Before you clean a gravestone, buy a mild, gentle soap or a non-ionic soap. Next, mix your soap with water and dampen a sponge with the solution. Then, gently wipe down the surface of the stone with the sponge, using a brush for the tougher dirt. If there’s any natural growth on the stone, like lichen, mix 1 part ammonia with 4 parts water and use a sponge to scrub the affected area. Aim to clean the gravestone once every 18-24 months, or as needed.