[Jewelry making - Article 00215] - Re: [Orchid] Exotic alloys

[Peter W . Rowe]

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Bill,

    The japanese technique prevents the metal from just sitting in air
    as it solidifies, absorbing more oxygen.  With many alloys, oxygen
    absorbtion is not significant during melting as the flame protects
    it.  But the only oxygen or gas thats given off while the solidifying
    metal is under water, is gas that's merely dissolved, like CO2 in soda
    pop.  It doesn't affect any oxygen that has chemically combined with,
    for example, copper, to form copper oxides.  Copper containing alloys
    cast this way will have less oxides in them than those just poured
    into an open air mold, but they'll still have the same oxides in
    them from any oxygen absorbed during melting or pouring, until the
    metal reached the water.  it's better, but not perfect.  The water
    bath (actually for the metal, a steam bath) is also a controlled
    cooling rate which can be beneficial for a uniform crystal structure. 

    46irst, the 18K purple gold alloy is 18K gold, which is 75% gold,
    and 25% aluminum.  It's NOT a mix of 18K gold with aluminum. 
    Technically, the perfect mix is more like 22 percent aluminum, but
    that's not a big difference with a strict 18K proportion, and doesn't
    affect the color. 

    As to the japanese method of pouring ingots, I expect it might help
    some, but I doubt it would help enough.  i've not tried it.  When I
    did try to make this alloy, using a torch, I couldn't even get the
    alloy to melt clearly enough to pour.  Just the attempt to melt the
    two metals together formed, as they combined, a progressively cruddy
    gunky mass, not a pourable liquid.  Remember that aluminum has far
    more of an afinity for oxygen than does copper or precious metals,
    and the oxygen it finds combines with it chemically to form an
    oxide, it's not just absorbed, free to be expelled upon cooling.  I
    expect (but don't know for certain) that even only partial oxidation
    of the aluminum in the mix would prevent the proper formation of the
    ordered array structure that gives the exotic purple color.  so far
    as I know, all the folks I've heard of who've made this stuff with
    good results have melted the mix under truely oxygen free conditions,
    such as induction or electric melt furnaces with inert gas shield, or
    under vaccuum.  Note I say melted, not just the ingot mold, which is
    what the japanese method really is.  I don't think a torch is capable
    of doing it. Hydrogen fuel, maybe, but again, I've not tried it in
    many years, so I'm not really sure of that.  On the other hand, I do
    recall having little difficulty melting just aluminum itself, which
    is of course a much lower temperature, in graphite crucibles in a gas
    melting furnace, for varous casting projects. Excess oxidation didn't
    get in the way with that.  It's always possible that something
    similar might work.  But for my part, I have my doubts.  Try it,
    please.  I'd love to find I'm wrong. 20 

    I'll mention at this point one other reference to purple gold some
    may find of interest.  In the 70s, I bought a small paperback book on
    various patinas and the like which mentioned this alloy, by a
    metalsmith/teacher who's name I've long forgotton, as well as the
    book itself, which has long disappeared.  What he'd done was to raise
    holloware vessels in sheet aluminum, and gold electroplate them. 
    Then he'd fire them in a kiln, watching them carefully until the gold
    just started to dissipate into the aluminum surface.  Times right,
    he could pull the piece from the heat at just the point where the
    gold incorporated a suffient amount of aluminum to form that purple
    gold color.  Too much, and the gold was gone, too little, and of
    course there was just the gold color still. an interesting concept,
    but again, not one I've tried.  I don't recall the temps he was
    using, but expect it to be perhaps in the 600 - 800 range, perhaps? 
    Just a wild guess... 

Peter 

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