As discussed in Part II, there is a need to assess deteriorated, soiled traditional building materials and structures in order to determine their cleaning requirements, to evaluate all cleaning systems and to determine the details of how the selected systems are to be applied to the situation at hand. Masonry and soiling types and conditions are never the same from project to project. More than 75 per cent of the success of a cleaning operation depends on those executing it on site.

The categories of cleaning commonly involve the following methods:

Many paint removers are based on methylene chloride. Graffiti, bitumen, mastic and grease removal products contain an even wider variety of organic solvents.  Chemical cleaning is most frequently used on materials with soiling which is not water soluble, i.e. sandstone, brick, unglazed terracotta and unpolished granite. In these siliceous materials cleaning involves degreasing and the breaking of siliceous bonds. It commonly involves an alkaline chemical followed by an acidic chemical. In calcareous surfaces, the alkaline cleaner is used to breakdown the greasy component of the soiling and undertake the cleaning. Chemical cleaning processes are selected for siliceous materials because this method is considered to be often the only method that does not have the potential for abrasive damage and one that can achieve a high level of cleaning. One drawback of the use of chemical cleaning is that chemicals are non-selective in the minerals and constituents they attack and their effect on certain constituents of the stone’s cementing medium, such as clays and other metallic minerals, can be damaging and undesirable. They can also cause grains to be loosened and removed from the surface by softening the cementing medium.. This is a particular problem where the cementing medium is highly soluble in the chemicals used. For example, the calcite of calcareous substances is highly soluble in HF acid-based cleaning products. Masonry materials that appear to be similar can be geologically and chemically quite different. A product that is hastily chosen may clean one material safely, but cause irreversible damage to others nearby.

Certain procedures are essential to the use of chemical cleaning; these include: pre-wetting whose purpose is to fill the pores and capillaries with water and thereby to encourage the applied chemical to remain on the surface in contact with soiling and not be absorbed.; rinsing off, which is applied at the end of the required dwell time. The time required to achieve thorough rinsing is often much longer than many contractors prefer. neutralization, whereby the alkaline used for cleaning is neutralized with acid; the resulting salts need to be rinsed off thoroughly.   Certain chemicals may mobilize coloured minerals within the stone, particularly those which contain iron and manganese and cause them to be deposited as stains on the surface. Others may dissolve coloured minerals to be washed out and cause bleaching of the surface. Alkaline cleaning materials are often blamed for the presence of efflorescence on surfaces during the drying out periods following cleaning. On the other hand, existing salt loadings within the masonry prior to cleaning may also be the cause. Efflorescence, though unsightly, however, can be good news as it indicates that the soluble salts have come onto the surface where the effect of their deliquescence and recrystallisation is far less damaging if they have remained within the masonry to cause damage. Salts on the surface can be easily removed by brushing or vacuuming,

Poultices are absorbent materials such as clay, diatomaceous earth, talc or chalk which are blended to a paste; cotton waste or paper pulp may also form the bases of a poultice. Most are bound with water, although other solvents are used at times.

Pastes of cleaning agents which, once applied, are not left to dry out on the surface are termed packs. The pack named More Pack contains 60 g ammonium bicarbonate, 60 g sodium bicarbonate, 25 g EDTA, 10 g surfactant disinfectant and 60 g carboxymethylcellulose (CMC) in 1000 ml of water. It is a clear sticky material applied in a thickness of 3-4 mm to a pre-wetted masonry 

The Hempel biological pack is based on attapulgite clay to which the active ingredients, a blend of 5 g urea to 10 ml glycerol in 500 ml of water, are added to form a thixotropic paste. The pack assists in the breakdown of sulphate crusts on marble and limestone as a result of the presence and activity of the micro-organisms it contains. The pack is applied to a pre-wetted surface ensuring good contact between it and the masonry beneath. Under the covering of a thin polyethylene film applied to prevent air reaching the surface, the pack is to be left on the stone for up to two months before being lifted and the surface brushed and rinsed. Many other poultices and packs have been prepared.

Soaps

Non-ionic soaps are most frequently used as they have better wetting ability and they do not introduce salts into the masonry. They are used in hot or cold water for the removal of greasy and superficial soiling on mainly glazed surfaces. Grease and oil can at times be removed, especially if present in small quantities, by gentle scrubbing of the affected surface with a warm solution of water and a suitable non-ionic soap. The soaps are diluted in water or white spirit in proportions of 1 part soap to 3-9 parts of the liquid. The diluted soap is applied by brush to the surface and agitated with a soft natural bristle brush or a plastic pot scourer. After few minutes it is rinsed from the surface with a with the help of low-pressure water and the surface dried with lint-free cloths.

Abrasive and Mechanical Methods

Air abrasive and mechanical methods involve the use of mechanical force through hand-held implements or mechanical equipment. Soiling is removed by physical force, cutting or abrasion. Air abrasive techniques involve directing particles of abrasive on the soiled surface in a stream of compressed air. The dislodging of the dirt deposits takes place by the breaking up, sometimes to a depth several millimeters of the surface layer beneath the deposits. Because of its speed, abrasive cleaning is frequently considered for large areas of metals or masonry which have few design features. Nozzle pressures of 20-100 psi are typical.

Dry, wet and pencil abrasive techniques are in common use. The range of abrasives extends from non-siliceous grits, copper slag, mineral slag, carborandum, aluminum oxide powder, olivine, dolomite, crushed wallnut shells, olive pips, nutshells, minute glass beads and flour. Inherent potential damage can come from type and condition of material being cleaned; size, hardness and sharpness of the grit particles; shape and size of the nozzle, effective working pressure; constancy of pressure throughout the cleaning process (one of the most difficult to achieve).

Surface roughening is the most common damage by air abrasion. The roughened surface will have a slower water run-off rate allowing water to be absorbed to increasing depth meaning that such areas will remain damp far longer, attracting more soiling and encouraging more organic growth. Abrasive cleaning on polished stone will dull the surface and will necessitate re-polishing.

Under fired bricks are only partly vitrified and are too soft fro abrasive techniques. The damage that heavy handed abrasive cleaning can cause is irreparable. Indeed the abrasive cleaning was developed to remove rust and mill scale from corroding iron and steel.

Conclusion

Selecting a cleaning process is undeniably complex. Water washing will not be a low-cost, low-risk system if the effects of saturation and water penetration are not prevented.

Chemical cleaning methods can be both relatively fast and extremely slow, depending on the number of operation, required.

Wet and dry abrasive methods are usually considered to be moderate-to high-speed operations with relatively low costs for material and equipment hire. However, necessary operative protection, dust control measures and the use of non-silica sand abrasive can alter the cost significantly.

Neither chemical nor abrasive cleaning methods are inherently bad. Both can produce good or bad results depending on the way in which they are being applied.