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Strange casting results, update


#1

We tried very hard to reproduce the results of that odd casting on
Saturday that came out of the flask pink and cupric oxide-free. We
put varying but still minute amounts of injection wax or tiny scraps
of paper (the only possible contaminants we could think might have
gotten into the first one) into the next few flasks we cast. Nothing
changed, other than we lost more pieces than usual to porosity,
probably from contamination, we are assuming. Discouraged by the
results, on the very last flask we cast, we added nothing. That
flask came out with pink spots on part of the sprue button and areas
on some of the charms (but not the entire casting). So, eight flasks
and about a 1/8 loss of the castings later, we’re no closer to
figuring out what we did than when it first happened.

In answer to various questions about our metal:

No, there’s no way it included any firescale-proof alloys. We have
never owned any of that, and we don’t melt down wire, chain, findings
or anything else that we did not cast originally. I don’t want to
risk melting down someone else’s nasty alloys, toxic metals, plating
or old solder by mistake. I don’t like inhaling toxic substances.

I’m certain that the pennies were solid copper. Zinc was first added
to the metal for American penny coins in 1982, so to be safe, we only
use pennies that were minted in 1980 or earlier. And we use marked
.999 fine silver bullion for the silver in our sterling mix, so we’re
as sure as we can be that the silver isn’t an alloy either. We
pre-melt the pennies and the bullion together, and the hot mixture is
poured into a tall 8" wide piece of heavy PVC pipe full of water to
create the actual bits of sterling casting grain (an exciting
procedure that everyone should try sometime!).

And we pour our castings as soon as the metal is thoroughly melted
in the electromelt. I couldn’t tell you the exact melting temperature
though, because the numbering on the blasted temperature gauge is so
small I can’t read it! I’ll have to check it with my Optivisor the
next time we cast…

So, there you have it. I guess we just had a tiny miracle occur in
that one flask that isn’t, at least so far, readily duplicable. Since
we don’t have a problem with actual “firescale/firestain” (apparently
I was misusing that term when referring to the more easily removed
black surface scale that is really cupric oxide), we’re not going to
worry about it. If it happens again, we’ll ponder it then.

–Kathy Johnson
Feathered Gems Pet Motif Jewelry
http://www.featheredgems.com


#2
  I'm certain that the pennies were solid copper. Zinc was first
added to the metal for American penny coins in 1982, so to be safe,
we only use pennies that were minted in 1980 or earlier. 

Not quite. Pennies prior to 1982 were not solid copper, but rather,
95% copper and 5% zinc. Pennies after that were made of an almost
solid zinc core, with the copper/zinc alloy being only the clad
exterior of the penny. These are essentially zinc coins with a
"bronze" shirt on. Mostly zinc. the previous coins were mostly
copper. But not entirely. So perhaps your situation may have had
something to do with the small percentage of zinc, which can act as
a powerful deoxidizer in an alloy, at least until it’s become
depleted. The other thought that occurs to me would relate to your
burnout. Some kilns are better ventilated than others. Some kilns
designed for enamelling, rather than burnout, don’t have quite as
good air circulation as might be desired, and flasks with a lot of
waxes in them may be harder to fully burn out. If your flask has
remnants of unburned carbon still in the investment, you might have
just by chance hit a percentage of carbon that acted as a sufficient
deoxidizer, without otherwise impairing the casting. This might be
too low a percentage to appreciably color the investment, at least
not the outer surfaces of it, so you might not have noticed slightly
discolored investment when you quenched the flask…

By the way, if you’re looking for a cheap and relatively pure source
of copper for alloying, try copper electrical wire. The heavy guage
copper wire used in “romex” house wireing wire, or the like.

Your use of pennies reminds me of a story told by a refiner I used
to deal with. Seems one of his big manufacturing clients, who bought
gold alloy from him, and had done so for decades, was finding
occasional castings in their line which weren’t quite up the the
standard for the karat they were supposed to be. This occasional
underkarating was driving them nuts, as they couldn’t find the source
of the problem. They knew the alloy they were buying was consistant
and up to standard, yet some castings were not. They examined
melting equipment to see if the alloy was segregating somehow, or
whether metals in crucibles were being somehow contaminate, or what.
All to no avail. When the quality control folks tried the equipment,
all came out as it should. Finally, they just sort of quietly
watched the normal operations for a time. that was when they
noticed that one of their older long time employees, who now and then
filled in for one of the casters when things got too busy, had a
novel habit in his casting process. As the metal melted and was
almost ready to cast, he’d throw in a penny for good luck. Said it
was an old tradition he’d learned as an apprentice way back when…
And indeed, the small additional fraction of zinc from the penny
might have helped a little with porosity, but of course, the guy
didn’t realize that the bulk of it, the copper, was lowering the
karat of the alloy down too much to be legal. When he’d started
doing this, it was the older more lenient karat content laws, and
then his castings were still within limits, but now, they weren’t…
He’d been doing it so long it never occured to him to question the
practice…

Peter Rowe


#3

I am curious, why would you chance the content of a penny when
copper is so cheep and readily available? It takes so much work to
get to the end result. Why blow it on something like this? Over and
over I hear about this problem. It seems to me that we should
recommend to newbies to stay away from pennies.