Salt weathering in the coastal environment : the deterioration of wall paintings at Delos, Greece
Salt weathering, apart from being an important geomorphologic agent, comprises a major hazard for both modern and heritage structures. Although its action is witnessed globally, it is particularly aggressive in coastal environments. The coastline attracted in antiquity a considerable part of human activity that has left valuable built traces. Conservation research is frequently called upon to define sustainability in this aggressive context. Wall paintings comprise an integral part of the built heritage. The particular importance of wall paintings and finishing layers derives from their unique aesthetic function in the building's integrity as well as the plethora of information that they carry. Whereas wall paintings are more susceptible than masonry materials, the tolerance against loss is much smaller due to their descriptive nature and scale. It is not until recently that international heritage organisations recognised the technical particularities of wall paintings suggesting that they must be investigated independently and treated in situ. This project aims to identify the particularities of wall paintings' susceptibility to salt weathering in the coastal environment. The methodology is composed of both in situ and ex situ experiments. The in situ investigation follows a comparative approach, guided by specific variables, in a number of monuments at the archaeological site of Delos island. The goal of the in situ investigation is to determine the optimal conditions for preservation, by modeling the salts interactions, in an effort to define sustainability against salt weathering in this aggressive environment. The ex situ approach comprises laboratory simulation of the weathering mechanism and aims to describe the particularities of the substrate that lead to the distinct loss of the external finishing layer, which carries the principal information. The results of the project underline the importance of kinetic deviations deriving from the solution and the substrate properties. Despite the limitations of determining the optimal conditions for preservation, the variables that directed the comparative approach permitted the generation of a periodic model in agreement with the phenomenological observations. The model suggests that the potential of salt damage in real conditions of various contamination pathways and sources cannot be restricted to a single resultant. Although the model follows a certain periodicity in response to the annual climatic cycle, random events and fractionated accumulation lead to the production of mixtures with variable composition. Additionally the results stress the role of solar radiation and air movement as evaporation accelerators. Consequently environmental control against salt weathering should be directed towards multiple components which in the case of coastal environments, mainly due to the presence of marine aerosols, cannot be achieved simply by hygrothermal management. On the other hand we traced specific deviations from the theoretical model of salts interactions, concerning mixtures commonly found in coastal regions that should also be taken into account. Besides we tested the hypothesis of salts accumulation at the interface of rendering layers, caused by hydraulic discontinuity, with a weathering simulation. The aim of this investigation was to provide evidence descriptive of damage. It has been shown that the external layer of wall paintings is particularly susceptible to marine aerosols and it can be damaged independently and in advance of the bulk mortar. The salts crystallise selectively under the lime wash layer causing gradually its detachment from the mortar. The results of the weathering simulation raise serious implications for remedial and preventive conservation practice and suggest that research must focus as well on the kinetics of particular cases.