[Jewelry making - Article 00028] - Re: [Orchid] Casting at high altitudes

[Peter W . Rowe]

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>     it just not physically possible to achieve a stronger vacuum than
>     24lbs at an altitude of nearly 7,000ft? 

    Nicholas, Vacuum is the absense of air pressure, so the maximum
    vacuum reading you could get would be the actual air pressure at your
    altitude.  I don't remember the precise readings for a given
    altitude, but I'd guess that at 7000 feet, 24 inches of mercury would
    be about the max.  But this should not be a major factor.  If you
    were in outer space, your vac pump would not be able to pull anything
    at all, since there would be no difference between the environment
    and within the bell jar, yet there would be no air in the bell jar,
    and bubbles of air would not be possible.  In your situation, the key
    is not the guage reading, but whether the pressure is low enough in
    the bell jar to allow the investment mix to boil, which acts to more
    fully mix it, as well as dislodging air bubbles. This behavier
    depends on the actual pressure in the bell jar, which is NOT the
    guage reading, and the higher you go in altitude, the EASIER task a
    pump will have in getting the pressure in the bell jar down to those
    needed levels. Examine the time it takes for the investment mix to
    boil when you're working, to determine if your pump is working
    properly or not.   If this seems in accordance with published
    procedures, then adaquate vacuum is not your problem.  You may also
    need to examine factors such as the water quality where you are,
    which may also cause investing problems, the temperature of the water
    you're using, the water/powder ratio, and perhaps just the brand of
    investment powder you use. also, check things such as the cleanliness
    of your wax.  Dirt mixed in the wax can cause problems.  I know of no
    reason why an attached bubble of metal in the casting, caused by an
    air bubble in investing, should automatically cause a pit, at least
    with a small bubble.  Dirt, on the other hand, might do it in some
    cases...  

Peter

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