I keep an extra spring on hand, just in case. It’s really a drag to
have a kiln load of flasks and have a broken spring. I’ve replaced
the spring before, but it was long enough ago that I don’t remember
how it was done, only that it took about twenty minutes and didn’t
require any tools that I didn’t have handy.
As far as vacuum versus centrifugal casting, I use both. I vacuum
heavier pieces without a lot of intricate detail, and throw those
things that have thin areas and of course, all platinum castings.
White gold especially in thin or intricate pieces can produce
non-fills using vacuum in my experience. No right or wrong here, I
just use what works for me. Porosity seems to be a function of
temperature more than anything else, for me anyway, although gating
thick to thin and back to thick again is often problematic. Get the
metal too hot for the piece being cast and you will get porosity,
regardless of pretty much anything else including casting method and
the most careful gating. Old metal is a concern, but I do a lot of
casting using customer’s old gold, and as long as I don’t get it too
hot, it works just fine, most of the time. Adding a tiny piece or two
of Re-Cast-It from Roseco helps a bunch.
Did you also know that if you take two identical waxes and cast
one spin and the other vacuum they will have different weights? In
today's market that can spell a bit on money.
I read this in an ad for a centrifugal machine years ago, didn’t
believe it and did an experiment to find out. Two waxes with
identical weights, one cast using centrifugal, one using vacuum. They
both cast to the same identical weight, weighed to the thousandth of
a gram on a carat scale, or at least within a file stroke or two.
Metal is pretty much incompressible, meaning that unless it is
subject to the forces inside a black hole, the specific gravity will
be the same regardless of the casting process used. Even for forged
metal, the only differences are in crystal structure, so the specific
gravity remains identical, at least for our purposes. The only
difference that might be measurable would be caused by porosity. Air
pockets are a lot lighter than metal, so a lot of air pockets or an
open crystal structure might cause a very slightly lighter casting.
But my guess is that a casting that is measurably lighter would be so
full of holes that it wouldn’t be usable. Bottom line, don’t look for
savings in metal costs by using one method of casting over another,
there are none. Use the method that gives you the best castings.