I have 3 stone buildings in France. Recently I have been visited by a couple of "traders" offering wall cleaning services. I don't especially trust unsolicited callers but let one of these spray a small part of a wall with a chemical. This stuff was magic. Within 30 seconds the sandstone faces had changed from blackened by age/mould etc to clean and yellow. I didn't take them up on their kind offer but would like to know what chemical they were using so I can do it myself. Anyone out there know?
Is the stone definitely sandstone ? If it was then it was most likely Hydrofluoric Acid (HF) Don't use it yourself unless you are kitted up well and no how to use it and how to neutralise it afterwards. It's a shockingly toxic acid to the body. The problem is it doesn't burn the skin that much but absorbs into the body and damages your nerves, bones and even you DNA. Works good though. :)
If it was another type of stone ( many Limestones can look gritty like a sandstone) and France has lots of Limestone, it may well have been just a strong caustic soda solution.
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