The Q&A Archives: Stained Paving

Question: I have a North-facing patio whose stones (real stone rather than concrete slabs) have gradually developed many small black spots over the last 10 years, with spot sizes ranging from 1mm to about 10mm. Because of the north aspect, they need cleaning regularly which I do with a jet washer, but this doesn't remove the spots. Nor does disinfectant, nor does thick bleach (slight improvement perhaps but no real removal). Scrubbing ineffective --it's in the surface of the stone. Any idea what the spots are, and any suggestions on how to get rid of them? They are making the stones look really unsightly now.

Answer: Based on your description, it sounds as though some mineral in the stone may be oxidizing or reacting to exposure to the air (perhaps a pollutant of some sort) or to some other element, perhaps through rainfall. You might want to contact a stone specialist or geologist who could help you identify the stone and what is causing the reaction, then you might be able to determine a way to prevent or counteract it. A stone specialty supplier might also be able to offer some insight, if it is a type of stone they carry. I'm sorry I can't be more helpful.

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