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- one of the most traumatic events in the history of any building, it should now be accepted that, certainly in respect of sandstone, there is no such thing as a safe method and all of them depend very much on the skill of the operater. Different technologies are appearing all the time for example, laser cleaning, but at present there can be said to be five basic types of cleaning -

Water-based - involves water sprays, with or without pressure, hot or cold, for varying periods of time. This removes surface dirt or softens it to make brushing effective. Most effective on limestones.
Blasting - usually involves blasting sand, both wet and dry, but various materials can be used including ice.
Abrasive - involves using abrasive tools, needle guns, discs and cones, all of which basically involve dressing back the surface of the stone.
Chemicals washing - use of acidic and alkaline solutions. Most commonly applied to sandstone.
Poulticing - applying pastes, acid, alkaline or neutral to facades.

There are a number of texts on the subject, the most rational of which is Nicola Ashurst's two volume "Cleaning Historic Buildings".