Unilateral NMR: A Noninvasive Tool for Monitoring In Situ the Effectiveness of Intervention to Reduce the Capillary Raise of Water in an Ancient Deteriorated Wall Painting
Portable unilateral NMR was used to quantitatively map in a fully noninvasive way the moisture distribution in an ancient deteriorated wall painting before and after an intervention to reduce the capillary raise of water through the wall. Maps obtained at a depth of 0.5？cm clearly showed the path of the capillary raise and indicated that, after the intervention, the moisture level was reduced. Maps obtained by measuring the first layers of the wall painting were affected by the critical environmental conditions of the second hypogeous level of St. Clement Basilica, Rome, and by the presence of salts efflorescence and encrustations on the surface of the wall painting. The morphology and the elemental composition of salts investigated by SEM-EDS indicated that efflorescences and encrustations were mostly constituted of gypsum and calcite. The presence of these salts is explained with the presence of high concentration of carbon dioxide and sulphur-rich particles due to pollution which, along with the high-moisture level and the extremely feeble air circulation, cause recarbonation and sulphation processes on the plaster surface. 1. Introduction Water is one of the major causes of decay to masonry materials, and particularly to building masonry belonging to cultural heritage sites [1–3]. The decay of porous stones, the detachment of wall paintings, the salts efflorescence are all phenomena due to the action of water. The moisture rises in the wall to the limit of capillary action or until it evaporates from the wall surface. When salts concentration activity is present in the soil or in the mortar, salts are drawn in solution into the porous network of the wall. Consequently, when salt concentration activities product is greater than the equilibrium constant and/or when the ambient relative humidity becomes lower than the equilibrium relative humidity of the saturated solution of the particular salt phase, salts crystallize causing fretting and crumbling of the wall. The knowledge of the water path and distribution through the wall is mandatory for determining the mechanism by which water triggers and accelerates damage [3]. An accurate diagnosis of the causes and extent of the moisture is a fundamental step in the conservation work. This is particularly true for wall paintings, where a number of special factors must be taken into account, such as the vulnerability of the extremely thin painted surface which is itself the interface between the plaster and the surrounding environment, the difficulty in controlling potential agents of deterioration such as
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