Lichens are often misidentified as dirt stains because they are black or dark grey in colour. However, lichen formations are perfectly round and are sometimes white. Lichens are a combination of a fungus and an algae, and, if left on a limestone surface, will permanently discolour limestone and cause roughness or etching. Lichens are difficult to remove, and harsh scrubbing will damage the limestone. Properly removing lichens must be done slowly.
Fill a bucket with a mixture of 10 parts cold water to 1 part household bleach. Pour the mixture over the entire area affected with the lichens, and extend it an additional 12 inches around the perimeter.
Allow the bleach mixture to sit for 30 minutes to an hour to kill the lichens.
Fill a large stockpot with water, and place it on a stove burner turned to high. Wait until the water boils, then pour 1 to 2 tbsp of liquid dish soap into the boiling water. Stir it with a long-handled spoon.
Carefully take the pot outside to the limestone. Pour the hot, soapy solution onto the limestone infected with lichen.
Put on rubber gloves. Scrub the area with a stiff-bristled nylon brush, using circular motions to dislodge the lichens from the limestone surface. Continue until no more lichens come free easily.
Rinse the limestone with a garden hose to remove the bleach and the soap residue.
Repeat the entire process every other day until all lichens are removed from the limestone surface.
If the limestone is not polished, a wire brush used with very gentle pressure can assist with removing lichens. However, do not use wire brushes on polished limestone or on gravestones or statues. If the lichen is on a limestone gravestone, dampen it thoroughly, and remove it with a gum eraser instead of a brush.