A study on the deterioration and conservation of wall paintings on globigerina limestone : the mural paintings by G. Cali at the Church of the Immaculate Conception in Cospicua, Malta

Abstract

Maltese wall paintings from the Baroque period onwards were mostly executed in oil or oil-based media applied to local limestone (Globigerina Limestone), a soft, very porous stone. Although these paintings are often found in a poor or very poor state of conservation, no major attempt had to date been carried out to study the deterioration affecting such paintings. The primary goals of this dissertation were to investigate the most common causes, effects and processes of deterioration related to such wall paintings, and to suggest general measures for their preservation. A case study was chosen to attain these aims. This was the secco painting on the dome of the chapel of the Crucifix in the parish church of Cospicua. Painted in 1903 by the local artist Giuseppe Cali and heavily restored in 1963, it is now in appalling conditions, with deterioration affecting both the stone support and the overlying painting strata. An interdisciplinary diagnostic approach, including extensive historical research, site examination and scientific investigations, was chosen. This permitted the gathering of information on the physical history of the site, its current state of conservation, causes and processes of deterioration. Scientific investigations focused on the analysis of the constituent materials, including Cali's and those belonging to other painting or restoration phases, and deterioration products. The role played by the environment in the current deterioration processes was studied through environmental monitoring. All evidence pointed to a long history of deterioration and repair of the dome. Soluble salt decay was identified as the main deterioration factor, being primarily caused by large amounts of sulphates, up to 41. 7%, and smaller quantities of chlorides, up to 3 .2%. The sources of salts were also investigated. Salts were found to have been transported and activated by prolonged and recurrent rainwater infiltrations, which have affected the chapel of the Crucifix for at least the last 40 years. Solar radiation, on the other hand, probably exacerbated the decay by increasing the number of salt crystallization cycles. The layered structure of the painting, consisting of a complex stratification composed of 17 layers belonging to 5 different painting phases, was held to be responsible for creating an interface where salts crystallize. Further water damage may have also been caused by the presence of restoration materials sensitive to moisture. An experimental laboratory test indicated that the least durable type of Lower Globigerina Limestone (soil) was probably used to build the chapel of the Crucifix. The definition of the main causes and processes of deterioration led to the elaboration of general guidelines for the conservation of the site. The prevention of water infiltration, to be achieved through monitoring the performance of the recent waterproofing membrane covering the exterior of the dome, was recognized as the most urgent and essential measure to be taken to ensure the long-term preservation of these paintings. Besides emergency treatments (such as facing) to prevent further loss of material, other remedial treatments need to be designed taking into consideration the substantial quantities of soluble salts, and must be performed only after the execution of a suitable laboratory and in situ testing programme.