Dear JJ Roberts, when I was a young apprentice, there was a small
square grindstone used by the hand engravers with an old-fashioned
orange label which read "The Original Tam 'O' Shanter or Water of
Ayre Stone". I've since found several in some old stuff of my
grandfather's (who was also a jeweller). They came in several
different grits - one is much finer than the other - and they seem to
be made from fine grained slate.
Being an engraver, I still use them for removing scratches or
miscuts. The traditional method is to lightly hammer the back of the
plate on a polished steel block so that the miscut is left slightly
raised. Putting a dab of water on the spot to be ground back and
simply rubbing the stone around in a circular pattern eventually
wears away both the slight raise and the miscut to leave the plate
nice 'n' flat. Although the Water of Ayre stone degrades slightly as
you work, the grit forms a supplementary paste which aids in the
grinding process. Finish off with fine emery paper and the plate is
ready for re-engraving.
If these little stones ever wear out, I'll get one of my lapidary
colleagues to cut up some fine slate for me. I've discovered that
some old roofing slates have a very fine grain and may be suitable
for this. If this is unsatisfactory, I'll explore some of the modern
ceramic whetstones which come in all shapes and sizes these days.
Hope this helps. Kind regards, Rex in Oz