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Stone cleaning
It is widely recognised that all types of masonry can be permanently damaged by cleaning methods in common use, however the Scottish experience demonstrates that the risk of damage can be so great that some types of masonry should never be cleaned.

The need to clean a building is commonly perceived as being critical for its well being. Such a physical act is driven by aesthetic considerations often without considering the after-effects or consequences properly. The decision is also frequently made on townscape or streetscape grounds regardless of the physical impact on the actual face of the structure.

Where buildings which are listed or in a conservation area are concerned, Scotland differs from the rest of the country. Here, acknowledging that physical change can occur, stone cleaning has been deemed an 'alteration' since 1992. As a result, all proposals to clean listed buildings now require Listed Building Consent or, in the case of unlisted buildings within a conservation area, planning permission. Such an approach has been determined necessary to ensure that the risk to the fabric is considered and that appropriate damage-limitation measures are adopted. Typical ProblemsUntil relatively recently, cleaning methods generally lacked proper specification and site control. This has resulted in a wide variety of techniques being offered by contractors without due regard to the full consequences of their effect, and usually little or no detailed consideration was shown by manufacturers or suppliers to this need. In attempting to deal with all types of dirt and surface coatings in one treatment, contractors and specifiers catered for worst-case scenarios, and over-treatment was the established norm.

In particular, insufficient consideration was given to the wide range of natural materials being dealt with and their relative susceptibility to deteriorate as a result. No building is homogeneous in its construction or detail. Materials such as sandstone, limestone, granite, brick and terracotta are liable to be bound by lime mortar. Some may be used in combination, and other factors such as variations in colour, texture, tooling and form are likely to be met. In many, the composition will vary, and different combinations can lead to the interaction of materials, one with the other. Decay may also be present and different patch repairs, with different substances at different times, may further complicate the issue.

A basic difficulty is deciding where to stop. This can lead to a form of facadism, with only the principal elevation being treated. However this approach has one advantageous side-effect: any change that subsequently reveals itself can be compared against the untreated return faces of the same stones at the extremity of the cleaned area.

The type of soiling also needs to be taken into account. In the cleaning debate, soiling is often pre-supposed only to be an external agent, with particulate deposition and reaction resulting from either wet or dry conditions. Damaging crust-formation can be evident on the surface limestone, but the prospect of benign mineral movement occurring from within the body of sandstone is rarely considered.

Biological surface soiling is equally complex, with bacteria, algae, fungi and lichens each seeking out the appropriate colonisation conditions within which they will flourish. Influencing factors in their growth can include atmospheric and micro-climatic conditions, fluid movement and concentrations, surface roughness and physical changes.

Once the building has been cleaned, incidents of resoiling, iron mobilisation, efflorescence, vandalism and graffiti further complicate matters.
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