Salt extraction by poulticing : an NMR study

The crystallization of salts is widely recognized as one of the most significant causes of damage to many cultural objects consisting of porous materials, such as monuments, sculptures, historic buildings, wall paintings, etc. A common response to salt damage problems are treatments aimed at reducing the salt content of the affected object, most typically through the application of poultices. Poultices are applied to porous materials in order to extract soluble salts. The process of poulticing is relatively simple in theory, but in practice the efficiency of the salt extraction is more difficult to predict. This study aims to develop a better understanding of the physical principles of the salt and moisture transport by which poultices function. A desalination treatment by poultice includes two main phases. The first is the wetting phase: water is transported from the poultice into the wall, where it starts to dissolve the salts. The second phase is the salt extraction. The dissolved salt ions travel in the form of an aqueous saline solution from the substrate into the poultice. This salt migration can be the result of two different processes. The first is generated by the existence of a concentration gradient between the substrate and the poultice. In this case the salt ions diffuse through the solution. The other one is realized by the capillary water flow from the substrate to the poultice (generally resulting from drying). In this case the salt ions are transported by the moving solution (advection). If salt ions are advected from the substrate into the poultice by capillary moisture flow, a concentration gradient will be established. Because of this salt concentration an osmotic pressure will develop. One of the aims of this study was to investigate the potential contribution of osmotic pressure to salt extraction during drying of the poultice. For this purpose we have conducted a series of experiments to investigate the influence of osmotic pressure on ion transport processes. Nuclear Magnetic Resonance (NMR) techniques were used to obtain information on the water and salt distribution in the poultice/substrate system during desalination. The results of the experiments show that the contribution of the osmotic pressure can have a significant influence on the desalination process. Poultices which contain different mixes of clay and sand were studied in order to understand the influence of each component on the drying behavior of the poultice. Desalination experiments in controlled environmental conditions were carried out on substrates with well known pore size distributions. NMR was used to obtain information on the water and salt distribution in the poultice/ substrate system during desalination. The study demonstrates the relation between salt extraction and pore structure parameters of the poultice/substrate system. It also shows the influence of some additional factors, such as an interventional layer between substrate and poultice, on the salt extraction during the desalination treatment.