[Jewelry making - Article 00056] - Re: [Orchid] Red gold pickle

[Peter W . Rowe]

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>        I wouldn't recommend this ..... Red golds , esp. 18k
>     gold/copper binary alloys need to be quenched while still very hot
>     , they should be quenched from Red Heat. They can harden so much ,
>     if cooled too slowly that you can have castings break ! 

Mark,

    That's hotter than you need, and can cause cracking just from
    thermal stress, especially in castings which may have a rather coarse
    crystal structure, and are thus weaker.   Red golds get brittle from
    formation of an ordered array structure upon slow cooling, but this
    structure doesn't start to form at your higher/red hot quench temps. 
    You can safely cool the red golds to around 800F or so with no
    hardening taking place yet.  Then quench, preferably in alcohol, or
    perhaps boiling water, so as to not cause stress cracking.  Either
    alcohol or boiling water will still chill the metal fast enough to
    prevent that hardening.. 

    If you look up a phase diagram for copper and gold binary alloys,
    you'll see two inverted parabolic narrow zones ,  one centered on
    around 18K corresponding to a formula of AuCU, which forms at temps
    below about 760F,  and another a bit to the left of that,
    corresponding to a formula of either AuCu2 or AuCu3 (I forget which,
    right off the top of my head...) which starts to fom at a slightly
    lower temperature.    Those are the stable zones for the ordered
    array structure.  

    By the way, just for fun, any of you who have copies of Diego
    Pinton's "Jewelry Technology", take a look at the diagram of the
    copper/gold phase diagram on page 327, figure 380.  Then tell me,
    "what's wrong with that picture?".  Unless I'm missing something
    essential, that diagram has a couple fundamental errors, which mess
    up a bunch of interpretations of the thing.  Anyone else see what
    I'm seeing there?  (grin)  If you're not sure, check another source
    for that diagram, such as T.K. Rose,  "metalurgy of gold".   (If
    anyone wants me to email them those diagrams, let me know.  I can
    scan them and send you the scans. Probably best with a higher speed
    connection...). 

Peter Rowe



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