slate spalling

Quoted text begins.Another question: I recently finished a piece in limestone and have
given it away to a friend who likes garden sculpture. He wants to know
if there is some way to prevent it from going all green with algae.
Aside form keeping it out of the rain and away from nutrients in the
soil and dripping off plants I don't know of any. anybody know how?
End of quote.

Norman: If it is allowed to get wet, then it will go green. Some green
microbes will exploit it as a substrate, finding their way into the
slightest, even submicroscopic, cracks and chinks on the surface. Others
will actually drill minute holes into the surface as they seek the carbon
in the calcium carbonate for photosynthesis. Persuade your friend that
this is all part of the rich tapestry of nature, or else that he should
keep it dry.

Tim