testing the damaging potential of a specific organism or group of organisms under controlled conditions. However, distinct authors used a different microorganism, time, temperature, light, etc., which make it impossible to compare between distinct studies. Therefore, a working group should be organized in order to develop guidelines or standards for laboratory-based experiments: for instance, if you have a biofilm with five species you would test the biofilm as a whole and each specie separately. Is also important to reproduce the glass or the glaze. It is important to define abiotic factors such as HR, time, temperature, light, the incubation system etc. Microbial glass deterioration causes serious damage to glass, not only from the chemical and physical point of view, but also from an aesthetic and iconographic perspective, since the glass loses its transparency. The research into the microorganism-glaze-ceramic interface is a crucial step for understanding whether microorganisms are indeed capable of causing the detachment of the glaze. Accordingly, biodeterioration should be carefully addressed whenever the conservation of historic glass and glazes tiles is of importance. Conservators must act to achieve the inactivation and removal of the microorganisms in order to halt their progression. The selection of cleaning products and biocides is a complex subject with few optimal solutions, due to the susceptibility of glass to corrosion and the complex biodiversity [28; 71]. Additionally, the mechanical removal of biofilms, whether with a simple manual scalpel or through complex laser cleaning, remains in question since microorganisms usually grow close to the glass corrosion layer. In the case of cultural heritage, they are considered part of the original material and also a corrosion-passivating layer; therefore, their removal needs to be carefully evaluated. Protective layers have been investigated, such as antimicrobial [72; 73] or anticorrosion [74; 75] coatings. However, few or no real case applications have been carried out due to the ethical questions regarding their reversibility, ageing, and interference with aesthetic properties. Further research to understand glass biodeterioration is essential to be able to develop proper conservation guidelines for stained glass and glazed tiles. The complexity of biodeterioration studies is due to the many synergetic and antagonistic factors that make drawing conclusions based solely on the investigation of colonised samples very difficult.