Biodeterioration of Glass-Based Historical Building Materials: An Overview of the Heritage Literature from the 21 st Century  
[PERSON]  
1 Departamento de Conservacao e Restauro, Faculdade de Ciencias e Tecnologia, Universidade NOVA de Lisboa, Campus Caparica, 2829-516 Caparica, Portugal, [EMAIL_ADDRESS]
2 VICARTE, Research Unit Vidro e Cernica para As Artes, Faculdade de Ciencias e Tecnologia, Universidade NOVA de Lisboa, Campus Caparica, 2829-516 Caparica, Portugal
3 Laboratorio HERCULES-IFAA, Universidade de Evora, 7000-809 Evora, Portugal; [EMAIL_ADDRESS]  
[PERSON]  
1 Departamento de Conservacao e Restauro, Faculdade de Ciencias e Tecnologia, Universidade NOVA de Lisboa, Campus Caparica, 2829-516 Caparica, Portugal, [EMAIL_ADDRESS]
2 VICARTE, Research Unit Vidro e Cernica para As Artes, Faculdade de Ciencias e Tecnologia, Universidade NOVA de Lisboa, Campus Caparica, 2829-516 Caparica, Portugal  
Stained glass, mosaics, and glazed tiles are an integral part of the structure of a historic building, and were designed to be a part of the architectural ensemble. The removal of these architectural elements from the original location signifies a considerable value loss; consequently, their preservation on site is of the utmost importance [5]. However, their preservation on-site raises concerns regarding their conservation, since glasses are particularly sensitive to environmental degradation, and are also very vulnerable to biodegradation [8; 9; 10]. Microbial colonisation is a major problem of these building materials due to the damaging consequences of biodeterioration [10; 11]. For decades, research has focused on the characterisation of biological communities based on the taxonomic identification of colonising organisms [12]. The interactions between the substrate and organisms to unveil deterioration processes and, more recently, the influence of environmental conditions on biological colonisation and biodeterioration are now being analysed due to advances in genomic and analytical techniques. In general, studies focus on the relationship between three main factors--organisms, the material, and the environment [13]. Therefore, the research on biodeterioration has been based on the analysis of the colonised substrate of artworks [14; 15], on laboratory experiments [16; 17], or combining laboratory experiments with case studies [18]. This work aimed to review the 21 st century literature regarding glass-based historical building materials, particularly stained glass and glazed tiles. Although glass has been used in mosaics tesserae, and some studies have mentioned biodeterioration, no consistent literature was found [2; 19]. Therefore, this type of application was not considered in the present work due to the lack of data. The 21 st century literature was analysed and discussed, focusing on the biodiversity and the biodeterioration mechanisms, and comparing case studies and laboratory experiments.  
Another goal of this work was to perform a critical review based on the 21 st century literature regarding stained glass and glazed tiles, that can be used to assess cultural heritage biodiversity and biodeterioration. This knowledge is crucial to understand biodeterioration mechanisms, and is also very important for planning laboratorial experiments. The analysis of colonised samples is a vital mechanism to understand biodeterioration, particularly for identifying the colonising organisms and deterioration patterns on real samples. In this work, the objective was to analyse the materials and complexity of colonised samples in order to determine which microorganism was responsible for the biodeteriogenic action. The authors compared the deterioration patterns described in case studies with laboratory-based colonisation experiments, showing that many deterioration patterns and corrosion products are similar. A working group should develop guidelines or standards for laboratory-based experiments on fungi, bacteria, cyanobacteria, and algae on stained glass and glazed tiles.