Physical principles and
efficiency
of salt extraction by poulticing
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
Pros
When sufficient time is available, diffusion based
extraction methods can have an efficiency of 100%
The method functions independently of pore size.
Consequently, the same poultice will work on any porous
material
Cons
Slow method: in general it will take weeks or months for
this method to be effective.
One has to renew the poultice frequently in order to the
keep on extracting salts.
Good hydraulic contact between poultice and object has to
be maintained throughout the treatment period.
The object has to remain completely saturated with water
for a very long time period. Hence this could result in
additional damage due to dissolution of the material,
swelling of organic components, chemical alteration of
pigments and binding media, biodeterioration, and other
water related decay processes.
At the end of the treatment the object is wet, and has to
be dried, during which any remaining salt may be transported
back to the surface.
4.2 Advection based desalination methods
Pros
Fast method, i.e. the time period for salt extraction is
of the order of days
Less moisture is introduced to the object
The object surface is dry after desalination—although
there is residual moisture remaining at depth within the
substrate, and so further salt and moisture transport to the
surface can potentially occur.
Cons
The method is pore size dependent, i.e., it will only work
if the poultice contains a significant quantity of pores
that are smaller than those of the object. Accordingly, the
poultice has to be adapted to suit the material on which it
is to be used.
Requires good hydraulic contact between poultice and
object
Due to the nature of advective transport, salts will only
be removed from the surface region of the object.
During the extraction the increasing accumulation of salt
will influence the drying rate by lowering the vapour
pressure of the saline solution within the poultice, hence
reducing the rate of moisture loss by evaporation. As a
result the rate of advection decreases.
Increasing salt accumulation in the poultice also promotes
the rate of back diffusion from the poultice into the
substrate (i.e. both poultice and substrate are still wet
but the Peclet number has dropped to below 1). Hence
renewed poultice application is necessary, the timing of
which will have to be determined by tests, i.e., one could
not leave the poultice on the substrate until both are
completely dry.
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.
An extensive description can be found in:
A. Heritage, A. Heritage, L. Pel, A review of
salt transport in porous media: assessment methods and salt
reduction treatments, Salt weathering on buildings and
stone sculptures, 22-24 Oct, Copenhagen, Denmark (2008)
Leo Pel, Alison Sawdy, Victoria
Voronina, Physical principles and efficiency of salt extraction
by poulticing, Journal of
Cultural Heritage11
59–67 (2010)
A. Sawdy, B.
Lubelli, V. Voronina , and L. Pel, Optimizing the extraction of
soluble salts from porous materials by poultices, Studies in Conservation55 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)
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