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Chapter 2 The soiling of building facades 2.1 Building soiling The soiling of building facades is a complex phenomenon which takes place at or near the surface of the stone and leads to a change in the appearance of the facade.This soilingcan, for convenience, be sub-divided into two main groups, soiling caused by airborne particles and biological soiling due to the presence of microscopic flora. In practice, both types of soiling are likely to be present on stone surfaces, either separately or in combination. It is well recognised that soiling may be one cause of stone decay, leading to a loss of surface material. Alternatively, the soiling may take the form of surface discolorationwhich, although sometimesunsightly, need not necessarily result in damage to the stone surface. Over the course of many decades of exposure to the elements, stones build up a patina on their surface which is not merely the accumulation of soiling material. Wetting and drying cycles cause mineralogicalchanges near the stone surface which, combined with external agencies such as soiling and pollutants, develop into a more or less stable surface zone of variable depth. Behind this patina, zones of varying mineralogicalcomposition are often formed. Removal of this patina is not necessarily damaging to the stone in itself, but it is sometimes the case that a hard surface crust conceals underlying decay. Removal of the crust in such circumstances may result in serious damage to the stone, necessitating extensive replacement or repair. Colour changes may occur as a consequence of mobilisation of previously stable m i n e d assemblages under the surface of the stone. Re-establishmentof the stable patina on a stone may take many years or decades. Soiling does not occur in a uniform manner across the entire surface of a building. The nature of the surface material and the presence of architectural features, as well as micro-climatic effects, influence the water run-off patterns on the facade. These zones of water runofftinadditionto more protected areas (forexampleunder projecting ledges) dictate the main areas of localised soiling, as well as, in some cases, creating localized areas of stone decay. In many instances the soiling over flat areas of facades is not uniform. Adjacent stones, apparently similar, can exhibit marked differences in soiling intensity. It is likely that this is influenced by the porosity, pore size distribution, capillary system, surface tension forces, and surface texture of the stone. These characteristicsaffect the absorption and evaporation of moisture in the stone. Little research work has been conducted on this phenomenon to date. From careful examination of the pattern of soiling on a building's facade, it is often possible to get some understanding of the reason why the building has soiled in the way it has. This understanding is important for two main reasons. Firstly, the distribution of soiling gives clues as to how the building is likely to be affected by cleaning. For example, areas subjected to frequent wetting which take longer to dry out, may remain discoloured followingcleaning.Inaddition, thedistribution of soiling gives some indication of the likely nature and pattern of resoiling following cleaning.

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Profile for Historic Environment Scotland

Guide for Practitioners - Stonecleaning PLU 7545  

The original practitioners guide based on stonecleaning research includes background on sandstone characteristics, the effects of soiling ag...

Guide for Practitioners - Stonecleaning PLU 7545  

The original practitioners guide based on stonecleaning research includes background on sandstone characteristics, the effects of soiling ag...

Profile for hspubs
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