management of all the activities that produce pollutants, a virtually impos-
sible task. Even limited management of these activities is difficult to justify
given the relatively poor understanding of the level at which pollutants can
damage stone (see the dose functions mentioned above). Current policies of
pollution reduction could have the side-effect of reducing pollutant delivery
to stone surfaces. The impact of these reductions upon stone degradation may
not, however, be immediate. The so-called ‘memory-effect’, the weathering
of stone by the salts deposited by past pollution, will cause degradation above
that expected from current pollution levels alone. This additional degrada-
tional effect will continue to operate years after pollution levels have been
reduced.
4.3 Barriers to Agents of Degradation
Another preventative method involves the use of barriers on or within the
stone that prevent weathering agents from reaching and interacting with the
stone. Limewash, a traditional method of protecting building stone is a barrier
treatment. Coats of limewash are applied to a building and act as sacrificial
layers protecting the underlying stone. The limewash weathers preferentially
relative to the underlying stone, as it is the surface upon which reactants land.
Removal of the limewash, however, means that the sacrificial layer needs to
be renewed frequently, requiring a monitoring and maintenance programme
for the building. Treatments such as infilling damaged areas with silica-based
Stone 231
Table 2Simple classification of weathering forms on buildings
Type Description
Detachment and Alteration of surface leading to detachment and removal of
disintegration material, but no major raising or lowering of surface, e.g.
crumbling, flaking and spalling
Deposition and Change in colour of surface
staining
Relief forms Alteration of surface resulting in major raising or lowering of
surface. Major is defined here as noticeable change in surface
height relative to the original surface as can best be determined
by the investigator, e.g. blistering, pitting and back weathering
or hollowing out of forms
Cracking Single or multiple cracks visible
Weathering products Products of weathering found on surface or recesses. These are
products of weathering and do not refer to the texture of the
surface, e.g. grains or powdered material
Repair and Material visibly distinct from main body of surface that observer
replacement can identify as repair or replacement work, e.g. plastic repair