[Jewelry making - Article 00509] - Re: [Orchid] Making casting grain

[Peter W . Rowe]

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    If you don't need the nice uniform size and shape of commercially
    made ones, you can do this just fine without the perforated crucible.
     Just pour it slowly to get a thin stream, and it does the same
    thing.  A couple other comments may also help.  commercially, in
    addition to the perforated crucible breaking the pour into thin
    streams (which then break up as it drops to fairly uniform sized
    droplets), it's common to have it set up with a reducing flame under
    that crucible, so the air space between the perforated crucible (or
    just the one you're pouring from, if you do it without that
    perforated crucible) lacks oxygen.  That gives you less oxidation on
    the grain.  Or, you can simply pickle the grain after you've poured
    it.  And if you want it to look nice and bright, toss it in a tumbler
    with a bit of tumbling soap.  No steel shot or other media needed. 
    Just the metal shot, tumbling against itself will be enough to
    brighten it up attractively.  Not needed, but looks better, for what
    that's worth. 

    More useful tips are to use a deep enough water container.  The
    stream of molten metal drops entering the water will take a moment to
    solidify, all the while sinking fast in the water.  You want it to
    fully solidify before it gets to the bottom.  If you pour into too
    shallow a container, the metal will still be molten when it gets to
    the bottom of the container, and it will fuse to the metal that's
    already there, so instead of separate grain, you end up with a big
    lump of fused bits.  Interesting, sometimes, but not what you were
    after. I've found I need at least about 12 inches depth to the
    container, and it's better to use more.   At one point i worked for a
    casting shop where we did our own grain. the setup there was a full
    sized (30 gallon size) metal trash can, with a motor clipped to the
    top rim, connected to a shaft going into the water at the side, and
    at an angle, with a propeller on the end.  The propeller caused the
    water to slowly swirl in the can, and the stirring action helped
    chill the grain quicker, as well as helping to keep the molten stream
    from just landing on the same place at the bottom, again preventing
    it all from just fusing together at the bottom. The trash can was
    also fitted with a stainless steel container (just something we'd
    found in kitchen supply, I think), which sat on the bottom of the
    can, and could easily be lifted out, so we didn't have to drain the
    whole container to get the poured shot out.    Also, whatever the
    depth of the water container, be sure to pour from a good enough
    height over the water surface.  The metal drops take a moment from
    when the stream breaks apart into separate drops, for surface tension
    to pull those drops together into compact grains.  Also, while the
    drops don't need to be fully solidified when they hit the water, if
    they're still too hot, the metal splashes as it hits the water, so
    instead of solid grains you get little shell like shapes.  That's OK,
    sometimes.  In fact, sometimes they're really pretty, and can be used
    to fabricate funky stuff (though the effect is kind old... It's been
    done a lot before, so don't expect such jewelry to seem new and
    innovative).  But more to the point, i it's splashing enough so the
    metal bits end up as flattened or cup shaped shell shapes, then some
    of them will also likely be hollow and closed over completely.  Those
    can trap a bit of the water, so you end up with little bombs.  If you
    try and remelt those grains that have trapped water in them again,
    they can sometimes explode from steam pressure, scattering your
    melting metal all over the place.  So unless you want to hand inspect
    all your grain to pick out any bits that may seem like they've done
    that, it's better to try and get the melting / pouring temperature,
    height of the pour, and other such factors down to the point where
    you get predictable solid grains. 

    Poured without a perforated crucible, you end up with a wide range
    of grain sizes sometimes.  You can use a bit of screening to separate
    out the very fine sized grains that may be harder to handle.  And pick
    out any large unwieldy lumps if you like.  These then just go back
    into the next melt and get repoured. What's left gets pickled,
    rinsed, tumbled, and you've then got grain good enough to sell as new
    casting grain, if you like.  If using it yourself, skip the tumbling. 

    Note that this applies mostly to sterling silver.  I'd suggest that
    if you're doing standard gold casting alloys,  you need to be a bit
    more cautious of doing all your own grain, since unless you're able
    to control for the degree to which deoxidizers burn away,  you'll end
    up with grain that can vary in casting characteristics from batch to
    batch. 

Hope that helps.
Peter


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