Is it possible to make your own flux? And will it react the same way as paste flux?
The usual white paste fluxes sold for jewelry (or industrial brazing
too), are rather more complex formulas than you’re likely to make
yourself. Very active stuff, but generally needs more attention to
good ventilation that the fluxes you might mix up yourself. These
generally contain fluoride containing components, though some in a
form where free fluoride gasses are not so much given off on heating
(or so they claim on the “fluoride free” labels).
Those fluxes are great for metals that oxidize very easily, the
various copper based alloys, and the like. If you’re working with
silver or gold, while those fluxes are useful for difficult joins
(generally meaning poor fits, or you’ve got existing oxides on the
metal making solder flow difficult, or the like), you usually don’t
need such aggressive fluxes. The downside to those aggressive fluxes
(the white paste fluxes in general) is that they will deplete or
burn off with more extended heating, and sometimes, while promoting
solder flow, may not actually prevent fire scale or fire stain.
As others have noted, simple boric acid or borax are both reasonably
good fluxes.
Long lasting, cheap, etc. But sometimes, if your goal is to prevent
fire stain on standard silver (not an issue with Argentium or
Sterlium + silvers), then you may find simple borax or boric acid
hard to get good coverage. (For gold, you don’t need more than simple
boric acid in alcohol, though most add some specific soldering flux
too)
For that, an easy to mix up flux is a mix of three parts boric acid
and two parts each of borax and Trisodium phosphate (sold as TSP).
TSP is sometimes hard to source these days. It used to be commonly
found in hardware stores sold as a fairly aggressive and alkaline
cleaning material commonly used to clean walls prior to painting. But
it’s a high phosphate material, and runoff can contribute to water
pollution in rivers and lakes (promotes algae growth and the like),
so currently, actual TSP is not so widely used. Instead, one finds
alternative cleaners sometimes labeled with the letters TSP as part
of the label, so read the contents carefully. If you cannot find
actual TSP, the alternative similar sounding stuff will NOT work. If
you can find Cascade dish washing powder (used in automatic dish
washing machines), the type in the familiar green box, this often is
based mostly on TSP, and if so, it works as well, or sometimes even
better, than straight TSP. But again, regulations to limit water
pollution can make this harder to find. The Cascade must be labeled
to show it is not free of phosphate, and I’m unclear just how to tell
the difference between old (that work) versions and newer phosphate
free versions required in some states.
But anyway. The mix is 3:2:2, with boric acid being the larger
component. I normally mix about 60 grams boric acid and 40 grams each
of the borax and TSP to a quart (or liter) of water. Boil the water
to fully dissolve. If not all dissolves, add more water. More dilute
is just fine.
This mix is called “Prips” flux, named for John Prip, a longtime
teacher of metalsmithing and silver smithing who first introduced the
mix to his students in the 1060s. I learned it in Fred Fenster’s
sophomore metals class at the University of Wisconsin back in '72.
Been using it since.
Prips flux is not best applied with a brush. Since the aim is
overall protection of the metal, not just soldering, you want the
whole piece of metal covered. So this is best applied with a sprayer.
Most effective are the simple small "two tubes with a hinge between)
mouth blown atomizers used by ceramicists to spray liquid glazes.
Some perfume sprayers work the same way. Also some air brushes.
The big advantages of these are no tiny orifice to clog with dried
flux, and a fine even spray. You heat the metal enough so a light
spray of the flux dries on contact to a white slightly crusty film.
Cover the metal with enough so the metallic shine is hidden. More
than that is not needed. Proceed to solder or anneal.
If this sounds like a fuss, well, it can be, but once you figure
this out and get used to it, fire scale and fire stain won’t be
problems again. Of course, you can also just switch to the more
costly fire scale and fire stain resistant silvers, like Argentium
and Sterlium plus.
Prips flux is not as active as the white paste fluxes, so a good
fit, and clean metal are even more important, but that’s standard
good soldering practice anyway. And while this may seem more fuss,
the raw materials of the flux are really cheap. I mix up Prips flux
by the quart about every year or two, and the boxes of chemical I
bought (the borax in the laundry aisle of the grocery, the TSP at
Home Depot, and the Boric acid at another hardware store sold as an
insecticide to kill roaches…) have lasted me a good ten years now.
A lot cheaper than Handy Flux or Dandix or the like…
If you search the Orchid archives for “Prips” flux (or “Pripps”
flux, a typo that seems to have sometimes propagated. Proper spelling
is one p at the end.) You will find other articles, pretty much like
this, by me or others over the years that may have more details that
I’ve left out.
Peter Rowe