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the relative contribution of different factors to the development of distinct types
and patterns of degradation. Viewing stone degradation within this framework
it becomes easier to identify the types of conservation measures that may be
appropriate to particular types of degradation.
2 The Degradation ‘Equation’
Degradation can be described by the following equation:
Where D is Degradation, s and t are space and time respectively, M is material,
P is process and E is environment. Any degradation of stonework, both its nature
and rate, is the outcome of the variations in space and time of thesethree inter-
related factors, material, process and environment. Degradation at one stage
influences the nature and rate of degradation at another stage and so, although
general patterns of change may be identified, there is no guarantee as to the
precise degradation pathway any particular building or building surface will
go through. Each of these factors is looked at separately below, but it is the
interaction of the three that produces the complicated nature of degradation
as the example of limestone weathering in an urban environment illustrates.
2.1 Material
Stones, or rather rocks, can be divided into three geological types: sediment-
ary, igneous and metamorphic. Igneous rocks can be extrusive, such as lavas,
or intrusive, such as granite. Differences in the pressures and depths at which
these rocks form, the nature of magma as well as the time taken to solidify,
influence the type and size of minerals found in igneous rocks. Larger miner-
als, in general, result from deeper pressures and longer solidifying times.
Igneous rocks contain primary minerals that have differing resistances to
weathering processes. The crystalline structure of different minerals, as well
as their arrangement, can have a bearing on how prone igneous rocks are to
degradation. Silica and silicate minerals, for example, are among the most
common rock-forming minerals on Earth, up to 90% of the crust and 75% of
all rocks exposed at the surface. Silicon and oxygen atoms can be arranged in
a number of ways, some involving the sharing of oxygen atoms with other
elements such as magnesium and aluminium, some retaining a tightly packed
structure with no sharing of oxygen atoms. The basic arrangements of atoms
are as chains, as double chains, as rings, as sheets and as three-dimensional
networks.The less oxygen atoms are shared, then generally the more resist-
ant the mineral. The different structures are prone to weathering at different
D(f(s, t (MPE)))