The Association for Gravestone Studies provides a good discussion of gravestone
rubbing on its web page <http://www.gravestonestudies.org/faq.htm>.
Some stones, particularly sandstone, slate, or marble markers that exhibit
cracking or exfoliation, should never be rubbed. Moroever, many cemeteries
have rules preventing all gravestone rubbing. As Ned mentioned in an earlier
post digital photography is a help in recording markers, as is the use of
mirrors to cast light obliquely across the carved face of the stone.
Photographers working without mirrors often have to time their visits to
cemeteries to make sure the light hits
the stones they want to photograph. The best times tend to be between 10:00 AM
and 1:00 PM.
Best,
Rich Veit