[Jewelry making - Article 00319] - Re: [Orchid] Twenty MuleTeam Borax

[James Binnion]

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Charles,

    With regards to Boric  Acid (H3BO3) Brephol states that the water is
    driven out at 70C  which decomposes to Metaboric Acid (HBO2) and he
    does mention that Metaboric Acid  has a melting point of 160 C and
    upon further heating to 500 C it decomposes to boron trioxide (B2O3)
    which at 577 C it forms a "fibrous cloak". However melting does not
    mean a low viscosity fluid.  If you will take a couple of
    tablespoons of boric acid and place them in a copper crucible and
    begin to heat the crucible in a kiln you will observe that contents
    of the crucible are a bubbling lumpy granular mass with large
    amounts of water vapor being released from it even at temperatures
    much higher than 160 C and it will not flow at all or cover a
    surface until reaching the 500-600 C range (even at this point it
    has the consistency of taffy) and it is not until much higher
    temperatures (900 C) are reached that it becomes so fluid that it
    can freely move on the  surface of the metal. It is at this higher
    temperature of 900 C  where the fluxing action (boron trioxide
    dissolving metallic oxides to form metaborates) begins. 

    Borax because it contains sodium  decomposes upon heating to sodium
    metaborate and boron trioxide (Na2B4O7 to 2 NaBO2 + B2O3) The
    presence of the sodium metaborate makes the liquid much more fluid
    at a lower temperature 700 C  this presence also provides an
    advantage over boric acid in that it mixes with the newly formed
    metaborates and transports them away from the metal surface where
    the pure boron trioxide from the decomposition of the boric acid is
    much more viscous and tends to hold the metaborates formed by
    dissolving oxides against the surface of the piece and not allow
    fresh boron trioxide to contact the surface of the metal so is not
    as good a flux. 

    So while at lower temperatures boric acid can be used as a glassy
    barrier to keep oxygen away from the metal as in pripps flux (if
    properly applied) it provides no reduction of oxides or fluxing
    action until much higher temperatures are reached. 

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160
http://www.mokume-gane.com
jbin AT mokume-gane.com
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