Here is some information I have on headstone cleaning, that I thought I would share.
Compiled from publications and information provided by The Association for Gravestone Studies, The National Trust for Historic Preservation, and The National Park Service.
TOOLS OF THE TRADE
Approved "tools" include soft natural bristle (e.g. tampico)brushes, nylon brushes, tooth brushes, Q-Tips, sponges (natural), soft wooden sticks (e.g. tongue depressors), plastic spatulas, spray bottles, and buckets. If using a hose, decrease pressure to a gentle flow.
DO NOT USE
SOAPS (e.g. Ivory) are rendered insoluble by calcium ions in masonry and hard water. They may also produce free alkali and fatty acid salts in masonry.
HYDROCHLORIC or MURIATIC ACID may result in ferrous chloride (rust) staining and the deposition of soluble salts.
SODIUM HYDROXIDE (e.g. Borax) mixed with water forms a caustic soda and can cause the formation and deposition of soluble salts and cause ferric hydroxide (rust) staining.
SODIUM HYPOCHLORITE (e.g. Clorox)can cause the formation and deposition of soluble salts. (There are safer choices - see CALCIUM HYPOCHLORITE)
TRISODIUM PHOSPHATE (e.g. TSP, Calgon) is not recommended for cleaning masonry due to formation and deposition of soluble salts.
"FANTASTIC" contains 2-butoxyethanol (butyl cellosolve), alkaline builders, water, fragrance, and dye. It is not recommended for cleaning any porous substrates (masonry) or polished surfaces.
"FORMULA 409" contains ethylene glycol, a non-ionic detergent, alkaline builders, water fragrance and dye and is not reccommendedfor masonry or polished surfaces.
"SPIC AND SPAN" abrasive cleaners, and products containing sodium chloride, sodium sulfate, sodium carbonate, sodium bicarbonate and ammonium carbonate will form and deposit soluble salts in masonry. Do not assume that the label of contents is a complete list of all the chemicals contained in a household detergent product.
USE WITH EXTREME CAUTION
PHOSPHORIC ACID (e.g. Naval Jelly) is not recommended for general cleaning however may be used in a 10% (by weight) solution on granite and sandstone for the removal of rust staining.
ACETIC ACID (e.g. vinegar) is less aggressive than other acids, solutions have been recommended for the removal of general soiling of limestone.
OXALIC ACID is not recommended for general cleaningbut is recommended for rust staining removal on light colored granites, one pound to one gallon of water.
CALCIUM HYPOCHLORITE (e.g. Chlorine) is effective for the removal of biological growth; mix one ounce to one gallon warm water. Available from swimming pool suppliers. Requires water hose for thorough rinsing and Hydrion Paper test strips for pH testing.
AMMONIUM HYDROXIDE (e.g. household ammonia) diluted 1:4, is recommended for cleaning light colored stones and is particularly effective for the removal of biological growth. Requires water hose for thorough rinsing and Hydrion Paper test strips for pH testing.
QUATEMARY AMMONIUMS (e.g. algaecides) are especially effective for the removal of stubborn black algae and biological growth. Also available from swimming pool suppliers.
PRODUCTS THE PROS USE
NON_IONIC DETERGENTS (e.g. Photo Flo - a Kodak Product, Triton-X 100, or Igepal)are recommended for cleaning gravestones. They are electrically neutral cleaning agents that do not contain or contribute to the formation of soluble salts. They provide better wetting of the masonry surface and therefor, successfully facilitate the removal of general soiling. Non-ionic detergents are available from conservation, janitorial, and photographic suppliers. A suggested cleaning solution is one-ounce non-ionic detergent to 5 gallons of water.
While it is the easiest way to do it, a pressure washer is not a good way to clean headstones. Good old fashioned elbow grease is the best thing, and they will last for generations to come.