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Re: [Orchid] Casting questions....
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Jeanne Rhodes-Moen Sunday, November 18, 2012
   
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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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