[Jewelry making - Article 00327] - Re: [Orchid] Technical question about firescale

[James Binnion]

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Marylin,

    Yes firestain can travel quite deep into the sterling with repeated
    heating. Oxygen is a very small atom compared to silver atoms and
    this allows it to diffuse into the spaces between the atoms in the
    silver matrix. Silver can absorb huge amounts of oxygen when heated,
    when molten it can hold 20 times its volume in oxygen. The silver
    will release the oxygen when cooled but the copper in the sterling
    will not as it forms a stable compound with oxygen. This red oxide
    of copper is the firestain you see. 

    Try using Prips Flux (look up recipe in archives). It will last much
    longer under the heat of the torch than the boric acid alcohol fire
    coat. I find the boric acid alcohol mixture to be almost useless in
    controlling fire stain for silver soldering of any but the smallest
    of pieces. But even Prip's Flux has its limits if too much oxygen is
    allowed to get at the work while hot. So to help with this try a
    fuel gas/compressed air torch rather than an oxygen/fuel gas torch.
    This type of torch gives you the ability to create a large bushy
    flame that is much cooler than a oxy/gas torch and you can keep most
    if not all the work covered in this flame while heating. By using a
    neutral to slightly reducing flame that is totally covering the work
    you will greatly reduce the amount of oxygen present and therefore
    reduce the fire stain. I use the National 3A-B torch
    (http://www.nationaltorch.com/Ntorch.html) along with a "N" series
    tip with propane and compressed air when I solder silver where I am
    concerned about fire stain. One word of warning, the torch can be
    used with either oxygen or compressed air but if it is ever used with
    compressed air then it must not ever be used with oxygen or it could
    cause a fire or even an explosion due to the oil and dirt present in
    compressed air that is deposited on the internal surfaces of the
    torch. 

    The other thing that can help if the firestain is relatively minor
    is to bring up a layer of fine silver by repeated heating and
    pickling of the work and brushing the work with a soft bristle brass
    brush and soapy water between cycles. This coating will not stand up
    to much polishing so is best if done after polishing and leave the
    work with a brass brush finish which is a fine, soft matte finish 

Jim

James Binnion
jbin AT mokume-gane.com
James Binnion Metal Arts
http://www.mokume-gane.com
360-756-6550

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