An investigation into the deterioration of a decorated Baldacchino tomb at St. Paul's Catacombs Complex, Rabat, Malta

Abstract

The present dissertation focuses on the study of a decorated Baldacchino tomb within Catacomb 17 in the St. Paul's Catacombs Complex located within Rabat, Malta. The complex is considered as an early Christian site (ea. 4th-6th century AD). The majority of the painted decoration on the Baldacchino tomb is obscured by a variety of surface phenomena of different colours and opacities. The dissertation includes documentation of the original technique and the condition of the Baldacchino, including the surface phenomena, and their implications on the conservation of the original surfaces. Microscopic observations revealed that the wall painting is composed of a lime-based plaster (applied on Globigerina Limestone) and typical pigments found in Roman wall paintings were used, including red ochre and possibly Egyptian blue. The internal environment has a high RH (100%), a stable temperature (13- 21°C) and low light levels (0.05-3.82 µmol photos m-2 s-1), which concur with other hypogean environments and provide ideal conditions for biological growth. Most of the surface phenomena covering the tomb seem to be of biological origin. The black layers covering the top half of the tomb might be of cyanobacterial origin, filamentous Cyanobacteria were observed within the green films, and purple spots seem to be composed of heterotrophic bacteria, whereas the white crusts mostly contain crystalline material along with coccoid cyanobacteria. Bioprecipitation of various forms of calcium carbonate was observed, possibly biomediated by filamentous Cyanobacteria. Penetrative behaviour in the underlying substrate was noticed in several films; this can lead to disaggregation of original materials. The LED lights close to the tomb lead to an increase in T on its surface, thus encouraging colonization by the green films. In light of the findings of this study, since a number of photosynthetic organisms were observed in various samples, it is recommended that monochromatic lighting emitting wavelengths not used for photosynthesis, is installed. Remedial interventions are not recommended at this stage, until further studies are carried out on the biological films.