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Boric Acid vs. Borax


#1

Hello all, A quick question if someone would please clarify for me…

I have always used boric acid + alcohol as a fire coat. Borax is a
different material isn’t it? I only use borax to cure crucibles
before melting…a very glassy coating.

My concern, from reading on this forum, borax can be the cause of
damage to some sapphires and rubies by etching the surface.

Would someone please confirm for me that they are different…Or
correct me if needed.

Thank you,
Mark


#2
   I have always used  boric acid + alcohol  as a fire coat. Borax
is a different material isn't it?  I only use borax to cure
crucibles before melting...a very glassy coating.

Borax and Boric acid are indeed different materials. But they are
related, and both contain boron. In use, borax melts, and becomes
active as a flux (dissolving oxides) at a lower temperature than does
boric acid, so it will etch rubies or sapphires more aggressively,
and at a lower temperature.

   My concern, from reading on this forum, borax can be the cause
of damage to some sapphires and rubies by etching the surface. 

Indeed it can. borax, boric acid, and any of the soldering fluxes
that I can think of, can all damage ruby or sapphire in this way.
But they won’t do it all at the same rate. Borax is worse than boric
acid, and some of the highly active paste fluxes are even worse than
just borax.

I too, use boric acid powder in alcohol to coat goldwork prior to
heating, as a fire coat. If working very near (as in retipping) a
sapphire or ruby, I try to remove as much of the boric acid as I can,
but generally, there will still be traces. Usually you can get away
with this if you’re careful not to direct the flame actually at the
stone, but just at the metal, so the stone remains at a low enough
temperature that the traces of boric acid don’t start to etch
noticeably. But if you actually get the stone glowing, then the
chances of damage are much greater even with just boric acid, not
flux or borax. It’s also quite dependent on how long it’s at those
temps.

Peter


#3

Boric acid is converted to borax when heated to soldering
temperatures.

Jim


#4
         Boric acid is converted to borax when heated to soldering
temperatures. 

Jim Say what?? Since Boric Acid is oddly enough Acidic and Borax is
Alkaline How can one be converted to the other, if this be the case
why do some casting fluxes mix Boric Acid and Borax together to bring
the mixture to a neutral PH, and why can you use Boric acid with the
Anti Fire Scale Sterling grain, but not Borax?? Or MATTS casting
flux Please help me with this one Ken


#5
Boric acid is converted to borax when heated to soldering 
temperatures. 

G’day; Boric acid is an acid, H2B03; or, hydrogen borate Borax is
SODIUM diborate, in other words, the hydrogen in boric acid is
replaced by the metgal sodium.

Cheers for now,
John Burgess; @John_Burgess2 of Mapua, Nelson NZ


#6

The normal form of boric acid is the orthoboric form (H3BO3) which
is hydrated boric oxide( B203) . When ortho boric acid is heated
slowly it loses water and becomes the metaboric form (HBO2). Borax is
the sodium salt of boric acid and is the primary natural ore. It
is not formed by heating boric acid . These materials are very
hydroscopic and the crystalline forms have various amounts of
water of crystallization. They are all glass formers which can
act as a flux to dissolve other oxides . Metalurgical fluxes
are generally borates , silicates , and could include phosphates.
All are glass formers. The more active flux mixtures will usually
contain a fluoride and or chloride salt. The proprietary fluxes
are all sort of “witches brew” mixtures . They are not a single
compound and there is no real “magic” right composition although
some maybe better than others.

The main use for these borates,silicates and phosphates is as
cleaning agents a their use as fluxes in glass making and in
metallurgy is relatively small in comparison . jesse


#7
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 mean, why not just use
straight borax in solution (alcohol)? Wouldn’t that do the same
thing (and possibly be less expensive)? 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?

Many thanks, Doug Douglas Turet Turet Design P.O. Box 162 Arlington, MA
02476 Tel. (617) 325-5328 eFax (928) 222-0815
anotherbrightidea@hotmail.com


#8

Boric Acid changes into Bortrioxide (B2 O3) at a temp. of 500 C.
Borax splits into Natriummetaborate (NaBO2) and Bortrioxide at 878
degrees C.

Bortrioxide is the active anti-oxidation part and dissolves
metaloxides into Borates, commonly seen as brownish/reddish slurry
on silverwork. Borax has already good oxidation-diffusing qualities
at temps from around 700 degrees C, before it is actually split into
the above. For the goldsmith all that counts is that Boric Acid is a
antioxidation agent (protective coating) and Borax is a flux (with
some antioxidation abilities).

Traditionally have mixtures of the two been used (3 parts Borax to 1
part Boric in methylated spirit and some water) to maximise the
effects and create a protective flux whith a meltingpoint of 620
degrees C. A word of warning: do not use this flux as a protective
coating on stones. Particularly corundums as the Borax tends to etch
the surface of these stones under heat. Straight Boric Acid and
methylated spirits mixed to a paste, will provide good protection on
diamonds, corundums and whitegold (especially Nickel-Whitegold).

At my bench I use a commercial flux like Fluoron or Auflux for all
goldwork and use the straight Boric Acid to protect the stones and
the prepolished work. For silver I now use Dandix, a non-fluoride low
temparature flux paste which works fine.

Hope this is not all too Bor-ing.

Juerg P. Muff t/a O R B I T Contemporary JEWELLERY
Award Winnign Designs
Q U E E N S T O W N
NZ


#9

Doug, I also use denatured alcohol (also purchased at Ace Hardware)
and boric acid (purchased at pharmacy) in a saturated solution and
after burning off the alcohol, mine pieces are covered in a white
powdery coating. Are you sure you got denatured alcohol? Can’t
imagine what is making it turn black.

Kay


#10
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


#11
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. 

Either the can you purchased is NOT just plain denatured alcohol, or
the powder you mixed with it is NOT boric acid, or the metal you’ve
been coating with it is already covered with some other substance.
some lacquer or something that will burn. The alcohol should burn off
cleanly. Try taking just a bit of it, by itself, say in a teaspoon,
the burning it. It should burn cleanly, leaving no trace in the
spoon. If that works, melt just a bit of the boric acid powder on a
bit of scrap metal. It should melt cleanly, staying a light color as
it melts. no black carbony look. The alcohol serves only as an easy
means of distributing a light even layer of boric acid on the surface
of the metal, which then melts to protect the metal when you heat it.
It shouldn’t turn dark, at least not on jewelry metals (silver and
gold alloys). Base metals might oxidize so aggressively as to just do
so in spite of the thing boric acid layer, and then you’d get a black
oxide surface. But that doesn’t sound like what you describe.

Peter


#12
        "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 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


#13
    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. 

I have had a similar situation using the 'Methylated Spirit’
available over here. It is the same mixture of Ethanol and Methanol
but also has a purple dye added and it is this dye which does not
seem to burn away. I would hazard a guess that the ‘black, sugary
coating’ is actually crystals of borax coated in the concentrated
dye.

Best wishes,
Ian
Ian W. Wright
Sheffield UK


#14

Hi Jim, I appreciate your knowledge on this subject and hence I am
taking the liberty to address my questions directlyto you and thank
you in advance for giving me the opportunity to do so. In fact I had
posted a query, but for reasons best known to the co-ordinators it
was titled as oxidation resistant metals ( whereas it was a flux
related post ). Well, my query is, what are the most common
chemicals, compositions, formulations which can/are used as fluxes (
for Silver alloys and Nickel Silvers) ?. Is there any other site
where I could get indepth as regards this subject. I
would really appreciate your feedback.Thank You Nilesh.


#15

Nilesh,

It is best if you get a copy of Brephol's "Theory and Practice of

Goldsmithing" (ISBN 0 9615984 9 2) it has more than any
other reference I know of on fluxes and their use.

Price: $70.00 Hardcover: 560 pages ; Dimensions (in inches): 1.75 x 10.00 x 8.00 Publisher: Brynmorgen Pr; (October 11, 2001) http://www.amazon.com/exec/obidos/ASIN/0961598492/theganoksinpr-20

Jim