[Jewelry making - Article 00118] - Re: [Orchid] Cement-like gunk on castings

[Peggy E. Pollak, Ph.D.]

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    More information was called for so here are all the gory details of
    how I achieve the lovely cement-like look on my recent castings. 

    I use plain old sterling casting grain and scraps from old castings
    (buttons, etc.) to cast organics coated with sticky wax for
    thickener/stiffener. I use Satin Cast 20 and distilled water (my tap
    water is VERY hard) vacuumed at 23 psi to remove bubbles. That's the
    max that will draw at the altitude here. I burn out at ramp of 300
    degrees and hold for one hour at each stop of 300, 600, 900 in a new
    programable kiln; spend 3 hours at 1250; and cast at 1100. I use
    borax for flux and a reducing flame. My castings tend to be large,
    treed structures with 100 or more grams of metal. None of these
    parameters are any different from what I used before the "gunk"
    appeared. 

    Now to the gunk. It is grey and appears as both bubbles and large
    surface areas on different pieces within the same casting. Sometimes
    but not always there is fire scale beneath the gunk. The gunk doesn't
    scratch with a finger nail but polishes off with 120 grit. It doesn't
    pickle off or come off in an ultrasonic cleaner. 

    I'm wondering if dirty metal combined with the new torch could be
    the problem. I often don't follow the rule of half-old to half-new
    metal, although I've not had any problem using old metal before. The
    reason that I suspect that the torch is a factor is that, because the
    torch is hotter than I am used to, I believe that impurities in/on
    the metal may be melting in before I can identify them as glowing
    spots and fish them out. Obviously I need to try a run with
    exclusively new grain which will be my next step. 

Thanks for any help.
Peg
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