The papers, particularly the rice paper that was popularly used by many,
leave behind particles during the rubbing process. These particles/fibers expand
when they get wet (ie rain). This in turn breaks down the stone's surface very
quickly.
I did many rubbings during my college days and wish I still hadthem. Many of
the stones are nearly unreadable now. Just take a look in the old cemeteries
in Salem, MA the next time you are there.
****Of course the acid rain isn't helping much either!
Dancin' Feet