[Jewelry making - Article 00033] - Re: [Orchid] Unresolved issues with casting

[John Burgess]

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>                 I worry about my crucible, which has been turning red
>     / deep brown inside. Till now, I have been casting Sterling. I do
>     not know the origin of the red substance. Could someone please
>     explain what it is and whether it is supposed to be there or not? 

    G'day.  The red-brown colour is a copper/oxy/boron compound  which
    is derived from the copper in sterling and low carat gold alloys.  
    There are two oxides of copper; one is red-brown cuprous oxide  and
    the other is black cupric oxide, and that is the one which produces
    the black, dreaded firestain on sterling. These oxides form when
    sterling or just plain copper is strongly heated in the presence of
    air, and the purpose of borax type fluxes  is to dissolve these
    oxides, thus keeping clean the surface of the metals be soldered or
    otherwise worked when heated.   The presence of this coloured borax
    related compound  is quite normal, but if the build up of flux in the
    crucible becomes too great the excess may be removed by heating the
    crucible very strongly indeed  and allowing the flux to slowly run
    out - it has the consistency of molasses on a cold day. 

>     Would it (still) be safe to use the same crucible for gold?; 

    The answer is, "probably" , but I think that one should keep a
    crucible for gold and another for silver to be sure; they aren't all
    that expensive. 

>         Working with a Little Torch, I use a tip with six orifices
>     (for propane/oxygen) This is certainly a big improvement, but it
>     remains problematic to melt 45 - 50 gr. of Sterling. It takes a
>     long time before the metal begins to melt 

    It occurs to me that you may be using the form of crucible which is
    the tall, hollow form. These are really better for use in a kiln, but
    for torch heating small amounts up to about 100 grams of metal, the
    type of crucible to use is a flat slab of ceramic which has a shallow
    hollow to contain the metal.  Thus the flame may be played directly
    on to the metal, and a sprinkling of powdered borax on the pool of
    liquid metal will instantly remove any scum of oxides which may have
    formed, and allow it to run freely like mercury.   This heating
    method is far faster and will result in a lower formation of oxides
    because you aren't heating it for so long. 

>     and, also, I have difficulties to obtain a reducing flame -
>     whatever I do, the flame doesn't seem to be a really reducing one. 

    If you could cut off a very small amount of air entering the torch,
    using say, copper or aluminium foil, you can control the
    oxidation/reduction properties of the flame, or in the case of an
    oxy/fuel gas torch, simply cut the amount of oxygen  fed in. 

>        I have been working in the garage and it was really cold there.
>     Could that have something to do with it? 

    No; the ambient temperature of the working area has little to do
    with melting temperature. -- Cheers for now, and may 2003 be extra 
kind
    to you John Burgess;   johnb AT ts.co.nz of Mapua, Nelson NZ  

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