What is the difference between Boric Acid and Borax for fluxing the
melt for casting? Is there a better choice?
What is the difference between Boric Acid and Borax for fluxing the melt for casting? Is there a better choice?
Chemically they do pretty much the same thing. The difference is that
borax melts at a lower temperature, and starts being active at that
lower temperature, but then also doesn’t survive as long at higher
temps. Boric acid becomes active at a higher temp, and lasts longer
at higher temps. Borax is more commonly used as a soldering flux
(typically a lower temperature range), while boric acid is more
commonly recommended as a metal melting flux. Some people I know use
a mix of the two, with more boric acid than borax, for melting. Not
sure it makes too much difference. I prefer straight boric acid for
melting metal.
Boric acid starts working at lower temps than borax. I normally use
only borax for casting and a mix of the two for soldering.
Jim
James Binnion
James Binnion Metal Arts
Hi Peter,
Chemically they do pretty much the same thing. The difference is that borax melts at a lower temperature, and starts being active at that lower temperature, but then also doesn't survive as long at higher temps. Boric acid becomes active at a higher temp, and lasts longer at higher temps. Borax is more commonly used as a soldering flux (typically a lower temperature range), while boric acid is more commonly recommended as a metal melting flux.
Where does this idea come from? It seems to be common in the craft.
Borax has a much higher melting point than boric acid.
Borax melting point is 741 C / 1365 F
I have found different melting points listed for boric acid but the
highest one is 575 C 1067 F I have also found this which think may
be more accurate. Boric Acid begins to disassociate (looses its
chemical water) at 170 C / 338 F and turns into metaboric acid which
melts at 236 C /456 F.
Jim
James Binnion
James Binnion Metal Arts
There was a huge discussion a few years ago on Orchid about this.
What comes to mind was the importance of not fire coating sapphire or
ruby with borax because it will etch the stones when retiping. If I
remember correctly boric acid & alcohol does not cause this same
problem to occur.
Where does this idea come from? It seems to be common in the craft. Borax has a much higher melting point than boric acid.
Thanks Jim for the correction. Not sure where the idea comes from.
Just always heard it as the reason for boric acid being more used for
casting flux and borax used a soldering flux, and that as a
flux/solvent for oxides, borax became active at a lower temp. Never
really questioned it. Apparently I’m wrong,… So thanks for setting
me straight. What, then, IS the difference, and what accounts for
that convetional customary usage difference?
Peter
Where does this idea come from? It seems to be common in the craft. Borax has a much higher melting point than boric acid.
Yes. My chemistry tables say borax melts at 741C, boric acid at
185C. The boric acid melt is complex, with some changes starting at
130C.
Al Balmer
Sun City, AZ
And also, the Borax…is the the supermarket 20 Mule Team Borax (in
the US) or some different/speciality grade?
Thanks,
Linda
Surprise surprise, but I found an old posting by the ever helpful
always posting Jim. Thanks for the explanation, and
book recomendation. I purchased it off amazon with your link.
Not to over simplify your explanation but the jest that I got was
that Borax was a better choice for the higher heat of casting and
that the added breakdown element in boric acid was better for
soldering.
There was a huge discussion a few years ago on Orchid about this. What comes to mind was the importance of not fire coating sapphire or ruby with borax because it will etch the stones when retiping. If I remember correctly boric acid & alcohol does not cause this same problem to occur.
There may be a difference in how aggressively the boric acid attacks
/ dissolves aluminum oxide, but boric acid does still present a
danger to sapphires. Perhaps not as much, though I’m not sure. But
I’ve had it damage stones just like borax (though I don’t often use
borax as a fire coat, so my experience comparing it with boric acid
is limited.) The actual soldering fluxes, especially those white
paste fluxes, are more more of a risk, mostly likely because as
fluxes specifically designed to dissolve metallic oxides, aluminum
oxide (sapphire) would be much more in their cross hairs…
Peter
I saw a few more posts to follow up on my question and I did give
the ever helpful ubiquitous Jim my thanks. Thought I would repost his
original since it was so helpful. (Jim, do you get a royalty every
time its posted?)
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.
thanks again Jim!
OK,
I guess I need to ask again. Is the Borax used for our purposes the
same Borax found in supermarkets (as in 20 Mule Team Borax)?
Thanks,
Linda Kaye-Moses
I guess I need to ask again. Is the Borax used for our purposes the same Borax found in supermarkets (as in 20 Mule Team Borax)?
Yes, there is a small amount of anti-caking agent in it but it does
not affect it’s use as a flux.
Jim
James Binnion
James Binnion Metal Arts
Hi Linda,
I guess I need to ask again. Is the Borax used for our purposes the same Borax found in supermarkets (as in 20 Mule Team Borax)?
According to 20 Mule Team’s own website
(http://www.20muleteamlaundry.com), their borax is 100% borax, ie
identical to the type you might buy from a chemical supply company -
and probably cheaper. So, yes, it will suit your purposes as a
jewellery-making flux.
Helen
UK
Yes, there is a small amount of anti-caking agent in it but it does not affect it's use as a flux.
Generically speaking: If you’re in a chemistry lab and you want
acetone (or borax), you are likely using it to do chemistry - atoms
and molecules bonding together to form new molecules - and any
impurities are going to affect or even destroy the outcome. Those are
the high purity chemicals, and why they are needed.
When you are using acetone to clean a paintbrush or borax on a
casting, just about any old reasonable purity will do - 20 mule team
borax, hardware store acetone, all of which is actually “technical
grade”. In jewelry it’s rare that we need any higher purity than
that. You might buy reagent ammonia or nitric out of convenience or
necessity, but you don’t NEED it like a chem lab does. If you’re
doing something that requires high purity, you’ll probably know it,
and know why, already.