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operates in one direction. Without an understanding of how the relationship
works, this type of error is possible. Second, deriving the curves and critical
thresholds has usually relied on extrapolating laboratory-based studies to field
conditions. This often does not take into account the jerky and discontinuous
nature of damage. Third, damage functions are often only derived for
processes that operate in a relatively simple manner. Figure 6, for example,
could be viewed as being a useful expression of the effects of sulfur dioxide
on limestone dissolution. Limestone on a building does not, however, only
weather by the action of dissolution. Physical weathering by agents such as
salts and frost can also cause damage and a different sort of weathering form
such as blisters are produced. The development and loss of material from
such weathering forms is not necessarily a continuous process nor necessarily
related in a simple manner to environmental variables such as pollution levels.
This means that it is problematic to represent damage functions as a simple
linear relationship between two parameters.
3 Microorganisms and Stone Decay
Microorganisms play a crucial role in mineral transformation in the natural
environment, notably in the formation of soils from rocks and the cycling of
elements such as nitrogen and sulfur. It is therefore not surprising that a wide
variety of microorganisms, especially bacteria and fungi, have been isolated
from rocks and the stonework of historic monuments and buildings (see
Figure 7). The complex interaction of numerous microbial types at a micro-
scopic level in intimate association with the mineral substrate is readily
observed, often reaching deeper than 3 cm into the stone. Microorganisms can
Figure 7Stone decay and crusts on decorative arches at Portchester Castle associated with
high counts of microorganisms