Our policy of only using water, soft brushes and no chemicals is confirmed by the following article. Better to be safe than sorry.
HOW TO CLEAN A GRAVESTONE?
A gravestone should be cleaned and maintained carefully so as to avoid getting it scratched or tarnished. Thus, it is best to use a soft-bristle brush, or at most a plastic scraper or craft stick, instead of a wire brush or abrasive pad.
Moreover, care should be taken so as to avoid cleaning with detergents that contain compounds like sodium bicarbonate, sodium chloride, sodium sulfate, ammonium carbonate, and trisodium phosphate (TSP or Calgon); they contribute in the build-up of soluble salts beneath the surface of the stone.
In addition, it is suggested to determine the general condition of the headstone, gravestone, or tombstone [before] taking measures to clean it.
In case a gravestone has flakes and cracks then you will have to be extremely careful while cleaning. You may avoid cleaning if the headstone is in a fragile condition.
Plus, avoid pressure washing a gravestone. Remember to wash the monument from the bottom to the top to minimize streaking and staining.
As for the detergent, do not use a household cleaner like soap, borax, Spic and Span, Fantastik, Formula 409, muriatic acid, phosphoric acid, and other strong acids and bases as because they tend to be corrosive.
You can use a non-ionic detergent (mix an ounce of the detergent in a gallon of water) because unlike most other detergents, it does not cause the deposition of soluble salts on the gravestone. You can find non-ionic detergents at janitorial, photographic, and conservation supplies.
Light colored marble or limestone tombstones can be cleaned with a cup of ammonium hydroxide or household ammonia mixed in a gallon of water to remove biological growth such as moss, mildew, algae, and lichen.
Avoid using chlorine bleach, though, as they tend to result in light brown staining after some time. The best and safest means of cleaning a headstone is soft water. So, avoid using cleaning solutions as far as possible.
If there appears lichen growth on the headstone in the form of grey and orange patches, then moisten the affected area and scrub it with a mixture of one part of ammonia and four parts of water. Finally, wash it with regular water.
Read more about the effect of lichen on man-made surfaces in this article from the British Lichen Society.
Besides, you can prepare a homemade poultice by mixing equal parts of water and glycerin (preferably vegetable glycerin) in dry porcelain clay until you get a smooth consistency.
Black algae growth can be removed with a solution of one lb granular calcium hypochlorite mixed in four gallons of water.
You can get more information about the process of cleaning a headstone on this page. If you want to gather details about gravestone cleaning and photographing, then read this.
Furthermore, keep in mind that gravestones made from materials like sandstone, marble, slate, etc. need not be cleaned and washed too often or else they may deteriorate soon. In fact, having cleaned a monument, you need not clean it again for the next ten years.
Tomb Stonez. (2017). How To Clean A Gravestone? Retrieved from https://www.facebook.com/TombStonez/posts/713751308749291