To clear up a common misconception, lichens do not eat the rock, rather they naturally grow on stone surfaces that are available to them, whether these surfaces are naturally occurring or are artifacts of human activity. You will not be helping to preserve the stones by removing the lichen. The gray and orange patches formed by lichens on gravestones give a distinctive character to an old cemetery. These attractive "time-stains" not only enhance the appearance of the churchyard but are often of some rarity for which, like many other organisms, the cemetery is a wildlife sanctuary. Many lichens require a particular type of stone on which to live and, in many lowland districts, the cemetery may be the only undisturbed location in the area for many of these types of stones.
There are differing views as to whether lichens damage the stone on which they are growing or whether they protect it. There is evidence that the acid substances produced by lichens can attack the stone, but this effect is limited to a very thin layer immediately under the lichen. Any small cracks present or caused by this process will probably be infiltrated by the fine root-like hairs (fungal hyphae) of the lichen and this may cause more damage. It has, however, been argued that any damage caused by these processes is less than would be brought about by the weather if the lichen was not present. The tough, rather thick, lichen can protect the underlying stone from the weathering effects of wind, rain and frost. On some soft stones in exposed sites the lichens may eventually cover raised areas where the surrounding stone has been eroded away by natural weathering.
In some circumstances it may be necessary to remove lichens and various methods have been used with success. You'll never get a crustose lichen off a rock and keep the rock's surface intact. Lichens cause differential weathering on the rock which is visible as stains. On basic rocks the lichens will stain the rocks by their acids. The lichens also shield the rock from radiation which can lead to differences in color even on acidic rocks. If the purpose is to enable an inscription to be read, other ways of doing this should be tried first before the removal of the lichens. These methods, to increase the clarity of an inscription, include wetting or looking at it in the twilight with a torch shone along the inscription on a gravestone at a low angle. This will enable many worn inscriptions to be read. If it is deemed that cleaning is essential, only the minimum area necessary should be treated. This may be done by physically rubbing the lichens from the surface. Where this is done on a smooth stone the result may be unsightly as it is almost impossible to remove many crusty lichens from the lettering of the inscription. The lichens remaining in the lettering and cracks will probably regrow but rare lichens may have been lost from the surface. Another physical method that has been used is to cover the area to be cleaned with black polythene. It may take some months for the lichens to die but they may then be removed with a brush.
There is a new product, BIO-LICHEN OFF, produced by Sunnz International Ltd,
P.O. Box 13-598, Onehunga, Auckland, New Zealand that is said to be a fast acting and effective concentrated product designed to remove all Lichens, Moss and Fungal growth from most surfaces. More information can be found at:
The Association for Gravestone Studies suggests that Calcium Hypochlorite (e.g., Chlorine, "HTH," "Shock Treatment") is effective for the removal of biological growth. It is a granular product that is not to be confused with "liquid chlorine" or sodium hypochlorite. Calcium hypochlorite is available from swimming pool suppliers. A suggested cleaning solution is one ounce calcium hypochlorite to one gallon hot water. Please keep in mind that this product should be used only when a waterhose with a good water pressure (e.g., 55 psi) is available. Any water pressure over 40 psi has the potential to cause significant damage to a stone, depending on the condition of the stone. Saving Graves recommends alternatives to this method if at all possible.
Whatever method is used care should be taken to treat as small an area as possible and not allow the chemicals to drip onto adjacent parts of the stone or statue. Before commencing try to get an experienced lichenologist to check that there are no rare lichens present. Remember, before you kill them, that these lichens may have been growing on the stone for many years.