The answer is to not get the firescale in the first place.
Pripps flux is a very effective way to do that. Properly
applied to the silver, your finished soldering operation will
leave a slightly frosted white surface, but without any
subsurface fire scale. It will polish up just fine. And the
gold surfaces, if prepolished, will stay that way. If you need
info on using pripps, let me know, and I'll email you the
article I wrote on the subject a number of years ago.
Well, several people wanted that info, so I dug it out of my old
files again, and may as well repost it now, for those to whom
this may be new. I first posted this, I think on the
rec.crafts.jewelry newsgroup, back some time in '95, I think…
Pripps Flux
Pripps flux is a mix you make up yourself, and does pretty much
the same as a borax coat which is the older more traditional
method. Classic silversmiths would often go through several
sequences of “burning on” a borax coat before annealing or
soldering, but it doesn’t work quite as easily or as well as
Pripps. I’ve been a gold and silversmith since college, and
learned Pripps from prof. Fred Fenster at the univ. of Wisconsin,
who proclaimed in that 1972 sophomore class that at other schools
people sometimes complained about fire scale, but “here at U.W.
it never gives us a problem”. This, by the way, was taught from
the first moment we were shown how to light a torch, just to
give you an idea of how important/basic a technique Fred felt it
was. It’s named after Jack Pripp, who taught at Rochester for
many years, and is considered one of the “fathers” of the
american metalsmithing community.
The recipe: A quart of water, 120 grams boric acid, 80 grams
each TSP and borax. Boil to dissolve (you might have to add a
little more water… It’s the 3:2:2 ratio that’s important, not
the concentration.) The Borax you can get at the supermarket,
in the laundry area. Borateem is just borax. The little green
flecks they put in there too don’t seem to matter. TST
(trisodium phosphate) is a strong cleaner/alkali often used in
cleaning walls and the like before painting. You can usually get
it in paint or hardware stores, but be sure it’s actually TSP.
Because it’s rather a caustic (though still reasonably safe)
material, some stores carry a substitute, which may be
confusingly labeled. Read the box carefully. The substitute
doesn’t work. If you happen to have a chemical supply house
around, you can also use Disodium or Monosodium Phosphates. But
the Trisodium formula seems to be the most common.
You apply it (and this is an important detail) by spraying it on
the silver while gently heating the silver up enough so the spray
dries on contact, as opposed to hitting as a liquid and
bubbling/boiling off. The best sprayers by far are the cheap
little two-tube-with-a-hinge mouth atomizers that ceramics folks
use for applying glazes. It gives a much finer and more uniform
spray than any sprayer bottle I’ve seen, and cannot clog. In
use, you’ll gently brush the metal with the flame, then in quick
short puffs on the sprayer, put the Pripps flux on a little at a
time. The idea is to coat the entire piece with a thin (thick
enough so the metallic reflections of the metal are no longer
visible, but not more) white crusty coating. Be careful as you
do this, not to either let the metal cool so much that the flux
stays liquid (it doesn’t coat evenly then), or the metal gets so
hot that it starts to discolor. Its easy enough, but takes a
little practice a first. You do all parts of your assembly, then
let cool, set up the joint, and with the addition of the
smallest amount of additional soldering flux only in the joint
(see below) and solder, do the soldering job.
Pripps is a much less “active” flux than the paste fluxes, and
doesn’t burn off easily (though with enough overheating you can
do it), so it gives continuous protection, and thereby completely
prevents fire scale. It will work as a soldering flux all by
itself IF your metal and solder are both completely clean before
you start, and if your “heat control” is good. Paste fluxes
such as “Handy” or Griffin, oddly enough, seem to provide little
or no fire scale protection. If fact, with some metals (like
white golds) you’ll find the fire scale is worse where the flux
was. This is why you don’t want to use much, and keep it only in
the join area. But they are so very active while still fluid and
“there” that they greatly promote solder flow, so many of us use
them anyway. Batterns self pickling flux is somewhere in
between. lasts longer, works fairly well, doesn’t give quite the
fire scale problem, but also doesn’t protect quite as well. In
my work, for simple repairs to already made silver jewelry, I
usually just use a boric acid/alcohol coat, solder with paste
flux, and clean up later, as most of these pieces usually already
have fire scale, and for a single quick ring shank solder job or
what have you, it’s not worth the trouble to bring out the
Pripps. But if I’m making something from scratch, with a few
exceptions, then every last annealing or soldering step is done
with pripps coating everything. The added time and bother is
more than paid back when it’s time to finish the piece, and
there’s no surface oxide and no fire scale, and the piece can be
polished out with the ease of gold work…
By the way, the coating, if you are careful and don’t pickle it
off after soldering, can usually last through several soldering
cycles, so for some complex assembly, if you’ve got everything
fitted before hand, you may only need to coat the parts once for
a number of sequential soldering steps. Also, since the sprayers
tend to cover rather more area than just your silver (like tools
and bench areas behind your soldering area, you will want to set
up some sort of simple shield behind the area you’re using for
spraying on the flux to catch that overspray. Saves a lot of
mess…
enjoy.
Peter Rowe
