[Jewelry making - Article 00144] - Re: [Orchid] Boric Acid vs. Borax

[Trevor F]

========[ Invite a Friend - http://www.ganoksin.com/invite.htm  ]========


>>     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?

    My favorite resource for this type of stuff is Erhard Brepohl's "The
    Theory and Practice of Goldsmithing", translated by Charles
    Lewton-Brain, edited by Tim McCreight (Brynmorgen Press, 2001, ISBN
    0961598492). 

    I've got a marker permanently set to the Melting section, page 123.
    There's a handy little chart there that plots the melting temps of
    the various compounds typically used in homemade fluxes. On this
    chart Boric Acid melts at 575 °C and Borax at 765 °C. My own tests on
    heating a plate with a little pile of Boric Acid and a little pile
    of Borax (not mixed together) has confirmed to my satisfaction that
    Boric Acid does indeed melt long before the Borax does. 

    As to the reasons for including both in a flux such as Prip's I
    quote Brehpohl, Chapter 4, p.123 ("Melting"): "While boric acid forms
    only a thin protective layer on metal at temperatures below 900°C,
    borax has the advantage of greater oxygen absorbing ability. Also it
    starts sooner, becoming active at about 700°C." And later: "It is
    important to remember that the elements of a flux are included either
    to increase the purifying action of the flux or to change the melting
    point of one of the ingredients." 

    There are more details in Chapter 8, p.299 ("Fluxes"): "Despite
    their similarity, borax and boric acid behave differently and
    therefore bring different things to the process. Both materials form
    a glassy melt at glowing temperatures and dissociate to make boron
    triodide, B2O3. This reacts with the oxides of the metals to form
    borates .... 

    At temperatures below 900°C (1650°F), boric acid is deposited as a
    thick glassy layer on the surface of the metal ... While this
    provides some protection against oxidation, the film is so thick that
    it prevents further penetration of additional boron trioxide, which
    would assist in the oxide protection.  This explains why boric acid
    is not recommended as a flux when working below this temperature." 

    When the temperature is raised above that point, as for instance
    when soldering brass and nickel silver, the glassy film becomes fluid
    enough to allow further penetration of boron trioxide. ..." 

    Borax, on the other hand, breaks apart to form boron trioxide and
    sodium metaborate when heated to it's melting temperature.  The
    boron trioxide dissolves metallic oxides to for metaborates.  These
    are dissolved by sodium metaborate and carried away so new boron
    trioxide can arrive at the oxide layer in order to work in the same
    manner ... The process will continue until the boron trioxide is used
    up or the oxide is completely dissolved." 

    As to the subject of boric acid (H3BO3) being converted to borax
    (Na2B4O7) Brepohl mentions that boric acid will, in the presence of
    sodium rich compounds such as soda (sodium corbonate Na2CO3),
    combine to form borax "so it's more efficient and cheaper to start
    with borax in the first place." I don't know if this is what Jim had
    in mind but it would appear that a sodium source would be required
    for the conversion to take place. 

    Finally, and no surprise I'm sure, I can't say enough good things
    about Brepohl's book. Invaluable! 

Trevor F


____________________________________________________________________
T h e   O r c h i d   L i s t
Open Electronic Forum for Jewelry Manufacturing Methods and Procedures
____________________________________________________________________
Orchid FAQ:
~ http://www.ganoksin.com/orchid/faq.htm
Orchid Archives:
~ http://www.ganoksin.com/orchid/archive
Orchid Galleries:
~ http://www.ganoksin.com/orchid/gallery.htm
Invite a Friend:
~ http://www.ganoksin.com/invite.htm
____________________________________________________________________
Tips From The Jeweler's Bench - Article Archive
~ http://www.ganoksin.com/borisat/tip_sear.htm
The Jeweler's Selected Bibliography List
~ http://www.ganoksin.com/jewelry-books
Buy Orchid Jewelry:
~ http://www.ganoksin.com/shop
____________________________________________________________________
-Unsubscribe:
-Email: orchid-request AT ganoksin.com Body=unsubscribe subject=blank
____________________________________________________________________