Jackie,
Congratulations on finding your ancestor's headstone - you are lucky!
I suggest taking a spray bottle with water and wetting the stone as that
will sometimes make it easier to read. Also, try reading it from different
angles. On one headstone we couldn't read, we carefully dabbed some damp
dirt (which can act as an abrasive if rubbed) into the engraved sections on
a stone making the words darker so they stand out more. Afterwards the dirt
can be washed out using the spray bottle of water. On stones that are
flaking, I would avoid using any sort of a brush or anything that touches
them (besides water). On studier stones, a very soft brush might not hurt
them to clean them up a bit. Taking pictures of stones at different angles
and in different lighting (bring a flashlight or mirror) then enhancing them
on the computer also helps.
Awhile back I read where headstones can now be photographed using thermal
imaging which is nondestructive and does an amazing job in making the
illegible become legible! Unfortunately, not a piece of equipment we can
throw in our bags, yet anyway!
Good luck,
Kathy