I have a couple of reconstituted marble statues in the garden, one is about
15 years old, the other is about ten years old.They both have a tendency to develop red patches or 'rashes' at various points of the surface. I suspect this is due to some form of air-borne algae settling and taking hold.
Tried cleaning with bleach and soapy water, but this tends to turn the markings a deep yellow colour. Anybody have any thoughts about a suitable method of removing the stains?
Yes. This is very much what I found on our sandstone.
I found that once I had neutralised the leftover HCL with bleach, it didnt come back. Seems to like acid conditions.
You are very safe with alkalis on marble..acid is a nightmare.
Try caustic as a stronger alternative to bleach, and pressure washing.
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Lots of marble also contains iron ore, which might be slowly dissolving out as rust.
Phosphoric acid is particularly good at removing rust, but I have no idea what it will do to marble, which is very sensitive to acids, so try it somewhere inconspicuous first.
You need to contact the Catholic Church. They'll buy them for a grotto in Lourdes.
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