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Posted: Sat Mar 17, 2007 4:20 pm
by Onegin
Hi
We have a sawn yorkstone terrace, which we pressure wash once a year to get rid of green slime, dirt, etc. Over the last couple of years the surface has become covered in rather unsightly black spots which do not respond even to intense pressure washing.
Does anyone know what has caused this, and whether it is possible to remove the spots?
Many thanks
Posted: Sun Mar 18, 2007 10:48 am
by TheVictorianCobbleCo
It sounds like it could be black algae, find a remote corner (out of sight)and scrape with a sharp blade to see if it will move and how deep it is into the sandstone. If it is black algae it will be removable but will need a proprietry fungal killer. You would need to contact an acid wash manufacturer to get advice on removing it. It is a common problem in pools when the ph/chlorine balance is not maintained, and is generally a *** to remove. Best of luck.
Posted: Sun Mar 18, 2007 12:23 pm
by Tony McC
This is an excxerpt from a new webpage I've been working on and hope to have finished some time later this week....
When pavements are left uncleaned for an extended period of time, circular or near-circular 'spots' that are mistaken for a stain are often observed to develop. Sometimes these spots will be dark or almost black, but they can also be creamy white, and they can be seemingly impossible to shift.
These spots are often a lichen, which is an unholy alliance between an alga and a fungus. The alga does the chlorophyll stuff while the fungi has penetrating root-like features referred to as 'hyphae' that are able to force their way into the substrate, enabling enhanced levels of mineral extraction along with a superb holdfast. The fungus swaps some of its mineral wealth with the alga in return for some of the chlorophyll-derived sugars. This is a classic example of a symbiotic relationship: both partners benefit and are better-off, ecologically speaking, than they would be on their own.
Which is all well and good for the botanists out there, but it doesn't half make them bloody hard to shift when they're disfiguring your paving with all their symbiotic goings-on.
As with the simpler algae, bleach is the answer, but it's likely that repeated, localised treatments using neat (undiluted) bleach will be required, along with a very stiff scrubbing brush, or, in extreme cases, a wire brush. There's a good chance that the surface will be altered in some way on a permanent basis. This can be due to over-energetic scrubbing, which inevitably scratches the surface to some degree, but it's also likely that the lichen will have chemically altered the paving material by removing minerals that contributed to the pigmentation.
So, the answer is little and often rather than massive dsoes of super-strong bleach and all-out wire brush warfare. A good strategy might involve bleaching the damned spot every other day, allowing the bleach to penetrate for 30-60 minutes and then washing off with boiling, soapy water and a good scrub with a nail brush or similar. It might take half-a-dozen treatments or more to remove every last trace: don't expect miracles on the first application. It's also a good idea to treat the surrounding area in the same way, so as to avoid creating a super-clean spot within a slightly stained pavement in place of an unsightly lichen blob. If the lichen(s) are on one paving element, say one block or one flag, treat the whole element rather than just the spot.
Posted: Sun Mar 18, 2007 10:21 pm
by Onegin
Thanks for both of these very comprehensive answers. It sounds like a lot of hard work is ahead. We'll try the fungicidal approach first to try to kill the blighters off, then the bleach approach to clean up the muck they've left.
Posted: Tue May 06, 2008 5:53 pm
by qkjb
Tony's post is by far the clearest I have seen on the subject and I started looking some three years ago when our patio had reached the stage when my wife wanted it replaced and it was only a few years old. Lots of research and a helpful man from the company who supplied the sandstone led me to a product called Ecochem - Algae and moss remover and it is made by Geocel. It also destroys those fungus spores which were causing the mess mentioned above. You dilute it, spray it on and leave for 24-48 hours. My patio is now completely clear of those spots and it couldn't have been easier. I have no connection with the company whatsoever, in case you think I am trying to boost sales. It just works. 
Posted: Wed May 07, 2008 12:30 am
by matt h
agree with qkjb, ecochem does do the trick and is friendly to the environment;)
Posted: Wed May 07, 2008 9:55 pm
by Tony McC
I shall endeavour to track down and blag a free sample of this EcoChem jollop to try for meself.