Cleaning oil-based wall paintings : the 18th century wall paintings of the Chapel, Palazzo de La Salle, Valletta, Malta

Abstract

This MSc dissertation deals with designing a site-specific cleaning treatment for different surface accretions obscuring the 18th century oil-based wall painting located in a private palace chapel, within Palazzo de La Salle, in Valletta, Malta. The surface accretions vary from inorganic (i.e. lime-based covering layers residues, dust accumulation, salts, pigment degradation products) to organic material (i.e. coatings). After information regarding the original technique, physical history, condition and accretion analysis was evaluated, this dissertation adopted an iterative approach. It initiated with extensive literature research focusing on oil-paintings and their cleaning. The dissertation then sought to understand better the accretions, their extent and solubility. The most widespread and aesthetically disturbing accretions showed sensitivity towards water, making water-based method promising. Consequently, the wall paintings themselves, were tested for surface water sensitivity and mechanical sensitivity. The sensitivity towards water and mechanical action increased with higher water delivery and prolonged mechanical action. Through the results achieved, the information was then assembled to set up cleaning parameters such as the intervention and performance criteria along with suitable working properties, upon which cleaning materials were chosen and evaluated. This compilation of information led to a series of testing, observation and evaluation. To minimize the water delivery and mechanical action for a safe cleaning method, hydrogels were tested (i.e. Agar and Nanorestore®). The results were ineffective unless mechanical action on the surface (e.g. swabbing) for clearance was applied. After evaluation, however, it was realized that the removal of gels from the surface and swabbing caused small original fragment losses and the cleaning results were still unsatisfactory. Hence, the study developed in another class of auxiliary materials: sponges. Three different sponges known to be sensitive towards paint layers, leaving minimal residues and good efficiency were tested (i.e. Sofft® sponge, Smoke sponge® and Elite models® make-up sponge). The sponges were tested both moist or dry, as clearance materials after delivering water to the painted surface with Agar. Areas containing water-sensitive pigments or salts were tested only with a dry sponge. For the final testing, nine different areas exhibiting different accretions were chosen. Cleaning trials were evaluated mainly by non-invasive techniques: visual observation, multispectral imaging and by a hand-held microscope. The evaluation also took place indirectly by studying the cleaning materials after use under a stereomicroscope. From all the materials tested, the moist Sofft® sponge or dry in the case of water-sensitive paint layers, resulted as the most promising cleaning material. Further analysis to assess any morphological changes were carried out with SEM. Future proposed work includes: further and larger cleaning trials using the Sofft® sponge, identification of removed material from the surface of the wall paintings and, identification and quantification of any extractable components from the Sofft® sponges when used moist.