I can think of two solutions one new and one old.
The new one, design your ring in a 3d program and then cut a
selection of sizes. Or,if you want to hand carve it, do that, then 3d
scan it into a computer via a Roland MDX15 and a Fourth Axis rotary
attachment and then recut smaller or larger. Not easy to learn or
cheap to set up but you could get someone else to do it and you then
have a set of carving wax models to tidy up, and build in casting
shrinkages if necessary. This by the way is a great way to do
opposite handed designs for earrings or larger for brooches, really
accurate. Roland are discontinuing the MDX-15 at the end of the year
so they should be cheaper soon, and although they are not as accurate
as the more modern machines, the scanning function is a huge
The old way, works in two directions, I have not done this but I
have seen others get good results, and worked with the mould. If you
have an RTV mould and soak it in naptha it will swell, you can then
inject wax into it before it shrinks back to its original size. The
opposite of this is to mix naptha with the rubber before you pour it
around your ring, let it set up in an enclosed container and then
cut the wax out and let the naptha evaporate. It will then be
smaller, the only thing is you wont know quite how much and will have
to experiment with the mixture, ie percentages of naptha. I believe
is quite jelly like and has to be treated carefully. Also not good
for you to have the naptha evaporating off.
I knew a jeweller who would make small waxes and then expand them on
a smooth ring mandrel in a bowl of hot water before the injected
waxes had cooled too much. If this would work for your design you
could miss out every other size perhaps. Again I have not tried it,
sounds like it would take practice, but is very cheap to try!
regards Tim Blades.