[Jewelry making - Article 00901] - Re: [Orchid] 2 Hours Casting

[Eddie Bell]

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Dear Fred,

>     I would like to know if disappearance of the grey and a change to
>     a "chalky appearence" of the pouring cup is a "sufficient"
>     indication of adequate burnout even in the large flasks. Otherwise
>     is there any clear indication of sufficient burnout other than
>     inserting a thermocouple or allowing more than enough time? 

    The white color of the investment in the area of the flask where
    metal will contact is a practical measure because we can look at
    every flask before casting. It is not a guarantee that the cavity is
    completely carbon free, but for the most part I look for other
    problems when diagnosing gas porosity if the flask was white before
    casting.

    Why do we care about a carbon free burnout? 

    Chemically carbon acts as a reducing agent in investment. When there
    is carbon present in the flask as a result of incomplete burnout it
    drives the reaction temperature of the gypsum used as a binder in
    the investment down below the casting temperature of most of the
    alloys we cast. Carbon free investment starts thermal reaction at
    about 1100C (2012F), which is high enough for our needs but if
    residual carbon is present the reaction temperature is about 700C or
    1300F. As the cast metal cools it gives up energy, heating the
    investment until the reaction temperature is reached and sulfur
    dioxide gas is liberated and absorbed by the hot metal. It is easy
    for metal that is between 1600 and 2000F to raise the investment
    from say 1000F to 1300F or higher.

    Gold is pretty inert but all the alloying metals react with SO2 and
    cause porosity and unwanted metallic/sulfur compounds and oxides.
    Silver casting is much more sensitive than gold casting because the
    silver contains about twice the energy per gram (it is better able
    to heat the investment wall) and the silver likes very much to
    combine with sulfur. A rotten egg smell on the casting as it comes
    out of being quenched is an indicator that SO2 is present. Another
    in-shop indicator is residual slag in the crucible after melting
    metal that has been exposed to SO2. However, melting previously cast
    items or sprues that are not completely clean of investment will
    produce slag and contaminate the metal too. Pickle has been found to
    be another source of contamination. It gets into the cracks and
    pores on the sprue and will only rinse out in an ultrasonic tank
    with clear water. If you see little green or blue spots on the metal
    after it has been pickled and dried, it is a sign that the pickle,
    which is a sulfur compound, was not completely rinsed away.

    Some casters use forming gas (a mixture of 93% nitrogen and 7%
    hydrogen) and that can be a problem too. According to Dieter Ott,
    Hydrogen is another element that will act as a reducing agent in
    investment causing the same reaction that carbon does, so if you use
    forming gas and have problems that look like gas porosity and
    contamination you might want to switch to argon.

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