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Noel, I recently had to do some casting for friends who needed
heavier components for flute production cast. With their stuff, we
were also getting the pitting and it was a real problem. After some
research, I found that, at least with silver, oxygen absorption into
the silver is one of the problems...silver sucks in oxygen when it
is melted, and it's that oxygen which causes the pitting. When I
learned to cast, it was centrifugal with a blow torch to melt in an
open crucible. we always added flux.... When I got my own set up, I
bought vacuum and an electromelt from Rio. The electromelts use
graphite crucibles, and theoretically, the graphite will burn up the
oxygen during heating rather than it being absorbed by the
silver....problem was, I was still adding borax/boric acid like I had
with the blow torch/open crucible, and this was coating the inside of
the graphite crucible, preventing it from absorbing the oxygen. I'd
not had much trouble with this with my filigree, but it is so fine,
that bubbles didn't get trapped as easily as with the heavier parts.
After discontinuing the flux, the pitting almost completely stopped.
The injection waxes should come out with a reasonably glossy
finish.... or at least as glossy as the original metal masters.
However, if they've been used over and over again, the molds can
wear out, making them less shiny. It may be time to make new ones...
silicon based rubber is great because it makes a much shinier finish
and does not require mold release spray. The molds seem to last
longer as well.
If the wax is old, or too many times recycled through the injection
pot, it can also loose quality. Not only can heating the plaster too
quickly cause the hairline fractures..... but you may want to check
how long they are letting it set before dewaxing....too long or too
short can affect the quality of the mold as well. You said the
plaster is new? how new is new? some estimates say the shelf life can
be as short as 6 months when you start loosing quality on the
investment... though I've cast with older plaster...it's always a
risk it won't cure properly.
considering this problem seems to be silver only, with the pitting,
I'd look at the oxygen issue..... it could be a matter of too little
or too much flux, or how it's melted...pouring temperature/mold
temperature can also affect things in terms of how much time the
oxygen has to escape from the silver..... Rio even suggested leaving
the vacuum on a little longer before removing the flasks...though I'm
not sure that does as much as cutting out my flux did.
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