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irreversible change in the dimensions of the stone. This type of behaviour
occurs at every scale. The idea of stress beyond a critical point of deformation
can be applied to whole stone facades, or to pores, or mineral grains. For degra-
dation, an important consideration is whatis causing the stress and wherein
the stone it occurs. Stresses from within the stone require agents to enter and
move about the pore structure of the stone. Agents in solution such as salts can
move about the stone, although their capacity to induce stress requires them
to be activated once within the stone. Similarly, ice-induced stresses can
occur from within the stone, but the conditions under which ice expansion can
produce damaging strain do not just depend upon it being present. It is the
combinationof material conditions and processes under particular environ-
mental conditions that produces degradation.
2.3 Environment
The environment within which degradation occurs is important for determining
both the nature of that degradation and its rate. Environment could be thought
of at the macro-scale, such as the climate or atmospheric pollution levels. At
this scale general relationships between environment and degradation can be
established. Figure 3 illustrates the general model of degradation with atmos-
pheric pollution in Western Europe. Increasing pollution through time acceler-
ates weathering rates and therefore degradation rates rise above a ‘natural
baseline’ level. Within urban areas, there is a spatial pattern to this change as
well. At the urban centre, pollution is likely to be higher than in the suburbs
and rural areas surrounding it. Zonation of land-use within urban areas can
Pollution Level1750
(pre-industrial)Today175 0 levelUrban RuralPollution Level
(a) (b)Figure 3(a) Spatial pattern of pollution with high values in central urban area, rising in the
industrial area, then declining to rural areas. (b) Temporal pattern of pollution
increasing from the Industrial Revolution to about 1960s in the UK. This curve will
also vary spatially being different for Eastern European and developing nations
(Modified from Cooke, 1989)