[Jewelry making - Article 00057] - Re: [Orchid] 2 Hour Casting

[Fred Sias]

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Dear Mich and JD:

    I am glad to see your testimonials on what I have called "Fast
    Burnout" in my book. I have been using a short, high-temperature
    burnout for about 5 years when I teach casting. This approach was
    suggested to me by the retired Director of Research at Ransom and
    Randolph. He told me that the key to success was keeping the flask
    moist just as you do. He pointed out that a partially dried flask
    might crack or even explode. I have had only one so-called explosion
    when a student used excessively thick investment and I believe it
    left cavities that filled with steam during early burnout and blew
    investment all over the oven. No real harm done, just a mess. 

    Most casting protocols are cookbook-like and don't explain what is
    going on. The objective of burnout is simply to get rid of the wax in
    the pores of the investment to make it porous and strengthen the mold
    so that it is more ceramic like than simply hard plaster. As you
    pointed out, when the pouring cup is chalky white instead of grey,
    all of the wax is gone. We can get to about 1350 F as fast as
    possible if the oven is already at that temperature instead of
    starting out at room temperature or 300 F as often suggested. The
    free water in the pores of the investment will keep the mold
    temperature from rising above 212 F (the boiling point) until all of
    the water has turned to steam. Then the temperature rises rapidly to
    almost the oven temperature to complete the burnout. I have placed a
    thermocouple in the center of a 2 x 2-1/2 inch flask and showed that
    the core temperature stayed at 212 F for about 20 minutes and then
    the temperature rose to almost 1350 F in about another 40 minutes. 

    I generally use relatively small flasks for my work but I have had
    commercial casters tell me of "pushing" their burnout cycle in an
    emergency. All that is necessary is that the core temperature reach
    around 1350 F and that will depend on the size of the flask.
    Successful burnout is indicated by the chalky-white pouring cup as
    you pointed out. I don't know whether there is any minimum time that
    the flask should be held at maximum (core) temperature but I suspect
    that it is long enough when all of the grey has disapeared. Does
    anyone reading this have any technical data on how long the flask
    core should be at maximum temperature? I am not referring the holding
    time for the oven since the core temperature obviously lags behing
    the oven temperature. 

    I will be interested in hearing any other experiences concerning
    fast burnout. Hope this contributes to the discussion.

Fred

 
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