An investigation into the biodeterioration of the wall paintings at St. Agatha's Crypt and Catacombs, Rabat

Abstract

St. Agatha' s Crypt and Catacombs in Rabat constitute the richest subterranean site embellished with wall paintings in Malta. The wall paintings present belong to three different periods and styles, being early Christian, Siculo-Byzantine and Medieval. These wall paintings were chosen to investigate the distribution of any surface alterations caused by the growth of micro-organisms, and how these alterations interact with the substrate. The environmental conditions which might help for the development of biological alterations were also investigated. Surface alterations appear as green and white films of different thicknesses and consistencies. Through electron microscopy it was confirmed that these alterations are biological in nature. Predominant biofilms can be divided into two types: photoautotrophic bacteria (cyanobacteria) and chemoorganotrophic bacteria (heterotrophic bacteria and actinomycetes). Associated with the white chemoorganotrophic organisms are also salt efflorescences and subflorescences. Microscopic observations revealed that both biofilms have a close interaction with the wall paintings' paint and plaster layers. Organisms grow inside pre-existing cracks and around particles of the substratum. EDS analysis confirmed that micro-organisms mobilise ions from the paint and plaster layers. Thus, evidence for both mechanical and chemical deterioration of the wall paintings is given. The biofilms also cause aesthetic damage due to their chromatic alteration covering the original surface of the paintings. The internal environment present i.e. a high and a rather stable relative humidity (96%-100%), frequent condensation phenomena, stable temperature (15°C-23°C) and the light available (less than 20 lux) provide the ideal conditions for the occurrence of biological growth. A strategy for the better preservation of the wall paintings is given at the end of the study.