Stone Deterioration of the Western Wall: Chemical and Mineralogical Characterization of Salts

Buildings and monuments made of carbonate rocks exhibit different rates of erosion. Chemo-mechanical processes are suggested as the main processes, yet quantifying them over long term periods is challenging. To constrain the variety of parameters controlling long-term limestone weathering, we studied the Western Wall, a Herodian-period edifice located in Jerusalem, Israel. The wall represents the outer boundary of the Herodian Temple precinct. Construction of the Herodian Temple precinct is thought to have been completed during the first century CE, and the wall is built entirely of locally quarried limestone. Deterioration of the limestone blocks is mainly associated with human activity and natural disasters, and include the formations of cracks and collapse from the wall. Applying non-invasive, semi-quantitative methods such as portable X-ray fluorescence (XRF) spectrometers, provide chemical information which allows to characterize and map the different materials that lead to the weathering of the limestone blocks. In this work, we find high concentrations of sulfur and chlorine at specific locations along the wall. These elements suggest the presence of salts (e.g., gypsum and soluble chlorides) as one of the promoters of the limestone weathering. Focusing on an ashlar that recently collapsed from the wall to the visitors’ area, our finding indicates the presence of gypsum in the remaining broken half, based on Infrared spectroscopy and XRF analysis. In addition, a survey of other Herodian stones, and later periods joint mortars and plasters that are found adhered to the wall, revealed that gypsum was present only in specific areas on the Wall. This elemental mapping suggests that the salts may originate from several sources (e.g., soil pollution, leakages, and sewage) and is not evenly distributed. The obtained results indicate on the possible use of non-invasive, semi-quantitative methods for detecting potential areas that are more susceptible to weathering in built heritage.