Optimizing the Extraction of Soluble Salts from Porous Materials by Poultices

The removal of salts from large non-movable objects such as architectural surfaces (e.g. wall paintings and stone masonry) presents a particular challenge to the conservator, since these objects require treatment in situ. One of the most common approaches to this problem is to use a poultice (Heritage et al, 2008), In general terms the application methodology for poulticing is relatively simple: the wet pouitice material is applied to the object surface and is kept in place for some period of time before being removed. Nevertheless, the actual physical principles by which poultices function are somewhat more complicated. It has been shown that, depending on the application methodologi poulticing treatments can be divided into methods based on diffusion (i.e. the transport of a component through a medium in response to a concentration gradient) and advection (i.e. the transport of a component within a flowing medium) (Pel et al. 2010). This is of key importance since, depending on which transport process is active, different conditions must be fulfilled for extraction to take place. The poulticing treatment itself can be summarized as having two main steps. The first is the wetting phase: water is transported from the poultice into the object where it starts to dissolve the salts. The second phase is that of extraction, whereby the dissolved salts ions travel in the form of an aqueous saline solution from the object back into the poultice. The cause of this salt migration is due to two different processes: it can be generated eithrt by the existence of a concentration gradient between the object and the poultice, in which case the salt ions diffuse