## 3 Results
### Laboratory Assays with Glass-Based Historical Building Materials
Fungi identified from stained glass are the most commonly used microorganisms in laboratory studies [18; 36; 47], although bacteria have also been tested [51]. This may relate to the fact that fungi were the most identified organisms on stained glass with biological colonization (Table S1). No phototrophic microorganisms were tested, since these have seldomly been identified on stained glass windows. Regarding glazed tiles there are only two studies: one using a mixture of phototrophic microorganisms [16] and another a fungal specie isolated from historical glazed tiles [17]. The inoculation of pure culture or multi-species has advantages and disadvantages. The use of pure cultures allows understanding 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. The majority of the studies were performed under high relative humidity conditions, which simulate better the conditions in which biofilms develop over historical glasses [16; 17; 18; 51]. The environmental conditions are usually kept stable during the experiment. Therefore, no daily temperature or relative humidity variations were tested. Although stable conditions favour the microbial growth, the simulation of temperature cycles could be relevant for simulating physical decay, since the variation of these parameters can induce volume variations of the microorganisms. The time span of the experiments varied from a few weeks to one year so, the comparison between the results of different experiments is difficult. After the incubation, morphological and compositional changes are investigated through several analytical techniques. On stained glass the most common biodeterioration forms reported in the laboratory studies were: micro-cracking, enrichment of some elements, pitting, hyphae fingerprint, depletion or enrichment of elements and deposition of crystalline compounds (Table 3). Similar to the deterioration patterns reported in the analysis of historical samples (Table 2). On glazed tiles, the laboratory-based biodeterioration showed that phototrophic microorganisms were able to grow chasmoendolithically on fissures of the glazes and imprints were visible on the surface [16]. In contrast, fungi could not cause physical damage on the substrate, but seem to cause deposition of crystals on the surface [17]. In most studies, crystalline compounds were formed on the surface of the glass, implying surface corrosion. The identified compounds were MnCO\({}_{3}\), CaCO\({}_{3}\), CaCO\({}_{4}\), NaSO\({}_{4}\cdot\)H\({}_{2}\)O, NaAlSi\({}_{3}\)O\({}_{8}\), CaSiO\({}_{3}\), SiO\({}_{2}\), K\({}_{2}\)CO\({}_{3}\), and CaC\({}_{2}\)O\({}_{4}\cdot\)H\({}_{2}\)O. In some studies, the mineralogical composition of the crystals was not determined; only the chemical composition was considered (Table 3). All these compounds were reported in field studies (Table 3). Ca-rich compounds were the most commonly identified, consistent with the fact that crystalline compounds are often detected on real samples with biological colonisation (Table 3). Only Ca-oxalates could be directly associated with biogenic action [60] due to the reaction of oxalic acid with calcium. Two laboratory-based experiments using fungi detected this compound on stained glass [22] and glazed tiles [17]. Although most of these compounds cannot be directly attributed to microbial activity, some authors have claimed that the precipitation was higher on the colonised samples than the controls. The precipitation of salts over glass is considered harmful, since their presence influences the corrosion rate [57], but these studies seemed to demonstrate the ability of microorganisms to also increase the deterioration rate [17; 18]. Regarding colorants, Mn-rich compounds were identified in two studies on the biodegradation of glass by bacteria [51; 61] (Table 2). [PERSON] et al.