Biodeterioration of mural paintings in a rocky habitat: The Crypt of the Original Sin (Matera, Italy)

The biological colonization of mural paintings in rocky habitats is a widely recurring phenomenon and should be considered for restoration activities. In order to plan conservation treatment of the Crypt of the Original Sin (Matera, Italy), biodeterioration problems were analysed taking into account their impact on the substrate and their relationship with environmental factors. The mural paintings showed a phenomenology of alteration varying from brilliant green, dark green, brown, and black duff patinas with powdery aspect, to rosy discoloration. These phenomena were linked to different microbial colonizations by Cyanobacteria (Chlorogloea microcystoides, Chroococcus lithophilus, Gloeocapsa spp., Gloeothece rupestris, Pseudocapsa dubia) and green algae (Apatococcus lobatus, Chlorella vulgaris, Chlorococcum sp., Muriella terrestris). The rosy discoloration was connected to pigment production by Actinobacteria related to Rubrobacter radiotolerans. Grey-green patinas, limited to more ventilated areas, were caused by lichen growth (Dirina massiliensis f. sorediata). Vascular plants (Adiantum capillus-veneris) were also detected in wall areas affected by rising damp. To control the growth of microflora, biocide treatment and intervention to reduce humidity and sunlight exposure of mural paintings were suggested. To choose the correct biocide, different products were tested, evaluating their efficiency on Cyanobacteria and algae as well as possible effects on the calcarenite stone.