[Jewelry making - Article 00245] - Re: [Orchid] Boric Acid vs. Borax

[James Binnion]

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>             "Boric acid is converted to borax when heated to soldering
>     temperatures." Hi Jim, That sounds reasonable but, if that's true,
>     why do we flux and/or firecoat with boric acid at all? 

    I knew as soon as I sent this I knew I should have been more clear
    and detailed. I apologize for not fully engaging brain before
    posting. 

    What happens is that both borax and boric acid break down into boron
    trioxide at high temperatures (1067F (575C) for boric acid and 1392F
    (765C) for borax). So boric acid does not really break down into
    borax rather they both decompose into boron trioxide and with borax
    there is also sodium metaborate produced as a part of the
    decomposition process. Boron trioxide is the active ingredient in
    the dissolving of metallic oxides. Copper oxides which are our
    biggest problem are converted into copper metaborate when they come
    in contact with the boron trioxide. These metaborates are water
    soluble and are dissolved away in the pickle  after soldering. 

    In the case of borax flux the  copper metaborate mixes with the
    sodium metaborate that was produced in the decomposition of the
    borax and is transported away from the surface which allows more
    boron trioxide to contact the metal oxides on the surface of the
    metal and further reduce the copper oxides. With boric acid on the
    other hand there is only the boron trioxide left after decomposition
    (boric acid H2B4O7 breaks down into boron trioxide and water vapor)
    which is sticky and very viscous at temperatures below 1650F (900C).
    While it still reacts with the metallic oxides to form metaborates
    it is so viscous that the metaborates are not transported away from
    the surface and there fore limits the amount of oxides that can be
    dissolved so it is an inferior flux at temperatures below 1650F
    (900C). On items like high carat  yellow golds there is so little
    copper oxide formed that the boric acid and alcohol fire-coat is
    sufficient to remove them. However on lower carat and red or nickel
    white golds better fluxes and fire-coats (Prip's flux is one good
    choice) will work much better at protecting the surfaces from fire
    stain and and promoting good solder flow. 

    Also as I am sure you know but for the record neither boric acid and
    alcohol or Prip's are soldering fluxes they are fire-coats. They
    provide protection from oxidation on the rest of the piece while
    soldering, you need to use a soldering flux at the area(s) you are
    joining. Many soldering fluxes like Batterns and Handy Flux have
    borax as the main component but they also have other compounds like
    chlorides, fluorides and carbonates added to both reduce the
    temperature that the fluxing action takes place at and to help in
    dissolving the more difficult oxides (like the silicon dioxide
    referred to in recent posts). A lot more information on fluxes ant
    their formulations and actions is available in Brepohl's "Theory and
    Practice of Goldsmithing". 

>      I mean, why not just use straight borax in solution (alcohol)?
>     Wouldn't that do the same thing (and possibly be less expensive)? 

    Unfortunately borax is not soluble in alcohol. Also its melting
    temperature is considerably higher than boric acid (see above) so it
    will provide less protection at lower soldering temperatures. 


>     While I'm asking flux-related questions, I'd like to add another:
>     is there a "correct" alcohol to add to the mix, for fluxing? I
>     seem to recall having been told to use denatured alcohol, each
>     time I've taken classes, but the batch I made up using a can
>     purchased at the local Ace Hardware (with boric acid) immediately
>     coats everything in a black, sugary coating, the second I heat it
>     with my torch. (This is after cleaning the pieces thoroughly, then
>     pickling and neutralizing the pickle in a baking soda-and-water
>     mixture.) Is it the alcohol? The torch (a blazer torch)? Something
>     else I'm not comprehending? Can you please lend a hand in
>     demystifying this for me? 


    I use denatured alcohol (ethanol with something added to make it
    non-drinkable, often methanol) that I get from the hardware store. 
    I don't have the kind of reaction you are referring to . I think
    ethanol is probably the safest and easiest to obtain type of alcohol
    to use (methanol is just too poisonous ).  I don't know what might
    be causing your black, sugary coating  but I wonder if you are
    completely rinsing the sodium bicarbonate solution off of the work
    before soldering. That might be a problem. I use it after pickling
    but follow it with an ultrasonic cleaning and rinse in pure water. 

Jim


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