The degradation of wall paintings and stone: Specific ion effects

Highlights

• Degradation might be associated to the synergistic effect of counterions and coions.

• Knowledge of RH_eq allows minimizing the crystallization of salts in the pores.

• The behavior of some mixed salts solutions is discussed.

Abstract

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 RH_eq, 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.

Introduction

Cultural heritage is crucial to improvement of society, as it conveys ideals and values that contribute both education and entertainment. Besides, the valorization of cultural heritage leads to economic benefits, impacting on tourism and job creation [1], [2]. Yet, works of art are subjected to degradation through a multitude of physical and chemical processes owing to the action of light, temperature, water or aqueous solutions, microorganisms, and even wrong conservation interventions [3], [4].

In this framework, science (in particular, colloid, soft matter and materials science) has a central role both in describing degradation processes and providing solutions to counteract the deterioration of artistic and historical objects [5], [6], [7••]. A great deal of effort has been put into understanding the role of salts, which are unsurprisingly ubiquitous in the degradation of works of art [8], [9•]. During the natural aging and weathering of plasters, the cyclic dissolution and re-crystallization of salts within the plaster's porous matrix result in flaking and detachment of the surface layers, eventually leading to the loss of precious artifacts (see Fig. 1). Both preventing salt formation and removing detrimental salts from the surface and the bulk of the works of art are central tasks that are hampered by the fact that several fundamental aspects in the physico-chemical behavior of the saline species found on works of art are still poorly understood. This reflects a knowledge gap of the specific effects of electrolytes in chemistry and biology [10^••]. In fact, interactions between ions in aqueous solutions are usually described only through classical theories of electrolytes that work quantitatively and predictively only when many fitting parameters are used [11]. Recently, theoretical research has focused on matching phenomenological set of rules (that explain qualitatively ion–ion and ion-surface site interactions) with models that take into account quantum mechanical dispersion forces, in order to provide a complete description of the specific ion effects on the behavior of aqueous solutions of salts [10^••].

The main purpose of this contribution is to shed some light on the detrimental crystallization processes that continuously occur within the porous network of plasters and stone. These phenomena depend on temperature and relative humidity fluctuations, which are commonly experienced by artifacts exposed both outdoors and (less frequently) indoors. Besides, added complexity is due to the fact that the crystallization of salts takes place within porous networks, and the pore size distribution has an effect on the crystallization pressure [12], [13].

The thermodynamics of binary systems (single-salt aqueous solutions) represents the general background for defining the best conservation conditions. However, this approach is not predictive of a variety of processes that usually occur in the real systems. In fact, stone and walls usually contain solutions of several salts, even at high concentration, and this originates some unexpected processes [14^••]. Research on this topic is far from being concluded.

Considering the importance of cultural heritage conservation in our society, we have been solicited by the editors of this special issue to focus, in this contribution, on processes related to the degradation of works of art that might be associated to the synergistic effect of counterions and coions, although not strictly related to the Hofmeister effects. Such interpretation of the artifacts' degradation has been overlooked in conservation science; therefore we hope that this simple review might contribute a stimulus for further investigation and for a reanalysis of the degradation of works of art.

Namely, we will try to highlight trends in the behavior of the electrolytes in terms of differential (specific ion) effects on the equilibrium relative humidity (RH_eq), i.e. the relative humidity of air in equilibrium with a saturated solution of the salt. The knowledge of RH_eq for different salts is critical in preventive conservation, as it allows minimizing the cyclic crystallization of salts in the artifacts' pores. The RH_eq values for single salts are well known, however the behavior of mixed salts solutions is much more complex.