Churches with grave brasses are careful to limit access to the tombs,
because too much rubbing will damage the brasses.  In the case of
tombstones, we may presume that the purpose of an occasional rubbing
is to preserve the inscription data for posterity. If we are careful
we should do no harm to the original.

In the old days you could get large sheets of carbon paper. Place a
piece of quality paper over the inscription, then place the carbon
paper face-down on it. A stiff piece of rubber or felt is then rubbed
over the carbon paper.

This is a method we used to copy stereotype mats when we were making
dummies at the newspaper plant where I worked 40+ years ago. Alas,
stereotype mats are museum pieces, and so are carbon paper sheets.
There is still a limited supply of fairly good white paper, but the
quality is dropping fast. We usually were able to get a pretty good
image of the flong for purposes of indicating to the compositor where
the cast  was to be placed.

You can get blocks of graphite, or artist crayon or pastels, which
allow you to make rubbings directly on paper laid on the inscription.
Soft crayon should not damage the stone.

Lately we have had good success photographing tombstones with a
digital camera, then fiddling with the brightness and contrast in
Photoshop.  It's amazing what you can do with digital images.  I just
finished a report that included unlighted pictures of the interior of
a factory, including the unlighted roof trusses.

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