On Tue, 10 Apr 2007 22:24:17 +0100 (BST), "Dave Liquorice"
wrote:

|!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.
--
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