1. Introduction
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 treatment itself can be summarized as having two main steps. 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 that of extraction, whereby the dissolved salt ions travel in the form of an aqueous saline solution from the wall back into the poultice. The cause of this salt migration is due to two different processes: it can either be generated by the existence of a concentration gradient between the object and the poultice, in which case the salt ions diffuse through the solution, or by capillary water flow from the object to the poultice (generally due to drying) in which the ions are advected within the solution. It can be demonstrated that the mechanism by which salts are transported during poulticing will strongly influence the efficiency of the treatment

    2. Salt extraction by diffusion
In order to remove salts from an object by diffusion the object is brought into contact with an aqueous solution with a lower salt concentration, i.e., in general close to zero. For the purposes of this argument, we will assume the concentration of the desalinating material remains constant (i.e. in the case when the object is flushed continuously with clean water, or the poultice is renewed very frequently). As an example, a simulation of salt extraction by diffusion is given in figure 1A, which illustrates the change in the salt concentration profile of a porous material, over daily time intervals. As can be seen the rate of salt extraction becomes slower with time. In figure 1B the same profiles are given, but scaled according to the square root of time.

Figure 1: A) Simulated concentration profiles taken at daily intervals over 10 days using a diffusion coefficient of 1x10-9 m2s-1.
B) The same profiles after applying the Bolzmann-Matano transformation, i.e., scaling the simulated profiles with t1/2.

As can be seen desalination by diffusion is a slow process. Hence, while it is possible to completely desalinate an object by diffusion, in general this is a slow process. Also it should be noted that for the purposes of this discussion we have taken a relatively high ion diffusion coefficient, while for many porous building materials the ion diffusion coefficient will be lower. Therefore, to speed up salt extraction process using a poultice, a faster ion transport process is required.

    3. Advection-based extraction methods
The term 'advection' refers to the transport of mass by a moving medium. A good example of advection is the transport of pollutants in a river: the flow of water carries the impurities downstream. This can also take place in a porous material, i.e., dissolved ions can be transported by the moisture flow. Hence if there is a flow of moisture from substrate into the poultice, then the substrate can be desalinated by advection. As advection is generally more rapid than diffusion, desalination treatments based on advection can be much faster. However in order for advection from the substrate into the poultice to take place, certain requirements regarding the pore size distribution of the poultice and of the substrate need to be fulfilled, in particular that the poultice contains a sufficient quantity of pores that are smaller than the majority of those in the substrate. These condition are schematically given in figure 2.

Figure 2: Schematic diagram illustrating the possible transport mechanisms (i.e. diffusion and advection)
by which aqueous ions can travel from a substrate into a poultice, depending on the substrate pore size relative to those of the poultice.

4. Pros and Cons of these methods

4.1 Diffusion based desalination methods


4.2 Advection based desalination methods


From this discussion it is therefore clear that there is no single ideal poulticing method for extracting salts, and that in practice one can never achieve complete desalination of a non-moveable object.  Indeed, it is therefore more accurate to refer to such interventions as ‘salt content reduction’ rather than ‘desalination’ treatments.

A. Sawdy, B. Lubelli, V. Voronina , and L. Pel, Optimizing the extraction of soluble salts from porous materials by poultices, Studies in Conservation 55 26-40 (2010)

          Leo Pel, Alison Sawdy, Olaf Adan, Principles and efficiency of salt extraction by poulticing: an NMR study, MEDACHS 10 La Rochelle, France (2010)

V.Voronina, Salt extraction by poulticing: an NMR study, Eindhoven University of Technology (2011). (Download 2.4 Mb)