[Jewelry making - Article 00935] - Re: [Orchid] Investment for detailed work

[Gianna]

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    The only problem with the "high altitude" recipies for investment is
    that they all assume that because the dial on the vacuum chamber
    doesn't go up as far as at sea level, that the vacuum is less. And
    for that reason they tweek with the recipe. In reality all that is
    happening is the dial is recording that the altitude has less
    pressure to be removed to get to full vacuum than at sea level.  All
    that is required is getting a full vacuum. 

    Longer times and hotter temperatures are important in cooking
    because of the chemical changes that involve starches and sugars at
    altitude are temperature dependant and at altitude the water boils at
    a lower temperature.  In investment it is only requisite that the
    chemicals in it get wet and nothing else.  At altitude a full vacuum
    has just as little air left in the dome as at sea level but the dial
    is only indicating the difference in pressure between the full vacuum
    and the outside air, therefore the needle is lower at altitude.  But
    it is still full vacuum. 

    To get a good job of investing, that has maximum strength and
    sufficient gas permeability after proper burnout it is best to follow
    the insructions of the manufacturer for times and temperature.  As
    long as your vacuum jar will boil water at room temperature within 30
    seconds of the time you close the valve on it, you have enough
    vacuum.  When the water is boiling, then mark the dial and that mark
    indicates full vacuum. Face it, when the air is gone from the dome,
    it's gone.  And what is outside the dome doesn't make any difference.
     Welcome to physics 101. 

    Lots of wierd formulas work because the investment is very tolerant
    and will let us get away with a lot.  But for the best results the
    guys that cast hundreds of flasks per week and have near zero rejects
    are the ones that know.  If you check with them you will find
    virtually all are using the mfg.'s recipe. 

    I'm not saying that other formulas are wrong or won't work, I'm
    saying that the mfg's. have spent years of time and zillions of
    dollars to get it right. Why waste all that testing to get results
    that are less consistent and not any improvement in quality? Best, 
    Bob Lynn


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