BCD 2019

PROTECTION & REMEDIAL TREATMENT 4.1 131 C AT H E D R A L C O MM U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 9 cleaning will mitigate the effect of soiling decay mechanisms and promote reuse of a neglected building. The balance of risks and benefits must therefore be considered for the long term (decades), not just before and after cleaning. PRE-CLEANING INVESTIGATION AND ASSESSMENT Firstly it is necessary to identify the masonry type and the soiling or coating adhered. In a straightforward case, experience alone may determine this correctly, perhaps with the aid of a x4-10 magnifier or USB microscope. In the laboratory, chemical analysis can determine soiling or coating constituents and thin section petrographic and paint microscopy is used to identify stone and paint types. XRF (x-ray fluorescence) equipment can be operated on site or in the laboratory to determine the precise elemental composition of almost any substrate or substance. Archival research may identify the age and source of the masonry and helps to establish its relative significance in heritage terms. Cleaning can be one of the most dramatic aesthetic alterations we make to a building, and great care should be invested in considering and undertaking the process. This will involve answering a significant number of questions honestly. A few examples: • Is there an overwhelming need to clean? • How large is the area, and in what timescale must it be completed? • Do we need to clean the whole structure? • Will the clean be durable? • Are there worktime restrictions – for example, high or low ambient temperature or noise – and at what stage of the project (before or after repair and repointing) can we clean? • Is there vulnerable adjacent or internal fabric that requires protection? • Can we confine the process and prevent egress of water, dust, chemical residue? • Is the masonry due to receive a coating or other treatment after cleaning? • Will the finished project be subject to periodic inspection and a maintenance programme? An effective way of reducing risk is to limit the anticipated degree of cleaning or paint removal. It could be said that 90 per cent of the risk is in removing the last ten per cent of the soiling or coating – highly unscientific but the principle is sound. SUBSTRATES, SOILING AND CLEANING GUIDANCE The principal published guidance for masonry cleaning is BS8221-1:2012 Code of practice for cleaning and surface repair of buildings – Part 1: Cleaning of natural stone, brick, terracotta and concrete . This provides a shortlist of the most common categories of masonry substrate; stone, clay and calcium silicate brickwork, terracotta faïence and concrete but not specifically render, plaster or cob. Each substrate description also contains advice as to which cleaning methods might be suitable, or indeed unsuitable. A list of specific deposits details their characteristics and how they might be removed. In broad terms, the techniques and precautions for cleaning un-weathered concrete are similar to those for limestone while those for brick and terracotta (excluding faïence) are akin to those for fine grained siliceous sandstone. Natural stones are subdivided geologically into three categories; igneous (principally granite and basalt), metamorphic (marble, slate and quartzite) and sedimentary (limestone, sandstone and alabaster). Their properties (grain size, weather resistance, porosity, hardness, water and chemical solubility) vary considerably and in turn affect their vulnerability. The broad determinant for the success of removal is whether the soiling or discolouration is on the surface or sub-surface, the latter being more difficult to remove. Calcareous substrates (limestone, marble, calcareous sandstone, concrete, lime render and pointing mortar) are particularly susceptible to pollution. In damp conditions acidic sulphurous gases convert the constituent calcium carbonate to calcium sulphate. This sulphate binds carbon soot and other airborne pollution particulates to the stone, forming a film of black carbon sulphation often referred to as a gypsum crust. Apart from being disfiguring, this can contribute to salt entrapment within the stone, although when exposed to sufficient rain the semi-soluble sulphate is rinsed away. Similarly, during cleaning the sulphate can be deliberately washed away, the process accelerated by regular brushing. Alternatively the sulphate deposition can be reversed by using a relatively mild poultice of ammonium carbonate, or abraded preferentially from the stone by the wet air-abrasive method. A number of effective options are therefore available for sulphate removal. The bond of carbon and hydrocarbons with siliceous substrates (those bound with silica – sandstones, grit stones and porous brickwork) is usually very strong. Separation normally requires correspondingly strong alkali-acid treatment or mechanical abrasion. For these reasons limestones are generally much easier to clean than sandstones, and calcareous sandstones (bound with lime) easier to clean than siliceous sandstones. A Karsten tube being used to determine the change of porosity before and after cleaning (Photo: Odgers Conservation) A micrograph made with a USB microscope at x50 magnification showing a partially cleaned surface with residual algae. (Photo: Odgers Conservation)

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