I’ve conducted a fair number if semi-formal experiments with
different fluxes on sterling and thought I might throw a slightly
different hat into the ring of this worthy topic.
Firstly I have to say that the Pripp’s flux mentioned and seconded
in this thread is worth the attention it gets. It’s pretty good
stuff! Others have made the point and I’ll re-iterate it: covering
the piece all over is a key step toward successfully banishing the
firescale demons.
Ok, that said, the question I had was “can we do better than
Pripp’s” assuming we’re mixing our own formulas. I’ll cut to the
chase and give a hearty recommendation for “The Theory and Practice
of Goldsmithing” by Dr. Erhard Brepohl, trans. Charles Lewton-Brain.
Starting in chapter four (p.122) and continuing in chapter eight
(p.295) Brepohl provides an invaluable examination of the chemistry
of fluxes in the melting and soldering processes. In a nutshell he
tells you how they work, when and why. You might think such a
discussion lofty and uninteresting but seeing his diagrams of oxides
forming and disappearing and then seeing the same thing happen on the
metal in front of you --finally I know what’s happening!-- can be
quite a heady experience.
Ok, the “hat” I promised: after much discussion and tweaking of
formulas Brephol recommends (p.300) what he calls a “melting” flux
of 2 parts borax, 2 parts potash (potassium carbonate) and 1 part
table salt. I mix a 1:1 solution using distilled water. Brepohl
indicates this flux is also good for soldering where firescale is a
concern but I just call it BMF (Brepohl’s Melting Flux).
BMF has been a welcome discovery for me and has much eased my work
in sterling. In my experience BMF seems to sheet better over the
warmed surface of the workpiece than most of the other fluxes. There
is very little puffing or popping while it’s being heated to
soldering temperature. It also seems to last much longer under the
torch. (If you’ve got a good torch this latter point may not be
relevant to you but since I don’t it has been quite important to me.)
BMF also seems to both protect the surface of the workpiece
throughout the soldering process and clean off any other oxides
present. It leaves surprisingly little residue after soldering and
thus cleans off much quicker than most of the alternatives: a dip in
water to quench the piece, a short bath in room temperature pickle
(20 seconds up to a minute or so), a rinse, and a dry. Now you’ve got
a very clean piece and it’s back to work.
BMF isn’t without it’s little drawbacks. First you have to find the
potash (is there a common, off-the-shelf source?). Second, you can’t
use any old salt 'cause things like sea salt are full of other gunk:
nice, clean, table or rock salt seems quite satisfactory. During the
soldering process the fluid solder doesn’t seem as active --as in
willing to flow a lot-- with BMF as compared to others. Finally, at
room temperature crystals tend to precipitate out of the saturated
BMF solution fairly rapidly. For this I warm a vial of the stuff in a
cup of hot tap water, kind of a double broiler effect. Grabbing a
fresh cup of hot water every hour or so is enough to minimize the
precip.
After having used BMF as my flux of choice for almost two years now
I was prompted by reading the earlier contributions to this thread to
mix up a wee batch of Pripp’s again and see how it compared. In my
experience Pripp’s puffs and pops much more than BMF, leaves more
surface gunk (mostly glass and carbony smut), takes longer to clean
off in the pickle, offers less of a surface cleaning action and
doesn’t last as long under the torch. The solder does seem to flow a
little easier with it though than with BMF. As ever, your mileage
may vary.
Trevor F.