replace like with like. Original quarry sources may have long since ceased
operation. It is often difficult to identify and acquire stone types of similar
physical and chemical properties. Even where such matches are available,
very little may be known about the weathering behaviour and, importantly,
alterations in colour of material upon weathering. For a period of time the
replacement stone may be visually sharply distinct relative to the rest of the
fabric of the building. Portland Stone, for example, when initially emplaced
is a bright white colour. As weathering begins, the colour alters and it is
hoped that it will eventually blend in with the existing fabric of the building.
Unfortunately, the weathering behaviour and associated discolouration of
stone is not clearly understood. Additionally, the conditions under which
stone weathered in the past are not the same as the conditions under which it
weathers today. Within the UK, for example, pollution control has severely
reduced smoke and sulfur dioxide levels in urban areas. This means that lime-
stone exposed today will not experience high smoke and sulfur dioxide
levels in its initial stages of weathering. What significance, if any, this differ-
ent weathering history will have for the appearance of the stone is unknown.
Choice of appropriate replacement stone is often based on the experience
of the architect or stonemason in charge of each project. The stone that
is used for reference is likely to have been weathered under environmental
conditions different from those in which the replacement stone is exposed.
Importantly, emplaced stone is likely to have been weathered in polluted
urban environments of the late 19th and mid-20th centuries.
5 Conclusion
Stone decay is a complicated process involving the interaction of a number of
different factors. Alteration of any rock, no matter the type, depends upon the
ability of agents of weathering to act upon the minerals of which the rock is
composed. A wide range of degradational forms may be present upon a sin-
gle building. Those present on each surface will reflect the relative degree of
adjustment to contemporary conditions and the expression of the underlying
characteristics of the stone. Processes of weathering have traditionally been
divided into chemical, physical and biological weathering. The environment
within which degradation occurs is important for determining both the nature
of that degradation and its rate.
The interaction between microorganisms and stone minerals demonstrates
metabolic diversity and versatility, combined with remarkable tolerance to
extreme environmental conditions. Biocides can be used to eliminate some
microbes but they should have extended activity and a wide spectrum of
action. They should be checked for detrimental effects on the stone, either
through colour change or internal crystallisation. The use of microorganisms
238 Chapter 9