growths, positive identification of the target organisms is critical. This allows
the selection of an appropriate chemical for the problem organism. Since few
biocides have a wide spectrum of activity or resistance may arise, combinations
or rotations may be necessary to achieve the desired effect. Toxic chemical
washes have certainly been used to eradicate or remove unsightly biological
growths from stone but these growths may be succeeded by mosses and higher
plants with greater damage potential.
Factors to consider. Success with biocides depends very often on the residual
effective concentration of biocides after application. This, in turn, depends
strongly on the nature of the stone and its growths. The density of any existing,
or previous, growths or dirt will directly affect penetration to target populations
and may cause inactivation, via adsorption reactions with the biofilm, or even
biodegradation. The stone itself may cause losses or deactivation by adsorption
to stone components; its porosity may cause it to act as a reservoir (if low) or
barrier (if high). In addition, leaching by rainwater, evaporation, photo- or
chemical oxidation will all reduce the concentration of the biocide that remains
to do its job. Maximum impact requires binding with minimum deactivation.
The persistence of biocides will be affected by environmental factors such as
pH and temperature and the type of environment they are used in, particularly
its microclimate and exposure. Re-application is usually necessary.
The desired biological action of biocides against unwanted growths on
stone, of course, also means that these chemicals pose a threat to the natural
environment if they cannot be restricted to the stone substratum. Thus the
chemical and physical behaviour, especially their solubility in water and
capacity to react with stone components, has an important effect on whether
there is a risk of environmental pollution. The interactions with the stone, or
indeed previous chemical treatments, could also lead to colour changes or
even mechanical damage through crystallisation.
4.6 Consolidation
Removal of weathering products may enhance the aesthetics of a building,
but removal may be detrimental to cohesion of the structure of that part of the
building. Statues, for example, may have sulfate crusts removed, but the crusts
may be the only solid component of that part of the statue. Calcium carbonate
may have long ago been replaced by calcium sulfate and so the details of a
statue’s face could be composed solely of weathering products.
Biocalcification. An alternative to chemical consolidation of stone is the
exploitation of a common phenomenon found in living organisms known as
biomineralisation, which produces shells in animals. Microorganisms can also
be isolated from the environment or stone surfaces that are able to precipitate
Stone 235