Before you touch that tombstone.......... 
check out these links brought to our attention by Jan Ramos.

This is the web site for the Connecticut Gravestone Network,
http://members.aol.com/ctgravenet/index.htm  The emphasis is on proper care, preservation, & restoration techniques that do no further harm.  It seems that a lot of well- meaning folks are hurting the old stones around the country - this includes some professionals hired by local cemetery associations for cleaning, etc.

The 2nd site is the Association for Gravestone Studies with emphasis on care, art, importance, symbolism, & more.  http://www.gravestonestudies.org

The word needs to get out - plain water, soft brushes only for old stones. No chemicals, no power washing (which is what the pros love to use - not only physical damage, but can change the color of the stone!).

I can't believe how much I've learned in a very short time thru contact with the former organization.  Glad I did a search & found them before I tried anything on the stones I was interested in.   
Thanks, Jan Ramos