Although there are professional services that remove lichens from gravestones, amateurs can get good results by gently scraping the lichens away with a wooden tongue depressor and water. It is crucial to use a scraping device that is softer than the stone. Hard objects damage the outer layers of the gravestone, making it more susceptible to the elements.
It is possible to use a diluted ammonia solution to clean a gravestone, but doing so requires the use of eye protection. To make the cleaning solution, mix approximately one cup of pure ammonia with one gallon of water. Always work from the bottom towards the top when cleaning a gravestone to prevent dark streaks. It is important to avoid haphazardly removing lichen or algae that is embedded in the rock. Doing so may cause large pieces of rock to become dislodged. Commercial gravestone cleaners are also available.
Lichens harm gravestones by trapping dirt, organic debris and moisture on the surface of the gravestone. Additionally, because gravestones are somewhat porous, the lichens also trap these contaminants under the surface of the stone. Lichen tissue sometimes penetrates the stone, which causes damage as the plant tissues swell and shrink in response to changes in humidity.