[Jewelry making - Article 01026] - Re: [Orchid] Silicon Sterling?

[Eddie Bell]

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Dear Andrew,

>     Does anyone out there in Orchidland have experience using silicon
>     sterling? I'd primarily be casting it, but I'd like to be able to
>     solder it, and it would be great to avoid firestain. Enamelling
>     would be nice too, if it's possible. I've used silicon bronze, and
>     appreciate its ability to melt cleanly without flux; does silicon
>     sterling act the same way? Any drawbacks I should know about? How
>     does it withstand tarnishing? Can anybody compare and contrast it
>     with Argentium silver or other de-ox alloys? 

    It is hard to generalize about alloys because there are several
    silicon bearing silver alloys on the market and I haven't cast all
    of them. What can be said is you should expect that the silicon will
    help protect the metal from forming copper oxides because silicon is
    a powerful oxygen getter according to the literature. Your
    experience with silicon bronze is not likely to be what you will
    find with a silver alloy. Large amounts of silicon can be alloyed
    with copper but silver and silicon don't mix. All the silicon-silver
    alloys I have cast are a bit softer than traditional silver-copper
    alloy and the tensile strength is not as high. Some of them have a
    tendency to have thermal cracks or ring shanks crack if they are
    stretched to round up and size. I have attached a picture of a tree
    that was cast with a silicon silver alloy that was brittle as glass;
    however, this degree of brittleness is unusual. 

    Argentium has germanium in the alloy. The element Germanium looks
    and acts a lot like silicon. Germanium makes metal more fluid for
    casting and is a powerful oxygen getter so preventing fire stain
    (copper oxide) and it allows for age (precipitation) hardening by
    heat treating at 300C. 

    For reference, quench annealed traditional silver-copper sterling is
    about 70 HV (Vickers hardness scale) and when age hardened get up to
    140 HV. The quench annealed hardness of Argentium is 55 HV and 70 HV
    when air cooled. After age hardening it reaches 130 HV which is
    comparable to Ag-Cu sterling. 

    Now I would like to bend your ear about fire stain. If you have fire
    stain on sterling it is because it was exposed to air when it was in
    a very hot state. Fire stain is deep seated copper oxide that is too
    deep under the surface to be pickled off. If no oxygen is allowed to
    come in contact with the metal when it is hot there will not be any
    copper oxide. If you are torch melting, put some flux in the bottom
    of the crucible and put the metal on top of it. Make sure your torch
    has a carburizing flame. When you start the heat, never take the
    torch away from the melt until it is cast and cover the sprue button
    with flux or charcoal as soon as possible. Make sure the investment
    is well burned out because any carbon in the investment cavity will
    cause fire stain in silver castings. Never melt silver that has even
    a trace of investment on it and make sure pickle solutions are well
    rinsed away before recycling the metal. Use feed sprues designed to
    allow the lowest possible casting temperatures to prevent investment
    reaction with the silver. 

    http://www.ganoksin.com/ftp/Si-Zn-Pd-Ag-brittle-cast.jpg 

Eddie Bell

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