[Jewelry making - Article 00106] - Re: [Orchid] Recasting old 18K...nasty alloy?

[Peter W . Rowe]

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>     Questions: 1. What might make an old 18k alloy smoke when melted?
>     I guess it is some metal burning off, but what? 

    My guess is a higher zinc content than is normal for 18K alloys. 
    That, combined perhaps with a higher copper content, if the alloy is
    rosier than usual, might easily do what you've described. 

>     2. What might cause the brittleness in the casting? 

    Too slow a cooling of the metal, if it's a rose gold, can do that. 
    Or, more seriously, if the metal is contaminated with iron, lead,
    tin, or some solders, you can also get brittleness.  I'd suggest
    melting the metal with a quite reducing flame (and very good
    ventilation), on a standard charcoal block (hollowed out cavity
    thereon), and when molten, adding a pinch of ammonium chloride, which
    won't melt, but which will skitter around on the surface causing much
    blue fumes.  As it does that, stir the melt with a carbon rod. 
    repeat this, then sprinkle on some boric acid and let the button
    cool.  The Ammonium Chloride will tend to form chlorides of baser
    metals, including iron, tin, lead, etc.  These chlorides are
    insoluable in the melt, and then slag off with the flux, resulting in
    less contamination.  It also removes zinc.  The downside to this is
    mostly the fumes, so do it with good ventilation, or outside.  I'd
    suggest then trying to recast it.  this time try a hotter mold, and
    heat the metal a bit hotter too.  Sounds like it wasn't fully liquid.
     Be sure to use a casting flux of mixed borax and boric acid, or a
    good commercial casting flux. The combination of the two works better
    than either one alone. 

    Also, on the possibility that the casting was brittle from slow
    cooling, after you'ver removed it from the investment, before doing
    anything else, anneal it. Reheat it to a low red, then at just the
    point where it looses that glow, which will be around 900 degrees,
    quench it IN ALCOHOL, not water.  This is a gentler, slower quench
    that water, as some alloys will tend to crack with a water quench.
    The alcohol quench is sufficient to keep it from age hardening as it
    cools.  If it's still brittle, then you'll just have to be careful. 
    In that case, cut off cast jump rings for the cross, and solder on
    new ones made with good wire.  If the heavier cross is brittle, it
    might still survive, but a brittle jump ring or bail is likely to
    break off. When you then cast this, again be sure you've got a gentle
    flame (you're using natural gas or propane with oxygen, aren't you? 
    Not oxy/acetylene, I hope? That can easily overheat your metal, also
    causing problems.  In golds, acetylene can also cause formation of
    carbides if the flame is too reducing, which are hard spots in the
    gold...) 

>     3.    Do you take in old gold to recast? If so, what kind of
>     guidelines do you use in deciding if it is appropriate for you to
>     accept? These rings did not seem to have solder on them. 

    The trouble with old gold is that even if it's the right alloy,
    which you never know, it's not as likely to give a good casting. 
    Usually, the cost savings for using old gold don't make up for the
    reduced quality of the finished piece, so the only real reasons to
    use old gold like this would be sentimental, not economic.  I'd
    actually charge the same as if I were supplying the metal, and in some
    cases, maybe even more, since sometimes you'll have to add a bit of
    your own to have enough for a sprue, which is then scrap you won't
    want to reuse again, but will have to sell the customer, or scrap
    out.  Customers should be warned that the finished casting will
    almost certainly have more porosity than normal, in some cases it can
    be quite serious, more spongy and full of holes than they'd expect. 
    That's what you tell em.  Along with possible brittleness and other
    casting faults.  If they then still want to proceed, they've been
    warned.  And though you've told them to expect porosity and crappy
    castings, as often as not, they turn out somewhat better than that,
    and sometimes are just fine.  Depends on the alloy and your luck that
    day.  If they're told there are risks up front, then no matter how
    bad it turns out, they've been warned, and any success is better than
    they may have been led to expect. 

>     4. What should I tell the clients to make them feel better about
>     this little disaster? I will tell them the truth, of course, but
>     in how much detail? 

    Tell em the truth.  No sense smoothing over it.  But you might want
    to retry it one more time before telling them it's a total failure. 

>      5. Is there any reason I should not feel badly about this? I
>     thought I could do it and that it would work ok, but now I'm
>     feeling kind of incompetent and fearful about trying it again. What
>     if there's cadmium in the alloy? Why risk my health just to attempt
>     to please some client? 

    There's not likely to be cadmium in the alloy, unless it's
    contaminated with solder manufactured since WW2.  Even then, amounts
    will be very small, and you probably burned most of it off the first
    time, if there was any there.  but there may be other metals there,
    and even with new good golds there are componants you don't want to
    breath, so you should always melt metal with good ventilation.  Zinc
    fumes, flux fumes, etc, aren't good for you.  I doubt your metal
    presents a special case in this regard.  As to feeling bad, well,
    that's normal.  But don't dwell on it.  You cannot predict how unknow
    metal will react till you try it.  Sometimes even supposedly good
    metal can "go south".  I recently had a commercially purchased bit of
    14K nickel white gold that was just terrible.  We'd cast it once,
    which was fine, but now attempting to fabricate parts with the sprue
    metal, was a total failure.  I couldn't even get the stuff to pour a
    good ingot, and I'm not exactly a beginner at this.  Hollow
    sections, big shrinkage voids into the side, sludgy pouring
    characteristics.  And the portions I could get to cast wouldn't roll
    into good metal either.  I've no clue what the hell got into that
    melt.  Something, I'm sure.  But beats me what.  The full ounce of
    that crap is in our refining bin now...  Not worth the time to fool
    around with it.  I just got another sprue button and used that, with
    no problems.... 

>     6. Is this why some jewelers don't take in old gold to recast? - 

    Yes 

Peter Rowe 

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