slate spalling

Quoted text begins.He wants to know if there is some way to prevent it from going all green
End of quote.

with algae

Hi Norman, apart from keeping the stone dry, as Tim suggests, you could coat
the stone with a microporous (breathable) acrylic sealer to keep the stains
off; another way is to wash the stone occasionally with pure soap.

I use Lux soap flakes dissolved in hot water and allowed to cool. Brushed on
with a soft brush and rinsed off, it is surprisingly effective at keeping
stone clean.

Simon