[Jewelry making - Article 00227] - Re: [Orchid] Investment inside casting pieces

[andy cooperman]

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>     Sounds like air bubbles trapped on the patterns when you pour your
>     investment. How are you debubbling your investment after pouring? 

    I've never seen air bubbles result in investment inclusions in a
    casting. The flaws described are classic investment pockets caused,
    usually, by a breakdown of investment. This can be many things: 

    -A thin or fragile finger or area of investment that breaks off due
    to the flow of metal as it enters the mold chamber or if the flask is
    droped or heated too fast. 

    Look for "filled in" areas of detail in the cast piece. 

    _A bit of the mold chamber wall has peeled back or crumbled off.
    This can happen when a bit of investment was weak or poorly mixed
    when investing or if the burned out flask has been dropped or shocked
    or if too rapid burn out weakens the plaster. Also if the invested
    flask was burned out at too high a final temp. which would begin
    disintegrate the plaster. (There would be other clues in this case,
    such as sulfur contamination and porosities.) 

    Look, in this case, for positives where there weren't any. Sort of
    like a rough tumor... Some hardened investment has broken off from
    the mold chamber wall and fallen into the piece. The "tumor" is the
    new area of the mold chamber (left vacant by the crumbling
    investment) which now becomes an extension of the mold chamber (and
    model) and fills with metal. 

    Another possibilty is that the bit of investment was introduced some
    time during the melt. Perhaps the "nose" of the crucible rubbed off
    some plaster from the button area of the flask. I've seen that
    happen... 

    When this first happened to me it was the second scenario. I saw the
    positive area on the casting and easily filed it off and
    recontoured, smugly assuming that I was out of the woods. What I
    didn't realize was that the investment had to go somewhere. It
    appeared as a tiny, jagged pit that only got bigger as I filed. It
    was, truly, the tip of the iceberg. 

    Always look for clues. It's really pretty basic. The hard part is
    figuring out what caused it. 

Hope this helps, Andy


 
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