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.

A homemade poultice an be produced using Dry porcelain clay mixed to a peanut-butter consistency with equal parts of water and glycerin. Small quantities of glycerin are available at most pharmacies; for larger quantities, search the Internet for soap-making supplies, floral supplies, etc. or check your Yellow Pages for "soapmaking supplies"; the large craft stores might carry it as well (Michaels, Hobby Lobby, etc.) Just be sure to stay away from "glycerin melt-and-pour" soap base. You'll need straight glycerin (you'll mostly likely find "vegetable" glycerin). Please be sure NOT to ask for NITRO-GLYCERIN. You will have every law enforcement agency in the country checking your personal history and watching your every move.

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:

http://www.sunnz.co.nz/

Hydro Clean Restoration Cleaning Systems produces HT-777 for use in cleaning Marble. This is a product that when mixed with water forms a creamy, non- acidic paste that will remove both organic and inorganic stains from polished marble and other substrates, returning the stone to its natural color and hue. It is made from mild, biodegradable detergents, clays and powders and will not etch marble or any other polished surface. The product's "wicking" action allows the cleaning chemistry to pull out stains that are deeply set in the stone. HT-777's Poultice Additive is a liquid that contains additional detergents and wetting agents that will enhance the cleaning properties of the poultice. The product is easy to use and virtually odorless. Large areas can be cleaned at one time. Additional information can be found at http://www.hydroclean.com/data_sheets/ht-777.htm .

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.