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The removal of soiling from a building can be achieved using water, chemicals, poultices, lasers, and also mechanical cleaning methods. Mechanical cleaning requires the use of force using hand-held tools such as brushes, sponges, scalpels etc. or with equipment such as vacuums, or dry air-abrasion. Mechanical cleaning can be low-impact, and is often used where water or chemicals are not suitable. However, stone can easily, immediately and permanently be damaged by mechanical cleaning.

Abrasive cleaning systems direct particles onto soiled masonry in a stream of compressed air of between 1.5 and c.7bar.  The systems work by abrading or eroding the dirt, however, the particles cannot distinguish between soiling to be removed, and the stone surface. Mechanical cleaning systems depend on using the lowest amount of force that obtains the desired effect without damaging the stone. A variety of abrasive particles (from talc and eggshell to glass beads and carborundum, as well as many forms of ‘micro-abrasive’ particle) are available, and the general rule is to use the least-hard aggregate which is effective. However, there are a number of challenges and risks.

Loss of stone detail

Air-abrasive cleaning of a stone pavement. These whorls appear on re-soiling of the stone surface.

A key factor is that not all stones are uniform, and the soiling may conceal that a softer stone or a softer stone element is present. If the stone, and its properties, have not been corrected identified, there is a risk that mechanical methods may over-clean, resulting in the loss of surface detail, detracting from the visual appearance of the stone, and in some cases accelerating stone decay. With a decaying stone surface coupled with surface soiling, it can be very difficult to distinguish between the soiling and the actual stone surface.

The risks to the stone surface include pitting of the surface, micro-fracturing (which can lead to colour changes), and an increase in surface topography or the opening of pores which can lead to higher rates of weathering and soiling in the future. Despite the risks, abrasive cleaning is effective on light to moderate soiling, but tends to be less effective on severe soiling and has little impact on staining. Used correctly and appropriately however, it can be an efficient option for cleaning in many situations.