Physical principles and efficiency of salt extraction by poulticing

The crystallization of soluble salts plays a significant role in the deterioration of porous cultural property. A common response to salt damage problems is to undertake treatments aimed at reducing the salt content of the affected object, most typically through the application of poultices. The process of poulticing is in theory relatively simple: the wet poultice material is applied to the surface of the object to be treated, and is kept in place for some period of time before being removed. However, in practice, the efficiency of the salt extraction, or even the location of salt accumulation post treatment is more difficult to predict. This paper examines the physical principles of salt ion and moisture transport by which poultices function, and shows how depending on the application methodology, these treatments can be divided into diffusion and advection-based methods. The maximum salt extraction efficiency, the depth to which this can be achieved, and the time scale required is estimated for each type of poulticing system, to gain a better understanding of their working properties and performance. Finally, the pros, cons and limitations of desalination treatments are discussed.