conservation treatments that depend on the context of the other materials.
Re-treatability refers to the possibility that new or different treatments can be
applied without any harmful effects on the building. Assessing re-treatment
implies knowledge of the effects of treatments on stonework and their poten-
tial interaction with different and new types of conservation treatments. It is
difficult to assess the interaction of existing treatments with each other unless
sufficient time has elapsed and good monitoring records have been kept.
Assessing the potential impact of new treatments is even more problematic as
there is no possibilityof obtaining data of interactions over time. Treatment
effectiveness can be defined in terms of slowing the rate of degradation and
increasing material durability. Both these concepts are difficult to judge with-
out sufficiently long exposures and monitoring. All the above emphasise a
very conservative view of conservation; most require long exposures and
detailed records to enable any judgement, other than anecdotal, to be made.
4.1 Identification
The first step in any conservation project should be identification. It is essen-
tial to try to identify the nature of degradation and its severity. This will influ-
ence the type of conservation practices undertaken. Of particular significance
is establishing the spatial extent of the types of degradation. This identifies
which treatments are needed and where they are to be applied on the build-
ing. A key part of identification is developing an appropriate classification
scheme. There are a number of different schemes in existence, many of which
rely upon visual characteristics of the degradation form for assigning features
to specific classes. A number of these schemes also assume that there is a
specific and known link between particular decay forms and a set of, or even
single, decay agents or processes. This link is usually assumed, rather than
established by theoretical understanding or even based on a vast amount of
empirical data. Table 2 presents a simplified scheme for identifying and dis-
tinguishing different forms of degradation based on a limited set of visual
characteristics. Although not ideal, the scheme does not assume a simple
process-form relationship and so does not assume that certain forms are pro-
duced by a particular process or a set of processes.
4.2 Prevention
One of the best means of slowing degradation is to prevent the agents that
cause it from reaching the building in the first place. Prevention is, however,
also one of the most difficult objectives to achieve. Preventing atmospheric
pollutants from reaching a building in an urban area would be possible if
atmospheric pollution levels were reduced. Reducing levels, however, requires
230 Chapter 9