On 11 Apr 2007 14:17:04 GMT, (Andrew Gabriel)
wrote:

|!In article ,
|! "The Simpsons" writes:
|!
|! "David Green" wrote in message
|! ...
|!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:
|!
|! http://www.davidwgreen.talktalk.net/DSCF0173.JPG
|! http://www.davidwgreen.talktalk.net/DSCF0174.JPG
|! http://www.davidwgreen.talktalk.net/DSCF0175.JPG
|!
|! 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.
--
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