Black 'stains' invading my 3yr old indian sandstone patio

[David Green]

I have noticed that many of the indian sandstone slabs I had laid 3 years ago are developing lots of unsightly dark 'spots', as shown in these images:

These marks cannot be removed by a pressure washer, and appear to reach down into the stone itself. I started to notice these stains last year, but this year there are a lot more of them. Not all the slabs are affected as you can see in the photos. Can anyone tell me what is causing this problem and what I can do about it? I'm worried that this will continue getting worse until my entire patio is black! Is there a name for this problem?

Is this a case of defective stone? Should I be demanding replacement slabs from the supplier or the company which laid (and supplied) the slabs?

I hope someone can help.

Sincerely,

D Green Cambridge UK

[EricP]

Looks like a normal lichen growth to me and little you can do except clean it off every so often, or use chemicals to destroy them.

I think it's the second or third oldest lifeform on earth.

[David Green]

Pressure washer has no effect. How else can I clean it off? And what chemicals do you recommend to kill it?

David

[The Simpsons]

In the third image I can see the same spots on the mortar joints, so it's not the slabs at fault, are you sure haven't spilt grease (from last years BBQ) on the slabs. Not sure how to remove the marks, others have used chlorine to clean patio slabs but not tried this myself.

F

[Dave Liquorice]

Live with it. Adds natural character and patina to boring clean stone.

[Dave Fawthrop]

|!On Tue, 10 Apr 2007 15:20:29 +0100, David Green wrote: |! |!>> Looks like a normal lichen growth to me and little you can do except |!>> clean it off every so often, or use chemicals to destroy them. |!> |!> Pressure washer has no effect. How else can I clean it off? And what |!> chemicals do you recommend to kill it? |! |!Live with it. Adds natural character and patina to boring clean stone.

In Yorkshire lichen/algae on York Stone is encouraged, to tone down the stark yellow colour of the new stuff. The recipe used to be a mixture of cow muck and soot mixed with water and painted on. The Clean Air Act made the soot redundant, and the recipe is now Yoghurt mixed with water and painted on. Both supply nutrients for any spores which land on the stone.

[The Natural Philosopher]

Algae like a slightly acid environment so a scrub of bleach and a pressure wash will remove it.

[Andrew Gabriel]

Another source would be spillage of lawn fertilizer/weedkiller containing iron sulphate. If so, phosphoric acid (rust remover) might help, but I've no idea what it might do to the sandstone, and it might just push the stain further in or change its colour.

[Dave Fawthrop]

|!>> |!>> |!>> |!> |!> In the third image I can see the same spots on the mortar joints, so it's |!> not the slabs at fault, are you sure haven't spilt grease (from last years |!> BBQ) on the slabs. |! |!Another source would be spillage of lawn fertilizer/weedkiller |!containing iron sulphate. If so, phosphoric acid (rust remover) |!might help, but I've no idea what it might do to the sandstone, |!and it might just push the stain further in or change its colour.

If the sandstone has a *reddish* tinge, this is caused by oxides of iron (ferric I think) which is the same stuff as rust, so it might turn the sandstone white.

[The Natural Philosopher]

According to my geologist friend, there is also a lot of manganese in sandstone.