Generally speaking, cleaning to historic surfaces is damaging and can cause erosion that will escalate the decay of historic fabric.
If the dressed surface of ashlar or the fireskin of a brick is aggressively cleaned then moisture will more easily enter the structure and cause a number of problems including rot, damp, rusting of ferrous components and freeze action that causes masonry to crumble.
In addition, in places like Salford, the industrial weathering to historic surfaces is every bit part of the narrative of its industrial past and can be significant to the buildings meaning.
There are circumstances by where cleaning could be advocated, particularly where vandalism has taken place or carbon built up has obscured architectural decoration. However, in such circumstances caution should be taken to select the appropriate system. The 'jos', 'doff' and 'torc' cleaning systems are most appropriate, with the 'jos' system designed for less stubborn dirt and the 'doff' and 'torc' systems for more aggressive staining.
The 'jos' system is a low pressure vortex process which uses a mixture of air, fine inert powder and relatively small quantities of water and is particularly effective for the safe removal of carbon, dirt, scale, micro-organisms and all types of atmospheric pollution on brick, wood and stone.
The 'doff' system complements the 'jos' system and is the natural answer to the removal of surface coatings such as oil, grease, biological matter, graffiti and vermin staining which have become bonded to masonry and other substrates. A unique combination of heat and pressure is used to cut through and remove unwanted matter without causing substantial erosion to the surface of the material, by using adaptable equipment. It is chemical free and is a uniquely efficient and gentle cleaning system as the substrate is left undamaged.
The 'torc' cleaning system is an evolution of the 'jos' system and is aimed at removing the most difficult staining, such as cement based paints, bitumen and lime scale. The 'torc' system creates a gentle swirling vortex using a mixture of low air pressure, little water and a safe inert fine granulate. This system uses less water and granulate than the 'jos' system.