Guide for Practitioners - Stonecleaning PLU 7545

Good cleaning practice Testing procedures as recommended in Chapter 6 should be carried out prior to cleaning.

It is essential when contemplating dry grit blasting to understand the physical characteristicsof the stone, and to note the presence of any decayed or spalling stone, since this will almost certainly be eroded by cleaning.Dry grit blasting should not beused on polished surfaces or on areas of delicate architectural detail or carvings.

The pressure used in dry grit blasting should be the minimum necessary to produce a level of cleaning consistent with the least damage to the stone. The hardness and size of the abrasives used needs to be considered against the softness of the stone. It might be the case that a range of different abrasive particle sizes are used on a particular building depending on conditions.

Dry grit blasting should be followed by low pressure water washing if any surface debris remains on the stone. This debris is unsightly and if left on the stone, may speed the resoiling process.

The residues from dry (and wet) grit blasting can block gutters, downpipes and even sewers. Care should be taken to avoid this problem.

Dry grit blasting produces considerable amounts of dust. This can ingress into buildings through the smallest opening. All necessary precautions should be taken to ensure this is avoided. The nuisance from dust can, to a large extent, be reduced by screening the scaffoldingwith sheeting and sealing off windows and other places where dust can ingress.

Abrasive cleaning can be very hazardous to operatives and those in the vicinity of the cleaning operation. Lung damage can be caused by inhaling airborne silica, dust and debris from cleaning. Even when a non-silica abrasive is used, silica can be released from the stone surface. Sandstones, granites and some limestones contain silica in the form of quartz and if the surface is abraded, this silica will be released into the atmosphere. Operatives must wear proper protective clothing, including "air-line" helmets which supply a constant stream of clean air inside the helmet.

Noise, particularly from the delivery nozzle and the impact of the air and abrasive mix on the stonework can be problematic and is difficult to avoid. Forewarning those likely to be affected by noise can at least be done. Compressors should be sited so as to reduce, as far as possible, noise from this source.

As dry grit blasting is a method which can cause considerable damage in the hands of unskilled operatives, training and proper supervision is essential.