The degradation of wall paintings and stone: Specific ion effects

Salts are ubiquitous both on the surface and in the porous network of works of art such as wall paintings and stone. Cyclic solubilization and crystallization takes place with fluctuating environmental conditions, inducing mechanical stress in the pores and the flaking of the artistic surface. The preventive conservation of precious cultural heritage would thus benefit from models able to describe quantitatively the behavior of electrolyte solutions. Besides the pore size distribution of the wall, cyclic crystallization depends on relative humidity and temperature. Whereas the behavior of single salts' solutions is known, that of mixed solutions (commonly found on artifacts) is still an open issue, owing to the specific interactions of counterions and coions. Classical theories of electrolytes need many fitting parameters to provide predictive and quantitative information, and research focuses on matching phenomenological set of rules with models that take into account quantum mechanical dispersion forces. Classical models have been used so far to describe the behavior of some mixed salts' solutions commonly found on murals and stone, in terms of their RHeq, which is the relative humidity of air in equilibrium with the saturated solution. Results indicate that environmental conditions deemed safe in the presence of single salts, represent indeed a threat to artifacts in the presence of mixed solutions, with other deviations due to the fact that the crystallization of salts takes place within mesoporous networks. We hope that the reviewed results might contribute a stimulus for further reanalysis of the degradation of works of art, where the synergistic effect of counterions and coions are taken into account. Such interpretation of the artifacts' degradation has been so far overlooked in preservation studies.