Hi folks, I am looking for someone with "tombstone expertise". I recently found the grave of one of my 5th great grandfathers. The tombstone is over 200 years old and the exciting thing is the man died at age 109 years, 6 months and 21 days....pretty good for 1792. But the large tombstone has patches of what I think is "lichen". It is gray in color but when you scratch at it there is green underneath. Can anyone tell me the proper way to clean this stone. I want to help preserve this historic stone but I don't want to damage it by doing the wrong thing. Would appreciate advise from any "tombstone experts" out there. Many thanks. Marion Bale JABandMcB@aol.com