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

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.

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

David

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.

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

|!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.

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

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.

|!>> |!>> |!>> |!> |!> 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.

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