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Posted: Wed Jan 19, 2005 5:43 pm
by alfiebines
We've been jet spraying a client's york stone terrace, and are left with a spotty black mould in patches.
We see this quite a bit, but this particular client is concerned about it. Clearly the mould penetrates beyond the surface.
Are there any solutions for removing it, or at least improving the appearance, chemical or otherwise?
Obviously reluctant to attempt anything radical as overall appearance of patio must remain consistent.
???
Posted: Wed Jan 19, 2005 7:01 pm
by Tony McC
It's a lichen, which is a 'marriage' of an algae and a fungus that work together in a symbiotic relationship that enables them to eke out an existance on what might appear to be a none-too-promising spot, such as stone flags, stone walls, roof tiles and the like.
They can be very difficult to remove as the fungi send hyphae into the interstices of the stone (in English: it sends special roots into the tny gaps between the individial grains that make up a sedimentary rock such as sandstone or limestone). A fungicide such as Jeyes Fluid or Armillatox will help, but it takes several weeks to kill off the organism and even when dead, it has a vice-like grip on the surface, similar to when old ivy is removed from brickwork.
As long as it is yorkstone, dip a paintbrush in brick-cleaning or etching acid and work it into the spot. 10 minutes later use a wallpaper scraper or hoe to scratch off the frazzled lichen and then use a stiff brush with hot, soapy water to scrub what remains. Don't use acid on Indian stone or any of the limestones, or on wet-cast concrete patio flags as the likely results are worse than having a few lichen spots to contend with!
Alternatively, a wire brush works, or even one of those round wire brushed that can be fitted to drills for cleaning-iup rusty metalwork, However, these tend to create a reverse problem, in that they clean the affected area so thoroughly, the spot now stands out because it is gleamingly clean and fresh and honed stone, while the rest of the surface remains "weathered".
The worst affected areas are better if sand-blasted. This cleans the whole area, so when the lichens are removed, the whole pavement looks as one.
Posted: Wed Jan 19, 2005 7:24 pm
by alfiebines
Thanks.
Speed and thoroughness of reply much appreciated.
Will report back.
Alfie B