A study on the desalination of paintings on Globigerina Limestone : the wall paintings at the former Jesuits' College, Valletta

Abstract

Maltese wall paintings from the Baroque period onwards were mostly executed a secco applied on local porous limestone (Globigerina Limestone). They are often found in a poor state of conservation, usually caused by the action of soluble salts. Although remedial treatments often need to be undertaken to prevent immediate loss, the long-term preservation of these paintings can be achieved only if substantial efforts are made to reduce and/or stabilize the salt decay. Some of the wall paintings at the former Jesuits' College in Valletta were chosen as a case study to examine the problems posed by the desalination of these paintings and to test several desalination methods. The desalination methods were selected and tested after documenting the deterioration phenomena observed on the paintings, and following the analysis of the constituent materials, the efflorescences and the stone powder likely to contain salts. Internal and external ambient temperature and relative humidity were also monitored over several months to understand the role played by the environment. Scientific analyses identified gypsum as the main component of the widespread efflorescences, and thenardite on limited areas of the paintings. After a series of trials carried out in the laboratory, several desalination methods were tested on site. The latter were mainly assessed for their working properties, effects on the paint layer and efficacy in removing the salts. The evaluation was achieved by visual means and scanning electron microscopy. Results revealed that anion-exchange resin Akeogel® (Syremont) applied using a brush and as a poultice achieved the most effective and safe desalination.