I have searched the archives and pursued the relevant forum posts
(or most of them). What I have found is a wide variation on belief,
theory, experience, and scientific (physics / chemistry approaches
if various threads.
ASSUMPTIONS (What I was Taught): Silver, Sterling Melting 1640
Specific Gravity 10.36 OzT / Cu In 5.457
Sterling (nothing exotic) Silver (Melting Temperature )
Flask sizes in the range of 2 D X 3 H
Types of waxes vary from signet rings to small charms with
filigree
Because I was experiencing low success rates with some very
difficult ORGANIC patterns (very thin elements), I sought some
advice from someone who I respect (not my original teacher). I
presented the above assumptions and was challenged regarding why I
cast at such a low temperature (high delta from the melting
temperature of the alloy). He mentioned that he always casts and no
more that 150f below the melting temperature of the alloy. So, I did
some experimenting and got some interesting conclusions. The first
of which was only use casting flasks don’t try to use tin cans, they
disintegrate at the higher temperatures.
Ok, here goes a technical question looking for a solution from two
perspectives:
A) The physics / chemistry approach (expert)
B) The practical approach (experience based)
QUESTIONS?
Why are we encouraged to cast in the range of 900f to 1150f for
Sterling?
Why does it seem (perception only maybe) that I get a lot less fire
scale at the higher temps?
Why do some individuals recommend not quenching but rather an
ambient temperature cooling then hammer or dig out the investment?
Why do some individuals recommend water soaking the invested flask
prior to burnout.
My Ron, you’re just a bundle of questions!! LOL Hopefully some of
those who know more than I will also answer, but I know some of it.
First, #3. For sterling silver there is no reason to air cool the
flask. There are some certain alloys that need that, but it’s stated
in the spec sheets. Sterling and the standard alloys of gold do not. #4 - I’d call that superstition, if anything. What purpose that
would serve I do not know. The chemistry of concrete, mortar, plaster
and investment is well understood, and the water you put into the
powder is part of a complex reaction - putting free water onto set-up
investment just makes it wet. As to #1 and #2. The reason we cast
{silver} at around 900F is because the book says so, and it works
well. Deeper than that, though, is what happens when you cast. The
alloy of copper and silver is a solid solution, and the copper is
only partially soluble into the silver. That solution needs to
undergo a phase-change - i.e. - it becomes a solid, and that solid
has a crystalline structure. My guess is that the higher flask temp.
is letting more copper into the crystals, thus less firescale. The
downside is that life is not about firescale. You’re probably
getting huge crystals due to the slow cooling rate - a massive grain
structure, too. Don’t depend on the look of it, stress it: pound it
on a ring mandrel, a ring stretcher, cut a ring open and see if you
can bend the shank back and forth, take 1/2 of a shank and
straighten it on an anvil and see if it breaks, run it through a
rolling mill, and finally, polish it and compare to cooler flask
silver. I’m very experienced at casting, but I don’t consider myself
an expert with all the technical side of it - again, hopefully some
of the real experts will weigh in. I believe that the 900F figure for
silver is a medium between good casting, good grain structure, and
good crytallization. You don’t want JUST less firescale, you want the
casting to have as many good qualities as possible. Finally, very
often in jewelry, especially with non-pros, you do get what amounts
to superstition. “First you mix it, then turn around three times,
slap your left hand on your right shin, then you pour it.” And it’s
important to be able to cut through that and just mix it and pour
it. And the secret of good casting, especially really fine items, is
probably not to cast sterling in a 1500F flask (Yeow!!), it’s
sprueing…