[Jewelry making - Article 00128] - Re: [Orchid] Cement-like gunk on castings

[Todd Hawkinson]

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    Here's the disclaimer. I have been casting dozens of flasks weekly
    since 1971, but am constantly challenged by the Orchid experts. My
    two cents... 

    Students in our casting class experience this many times. There are
    a few reasons in my observations. I don't have all your details about
    process, but here are a few thoughts. 

    Incomplete burnout. Peak temperature should be 1350 f. No grey color
    in the investment upon quenching will show you this. Larger loads
    need more time at hottest temperature. This assures the flask will
    breathe when the molten metal goes in. The air must travel out. 

    Some foreign matter gets cast into the flask. If the metal looks
    good in the melt (real shiny smooth & clean surface) cast as quickly
    as possible. A pinch of borax flux is plenty on most melts. 20 mule
    team borax works fine and is cheap. (Laundry detergent & almost pure
    borax) Stir when molten to mix metal as well as skim the crud off the
    molten metal. 

    Make sure any old investment is removed from a re-melt. Deox metals
    work the best and cast a second and third time with little problems.
    Make sure you have a decent spru button. Not enough metal will have
    the tail end of the pour go into the flask. This can be a problem. 

    Sequence is- melt, flux, stir & cast as quickly as you can. The
    metal when stirred should feel fluid. Vac casting or spin casting? If
    vac cast pour the metal through the flame as it goes into the flask.
    Vac flasks will cast better if there is vent at the inner flask edge
    (spur wax works fine). 

    Use a neutral flame for melting. Not reducing or oxidizing. Propane
    oxygen is the best fuel combination with a larger torch. Bigger than
    the bench torch. Not the Little Torch. 

    Old investment? Bottom of the barrel is usually the worst casting
    unless you mix the unused investment once in a while. 

    Be safe with proper gloves, goggles for melting and vent/mask for
    any investment contact. 

Kindest Regards,

Todd Hawkinson
Jewelry Dept
Mpls Comm & Tech College
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