Any help would really be appreciated. And one additional
question, how does flux promote the flow of solder and does oxygen
play a role in that process?
Carol, these are indeed a couple of the key points.
Oxygen combines with sterling silver to cause copper oxides. It
does the same with silver solder. The oxides will prevent solder
flow, and any solder that attempts to flow over the oxides, not only
won’t want to do so, but if forced physically over them, won’t stick
properly. You NEED the flux to not only prevent oxide formation, but
to dissolve even traces of oxides that may already be there. Solder
flows properly only when both it, and the silver, are chemically
clean. Dirt, or oxides, will prevent solder flow. so thus you need
flux of some sort. Oxygen, in this case, is the enemy. fire
scale, the black scale you see right away, are surface copper oxides.
They get in the way of soldering, but will come off in the pickle.
Fire STAIN, is the oxides that penetrate into the silver leaving a
discolored and blotchy surface. These, being penetrating, are not
removed well by pickle.
Your original class, using probably batterns flux, was thus using a
flux high in boric acid, which lasts well on the metal under
elevated temps, and doesn’t burn off that quickly, so it protects
better from fire scale and fire stain than does Dandix. But it’s not
at all as active (being formulated more for gold work, which doesn’t
need as active a flux), and won’t work as well to actually remove
existing oxides and contaminants on the silver, so then it doesn’t as
actively promote solder flow. If the silver is very clean, and
you’ve got enough Batterns on the work, it will prevent fire scale
fairly well for simple (short time periods) soldering, and the solder
can flow OK. But it’s not an especially effective flux for promoting
solder flow with silver. Dandix, on the other hand, melts at a lower
temperature, and becomes MUCH more active at dissolving oxides and
various contaminants, plus because it’s a paste flux, you generally
end up with more of it on the joint. For that reason, it’s a lot
more effective at promoting solder flow. But when you did not coat
the rest of the piece, you let it oxidize deeply, and there is the
source of your fire scale. And because dandix is such a good solvent
for oxides, and it’s lower melting point, it also tends to absorb
oxygen from the air too, and somewhat quickly becomes depleted, so
even coating the work with dandix will sometimes leave you with
substantial fire scale and fire stain.
The best answer to fire scale and fire stain are simply not to let
them form. Cupronil will work, but I think an even more effective
solution is one that I mention here in Orchid every year, or more
often, when people don’t find it in the archives. Prips flux. It’s
been well documented here in Orchid (you can find my writings on it in
the archives, as well as that by others), or in the jewelry making
books, for several decades now. But you don’t usually find it in the
catalogs, so those jewelers who didn’t learn silversmithing in
college level courses (where Jack Prip first introduced the recipe,
and where it most frequently was passed on), often don’t hear of it,
or believe in it’s effectiveness, since they either aren’t using it
right, or simply haven’t heard of it. many commercial jewelers and
goldsmiths have never heard of the stuff, and many self taught smiths
are also unaware of or unfamilier with it’s use. Properly using prips
flux can be a bit of fuss and bother, but IT WORKS. The only times
I’ve ever had trouble with fire scale and fire stain were those
times when I got lazy and didn’t use the Prips flux.
The reason prips is not in the catalogs is that it’s a simple recipe
that you make yourself with boric acid, borax, and TSP, dissolved in
water. In a quart of water, dissolve 60 grams or so, of borax, and
60 grams of TSP. Add 90 grams of boric acid. You can play with
these amounts as you like. The important point is that there are two
parts each of borax and TSP to three parts of boric acid. Note that
it has to be actual TSP, trisodium phosphate, or another of the sodium
phosphates (monosodium phosphate or disodium phosphate), rather than
one of the other various cleaning agents sold in the paint
departments of hardware stores which substituted for TSP. Since TSP
is a phosphate, it can cause water pollution problems (algae growth),
which led to the marketing of the many substitutes. But in most
parts of the country, TSP is still available. Just read the labels
carefully. borax you get in the laundry aisle, labeled borateem and
sold as a detergent. Or, some of the store brand cheap detergents
are the same, for even less money. Boric acid is sometimes sold as
roach powder, or you can buy it in pharmacies or jewelery tools
supply places.
They key to using this right is to apply it with a sprayer, to metal
that you’ve already warmed up enough that the spray dries to a thin
even white crust as it hits the metal, rather than going on wet and
boiling away. You put on a thin even white crust on all surfaces of
the sterling, and THEN add just a little paste type (or other)
soldering flux to just the joints. Unless you heat the sterling WAY
too hot, the prips flux glaze that forms when it’s heated will
protect the silver from any oxidation, even through several
soldering steps if you don’t quench the piece or otherwise wash off
the flux.
The time and effort you spend applying the flux will be many times
paid back when you end up with metal that stays clean and doesn’t
require you to remove fire stain. Use the prips any time you’re
heating the metal, including annealing.
Feel free to email me if you need more details.
Peter Rowe