Ben,
This was a tough concept for me, too. But here’s how I understand it
(which works for me).
When you cast metal, there is a small amount of shrinkage in the
casting process (5 - 10%, depending on the metal). When you then mold
the metal and run waxes from it, that molding process introduces even
more shrinkage into the process. So your finished pieces from molding
may be anywhere from 10 - 15% smaller than the original you started
with.
On the casting side, the shrinkage occurs because the metal is very
hot (heat expands because the molecules are moving around much more
in space) when it fills the cavities in the investment. As it cools,
those molecules slow down a lot and take up less space, so it shrinks
and becomes more dense. If you were to completely air-cool a flask
then cut it in half, you’d find that the metal has pulled away from
the investment a bit, representing that shrinking. As most of us
quench flasks hot (unless doing stone-in-place casting), we never see
that pulling away action.
For bezels, I’ve found that it generally works (for decent-sized
stones, at least) to wrap the diameter of the stone about 2 times
around with masking tape, then using that to carve the bezel. It’s
usually pretty much right on the money for silver, although I’ve had
to clean it up a tiny bit with burs on occasion.
Make sense?
Karen Goeller
No Limitations Designs
Hand-made, one-of-a-kind jewelry
www.nolimitations.com