Over the last 30 or so years we have done over 15,000 single
castings, mostly in 18 and 14 karat gold but a few hundred in
sterling silver. If we get porosity it is because we have done
something dumb like improper sprueing, too high a flask temperature,
or too much old metal in the melt. Of course, you should always cast
as cold as you dare and still fill the hole. We use a Kerr
centrifugal casting machine that has been modified with extra weight
on the end of the flask arm to cut down on the acceleration a bit.
This helps to keep the investment from breaking when casting delicate
Our flasks are 1 1/2 inches in diameter and from 1 1/2 to 3 inches
long. These will accomodate almost all rings. We make our own mold
bases out of vulcanized rubber.
Smaller flasks can be a real advantage, since they have less mass we
can use a shorter burnout cycle. We routinely go from mixing plaster
to casting in 4 hours. We have found our casting methods to be be
very forgiving and we rarely get a bad casting.