[Jewelry making - Article 00064] - Re: [Orchid] Overheating Yellow Gold

[Peter W . Rowe]

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>      following some threads in the archives I had found that one should
>     either heat gold up to orange for 30 seconds or 30 minutes so had
>     sort of split the difference, to my regret because cracks
>     invariably formed on rolling out. My question is, once yellow gold
>     has been overheated, is it possible to start again and make another
>     ingot or does the gold have to be sent off to a refiner. It is only
>     about half an ounce so.. 

    Peter, overheating the metal during annealing causes the crystal
    structure to grow in crystal size. That metal, with large crystals,
    is not then as strong as when one anneals correctly. But it should
    still be workable, just as you can roll a newly poured ingot, which
    may also have unevenly sized or larger crystals. If you've got
    cracks forming unduly, try less of a reduction between annealing. 

    If this still doesn't work, then re pour the ingot. Over annealing
    by itself shouldn't cause such harm to the composition of the metal
    as to require refining. Be sure, when you melt the metal, that you're
    not overheating the melt, and that you're using a proper flame
    (usually slightly reducing is what you want) as well as suitable
    melting fluxes.

    Now, if in originally pouring your ingot, you mixed in foreign
    materials, or heated the melt so hot that you volatilized some
    components of the alloy, you might have significantly altered the
    working properties. I've had some white golds in particular, that got
    contaminated in some unidentified part of the process, and which I
    simply could not get to roll or draw out at all.  So it's 

    But I'd suggest you first try just re pouring the ingot and rolling
    again. Note that it's not all that unusual to have a few bad spots in
    an ingot. We all get them. Often one can just work around them, or
    trim them off as they become apparent, and continue rolling or
    drawing. With Yellow golds, they are often more salvageable than
    white golds or platinum if you've messed up the alloy. 

    It can help to melt the metal in a hollow carved in a charcoal
    block, and in addition to using normal borax or boric acid flux, add
    a little bit of ammonium chloride after the metal is fully melted. It
    will skitter around the surface of the metal for a moment, then is
    gone, at which point you pour the ingot. The ammonium chloride is a
    "refining flux", which is able to bond with some baser metals in an
    alloy, forming metal chlorides. In general, these are insoluble in
    the melt, and slag off. The components mostly affected are those
    which are most reactive, so the ammonium chloride can help remove or
    reduce the amount of such contaminants as tin, iron, or the like,
    from your melt should these have gotten in there by accident. And
    the charcoal block combined with a reducing flame and your melting
    flux will do a good job of greatly reducing any dissolved oxides in
    the melt. 

    If melting on a charcoal block to clean up "dirty" metal, I usually
    melt it till it's a clear liquid pool, then add more borax/boric acid
    mix (I use a 50/50 mix of the two) to form a glaze over the button,
    which is left on the block to solidify because it's harder to pour
    cleanly off a charcoal block. When solid, it should then be clean
    metal, to be transferred to your normal pouring crucible and the
    ingot poured. The charcoal block may be overkill, but on some
    occasions I have it salvage a bad piece of metal, in conjunction with
    the ammonium chloride. 

    Also, you may find that if you're using an open faced ingot mold,
    your metal will be more prone to crack than if you use a closed
    mold. The type with two sliding plates that pours either an
    adjustable width flat ingot, or with the plates reversed, one of
    several sizes of round rod, is my favorite type of ingot mold. Ingots
    poured in these are a lot easier to get to roll out without cracking,
    than the variable thickness, oxide damaged surfaces, on the ingots
    poured into open rod or plate molds. 

Hope that helps.

Peter Rowe

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