Boric acid is converted to borax when heated to soldering
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
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
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!