The multi-disciplinary approach to determine and monitor the state of conservation of the wall painting of Our Lady of Mellieha (Malta) before, during and after the restoration works

This contribution aims to describe the approach followed during all the steps of the diagnostic project of the wall painting of Our Lady of Mellieha (Malta). The diagnostic activities started before the restoration interventions in order to understand the state of conservation of the wall painting, define the nature of the original materials, the presence of previous interventions and the processes of degradation. The activities were extended during the restoration intervention to acquire an immediate understanding of the decay phenomena and after the restoration to monitor the quality of the intervention, its effectiveness and durability The Siculo-Byzantinesque wall painting of Our Lady of Mellieha depicts the Virgin Mary and Child, it is found in a semi-hypogea environment on the rock-cut apse aedicule of the Sanctuary of Mellieha, in the north side of Malta. It is painted directly on the local carbonate rock with iron based pigments and organic black. Along the years, the environment underwent several changes and interventions (partly documented), owing to its use as a sanctuary for worship. Before the restoration intervention the painting was heavily altered by the presence of an acrylic product, applied during a previous restoration. The acrylic product was of consistent thickness, numerous over paintings were also present. However, the most important conservation problem was related to the substantial presence of salts acting on the cohesion of the plaster layer causing pulverization. The environmental conditions (temperature, relative humidity, moisture content and the presence of salts) and materials have been investigated through the use of non-invasive and micro-invasive techniques in order to determine the factors causing the deterioration and to suggest methods of prevention. The approach presented in this study was developed on a preliminary survey based on non-destructive investigations using infrared thermography, evanescent field microwave dielectrometry and imaging techniques (UV) together the thermo-hygrometric monitoring of the environment. The results of this preliminary survey highlighted areas characterized by a high index of decay (humidity, presence of salts and of organic products related to previous restorations). Results from the scientific investigations have led through targeted micro-samples in areas of interest, for further laboratory investigations (FT-IR, XRD, microscopy, SEM-EDS). The type of salts identified, (chlorides and nitrates) and the environmental conditions clarified the critical factors for the preservation of the painting and the development of a conservative intervention. The most significant results of the project will be discusses during the presentation.