Soot, stone and soft sandstone
Old stone and brick often carry black soot from the industrial past, plus algae from the damp. Hard engineering brick can tolerate more, but soft sandstone and lime mortar are easily eroded by a strong jet. A cleaning that removes dirt by stripping the surface is not a good trade.
We test a small area, use low pressure and gentle detergents, and stop if the stone begins to crumble. Listed buildings and parts of the town centre sit in conservation areas, so check with the council before cleaning historic masonry. We are glad to set out the method in writing, which helps when you speak to the council.
