polymers can also prevent agents of degradation reaching the surface. These
treatments do not act as a sacrificial layer, but rather block the path of any
agent to the reactive stone surface. The surface of the stone may be protected
from external agents, but unless the protective layer is able to let internal
agents through it, there is a possibility that the stone will be damaged below
the protective surface. At the boundary between the stone and protective
layer, agents such as salts may, if present, build up and eventually create such
high stresses as to cause a catastrophic failure of the stone surface.
4.4 Removal of Degradation
Where degradation has occurred the products of degradation can be removed,
but, importantly, the question of whether it is essential to remove these prod-
ucts needs to be asked. Where a surface is discoloured through, for example,
the accumulation of dust or by the development of a sulfation crust, removal
of the dust may require contact with the surface by abrasive agents, such as
brushes or by water. The process of removal may have an impact upon the
stone surface causing alteration. Inputs of large quantities of water, for example,
could result in water penetration into the stone and reactivation of salts stored
relatively harmlessly in the stone (the ‘memory-effect’). Likewise, abrading a
stone surface could remove both the accumulation layer as well as ‘sound’
stone underneath. Unless a highly-selective removal method is employed it is
likely that the stone surface remaining after treatment will have suffered
some physical alteration at the microscale.
A common method of removal is the use of poultices. The term has been
adapted from its original use in medicine where it indicated the application of
a treatment that removed infection from a body. For building materials, the
application of the treatment enables contaminants or staining to be ‘drawn’
out of the material. A poultice has to have the ability to mobilise the contam-
inants and then actively encourage the movement of the contaminants into
the poultice. Common poultice media are clay and paper fibres mixed with
water. The clay enables the mixture to adhere to the material, while the water
acts as the medium for mobilisation. Salts are drawn into the poultice by capil-
lary action as the poultice dries, although it may take a number of repeat treat-
ments to removal all the contaminants. Specific additives may be mixed into
the poultice, such as EDTA, which will target particular contaminants that are
not water-soluble, in the case of EDTA, copper and iron stains. Using poultices
with additives usually requires a moist surface for application to prevent deep
penetration of the additives as well as through washing of the surface after
application. Although poultices can be used for removing contaminants from
whole surfaces, other treatments such as water spray are often cheaper. Poul-
tices do, however, provide a degree of control in application of active agents
232 Chapter 9