Quoting from findagrave.com regarding . . .
How do I clean a headstone or grave marker?
Unless you are related to the interred on the headstone in question, DO NOT do anything to the headstone.
Gravestones should never be cleaned with anything but water and a soft brush. Slate gravestones from the Revolutionary era and Pre-revolutionary era are best left alone due to their delicate nature and tendency to erode.
Never apply bleach, ammonia, shaving cream, chalk, flour, baking soda, cornstarch, firm pressure or use anything abrasive.
