## 3 Results
### Laboratory Assays with Glass-Based Historical Building Materials
In the field of conservation studies for understanding susceptibility to biodeterioration depending on the degree of ageing, this is very relevant for preserving cultural heritage. In general, the selection of microorganisms to be used in the biodeterioration laboratory experiments seems to have been based on the biodiversity found on historical materials. Studies have often used microorganisms directly isolated from colonised materials. Fungi identified from stained glass were 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 colonisation (Table S1). No phototrophic microorganisms were tested, since these have seldom been identified on stained glass windows. As previously mentioned, only two studies have focused on glazed tiles. One used a mixture of phototrophic microorganisms [16], and the other a fungal species isolated directly from historical glazed tiles [17]. Pure culture or multi-species inoculation have advantages and disadvantages. The use of pure cultures allows understanding of whether a specific microorganism is a biodeteriogen. However, multi-species colonies better mimic the real conditions under which biodeterioration take place, since colonisation usually occurs in a community of organisms. The majority of the studies were performed under high relative humidity conditions, which are optimal for microbial growth and better simulate the conditions under which biofilms develop over historical glasses [16; 17; 18; 51]. The environmental conditions were usually kept stable during the experiment. Therefore, no daily temperature or relative humidity variations were tested. Stable conditions favour microbial growth, however the simulation of temperature cycles could be relevant for reproducing physical decay. The variation of these parameters could induce volume oscillations of the microorganisms that can induce physical tensions on the substrate. The time span of the experiments varied from a few weeks to one year, making comparisons of the results of different experiments difficult. After incubation, morphological and compositional changes were investigated through several analytical techniques. On stained glass, the most common biodeterioration forms reported in the laboratory studies were as follows: micro-cracking, enrichment of some elements, pitting, hyphae fingerprints, depletion or enrichment of elements, and deposition of crystalline compounds (Table 3). These were analogous to the deterioration patterns reported in the analysis of colonised 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 did not cause physical or chemical damage to the substrate [17]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Material** & **Organisms** & **Time** & **Type of Silicate Glass** & **Alt. Products** & **Biodeterioration** & **Ref.** \\ \hline Glazed tiles & Phototrophs & 12 m. & Pb-rich & \begin{tabular}{c} Pristine \\ Corroded \\ \end{tabular} & \begin{tabular}{c} CaCO\({}_{3}\) \\ CaCO\({}_{3}\) \\ \end{tabular} & \begin{tabular}{c} Penetration into fissures, \\ in-prints and surface deposits \\ \end{tabular} & [16] \\ \cline{2-8}  & Fungus & 12 m. & Pb-rich & \
\begin{tabular}{c} Pristine \\ \end{tabular} &
\begin{tabular}{c} CaCO\({}_{3}\) \\ CaO\({}_{4}\) \\ CaSO\({}_{4}\) \\ Ca-Oxalate \\ CaCO\({}_{3}\) \\ CaSO\({}_{4}\) \\ Ca-Oxalate \\ Ca-Oxalate \\ \end{tabular} & [17] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._In general, the selection of microorganism to be used in this biodeterioration laboratory assays seems to be based on the biodiversity found on historical materials. Studies often use microorganisms directly isolated from colonized materials.