The bottom line here is that trying to get fire stain/scale
protection from borax in denatured ethyl alcohol or grain alcohol
(180 proof everclear) is pretty much a waste of time and materials.
Boric acid on the other hand works just fine and both work much
better in methyl alcohol
Without arguing at all with your solubility notes, I might add two
more. One is that whatever the usefulness of boric acid over borax,
it should be noted that almost universally in the jewelry trade, the
mix used is ordinary denatured alcohol (since it's available and
cheap) along with boric acid powder.
The solubility of either chemical in the alcohol is actually only a
minor concern most of the time, since most of us (well, at least the
jewelers I've known, and sources I've read on the subject) don't use
this mix as a clear solution of dissolved chemical in alcohol.
Rather, using powdered boric acid, we usually mix a slurry. Some of
the boric acid may dissolve, but it doesn't matter, since the alcohol
is mostly there as a carrier, which carries the slurry of
undissolved powder just as well as it carries the percentage which is
dissolved. This practice means that any of the alcohols would work
about the same, unless one is expecting good protection from only the
clear alcohol without suspended undissolved particles. And I've found
that doing that sometimes doesn't put quite enough boric acid on the
piece to give the full level of protection one might wish (though I
admit I've not tried it with methyl alcohol). Doing it that way might
be the practice and sufficient for some, but it's not what I've
observed in commercial practice. Mixing the alcohol and powder to a
slurry means it would work just as well to put a coating of borax
powder on the metal too, if starting with sufficiently
ground-to-a-powder material. And the alcohol is used primarily
because it either evaporates quickly, leaving a protective coating,
or can be burned off even quicker. Burning it off has the advantage
that then you don't have as much annoying and potentially toxic
alcohol fumes floating around. The alcohol also perhaps offers some
effect as a cleaning/wetting agent, allowing the boric acid to not
have to fight through fingerprints or minor oils, etc, as much,
though I'm kind of guessing there.
Also, regarding RER's comment to not use laundry aisle borax, I'd
suggest that even though it has additives, those don't seem to make a
difference. I've used that very cheap grade of borax (20 mule team
brand from the laundry aisle) for making prips flux since I first
learned how to make it in '72. Never had any complaint, nor noticed
any difference between my mix made with that, and the mix made by me
or others (such as in school) made with technical grade chemical.
The main difference I see is that with the laundry aisle stuff, when
you go to pour some from the box, it pours without you needing to
work so hard to break up the big lump the box would otherwise have
turned into while sitting on the shelf... The same, by the way, can
be noted with regard to useing hardware store sodium bisulphate, sold
for pool and spa water conditioning, as pickle. It too has
anti-caking additives so it doesn't form clumps. But those don't
seem to affect it's function as pickle.
If Its a question of what works best in the studio borax is the
... In fact many silver solders melt at temperatures below the
melting point of borax so using pure borax as a flux for silver
soldering will often be a frustrating experience.
True. But here, RERs comment has substantial historical precendent,
as the use of a just plain borax slurry or paste, obtained by
grinding a borax cone into a slate dish with water, is indeed a
longtime traditional silver soldering flux, the use of which can
still be found mentioned in some contemporary books on silver work.
Whether it's the best way, of course, is a whole other question,
which you address well, as usual. :-) But I recall in my very first
jewelry classes, in high school in (I think it was...) 1968 (whew.
that long ago?), that borax cone and slate dish was the flux we most
often used. There was also some blue liquid flux the teacher had,
and would bring out if someone was having trouble, but most of the
time it got hoarded since it cost more...