[Jewelry making - Article 00444] - Re: [Orchid] Flux & Firescale

[Trevor]

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    I've conducted a fair number if semi-formal experiments with
    different fluxes on sterling and thought I might throw a slightly
    different hat into the ring of this worthy topic. 

    Firstly I have to say that the Pripp's flux mentioned and seconded
    in this thread is worth the attention it gets. It's pretty good
    stuff! Others have made the point and I'll re-iterate it: covering
    the piece all over is a key step toward successfully banishing the
    firescale demons. 

    Ok, that said, the question I had was "can we do better than
    Pripp's" assuming we're mixing our own formulas. I'll cut to the
    chase and give a hearty recommendation for "The Theory and Practice
    of Goldsmithing" by Dr. Erhard Brepohl, trans. Charles Lewton-Brain.
    Starting in chapter four (p.122) and continuing in chapter eight
    (p.295) Brepohl provides an invaluable examination of the chemistry
    of fluxes in the melting and soldering processes. In a nutshell he
    tells you how they work, when and why.  You might think such a
    discussion lofty and uninteresting but seeing his diagrams of oxides
    forming and disappearing and then seeing the same thing happen on the
    metal in front of you --finally I know what's happening!-- can be
    quite a heady experience. 

    Ok, the "hat" I promised: after much discussion and tweaking of
    formulas Brephol recommends (p.300) what he calls a "melting" flux
    of 2 parts borax, 2 parts potash (potassium carbonate) and 1 part
    table salt. I mix a 1:1 solution using distilled water. Brepohl
    indicates this flux is also good for soldering where firescale is a
    concern but I just call it BMF (Brepohl's Melting Flux). 

    BMF has been a welcome discovery for me and has much eased my work
    in sterling. In my experience BMF seems to sheet better over the
    warmed surface of the workpiece than most of the other fluxes. There
    is very little puffing or popping while it's being heated to
    soldering temperature. It also seems to last much longer under the
    torch. (If you've got a good torch this latter point may not be
    relevant to you but since I don't it has been quite important to me.)
    BMF also seems to both protect the surface of the workpiece
    throughout the soldering process _and_ clean off any other oxides
    present. It leaves surprisingly little residue after soldering and
    thus cleans off much quicker than most of the alternatives: a dip in
    water to quench the piece, a short bath in room temperature pickle
    (20 seconds up to a minute or so), a rinse, and a dry. Now you've got
    a very clean piece and it's back to work. 

    BMF isn't without it's little drawbacks. First you have to find the
    potash (is there a common, off-the-shelf source?). Second, you can't
    use any old salt 'cause things like sea salt are full of other gunk:
    nice, clean, table or rock salt seems quite satisfactory. During the
    soldering process the fluid solder doesn't seem as active --as in
    willing to flow a lot-- with BMF as compared to others. Finally, at
    room temperature crystals tend to precipitate out of the saturated
    BMF solution fairly rapidly. For this I warm a vial of the stuff in a
    cup of hot tap water, kind of a double broiler effect. Grabbing a
    fresh cup of hot water every hour or so is enough to minimize the
    precip. 

    After having used BMF as my flux of choice for almost two years now
    I was prompted by reading the earlier contributions to this thread to
    mix up a wee batch of Pripp's again and see how it compared. In my
    experience Pripp's puffs and pops much more than BMF, leaves more
    surface gunk (mostly glass and carbony smut), takes longer to clean
    off in the pickle, offers less of a surface cleaning action and
    doesn't last as long under the torch. The solder does seem to flow a
    little easier with it though than with BMF. As ever, your mileage
    may vary. 

Trevor F.




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