My flux keeps peeling off as I heat the metal.... ... Would
appreciate any ideas or suggestions.
As you note, having the metal clean and grease/oxidation free in the
first place in necessary simply so the flux can initially properly
wet the surface. But even when it’s clean, you may still get the flux
pulling away. Sometimes this is due to too thick a flux layer. Try to
get a uniform thin layer so the surface, as the flux initially dries,
is uniform and a thin, almost translucent layer, rather than anything
thick. This situation is one where more is not better.
But most soldering fluxes simply aren’t designed for this use. The
surface tension of silver causes the flux layer to want to ball up if
it can manage to do so (and thicker layers do it more), and many
soldering fluxes don’t survive higher temps well, which means even if
they don’t ball up, they may become “burned up” and you end up with
fire scale and fire stain anyway…
Or, see the Orchid archives for many posts, including mine, on how
to make and use Prips flux. This is a flux specifically designed for
this type of use, the protection of a silver surface while heating
for annealing or soldering. It’s only so-so as an actual soldering
flux, so often one would add just a little paste or other soldering
flux just to the joints, but leave the protection of the main mass to
the Prips. This is good too, because many of the more active fluxes
like the various paste fluxes, while excellent at promoting solder
flow, are not actually so good at preventing fire stain and fire
scale. They simply get “used up” too quickly, and sometimes just seem
not to really protect all that well even when they don’t look burned
up (varies with brand and type). (metal looks fine after soldering
and pickling, but still shows fire stain when you go to polish…)
Prips is less active, and lasts through higher temperature ranges,
which allows it to protect the metal better. It does work as a
soldering flux, especially for harder grades of solder, but
preparation such as clean metal, clean solder, good fit, are more
important, because the flux doesn’t work so actively to encourage
solder flow. But by the same token, it then doesn’t become burned
up, so the metal stays protected.
It’s relatively cheap, since you mix it yourself. Borax (grocery
store stuff, Boraxo, Borateem, etc is fine, even though it’s not
totally pure), boric acid (hardware store roach powder), and TSP (for
precleaning prior to painting, Home Depot sells it, make sure it’s
actually TSP rather than one of the phosphate free alternatives). 120
grams (3 parts) boric acid to 80 grams (2 parts) each of the other
two dissolved in about a liter of tap water makes a good mix.
Now here’s the key, which is how you prevent it too balling up and
pulling away from area. You don’t apply it wet with a brush or a dip,
etc. Rather, you lightly preheat the metal and spray on the flux, so
it dries on contact. What you want is a thin lightly white coating
which you can almost still see the metal through. Any thicker than
that and it too can ball up, but if it does so as you’re heating,
simply spray a bit more on those spots as it happens. Ceramics style
mouth blown atomizers are the best sprayers since they give an even
fine spray and cannot clog the way trigger sprayers can do. Hobby
style (cheap, external mix type) air brushes work too (Harbor freight
sells a cheap one).
I know this all sounds complex, but it isn’t, other than finding a
suitable sprayer (like I said, the mouth blown atomizers the ceramics
types use for spraying glazes). Once you get used to the method,
problems with fire scale and fire stain (the whole reason to bother
coating the silver in the first place) pretty much go away.
Then of course, there’s the other solution. Don’t use standard
sterling silver, but rather, try argentium or a similar fire stain
free silver alloy. These may work differently than standard sterling,
but are formulated so as not to form a fire stain layer when heated.
So after heating, etc, you simply pickle it, and the result is clean
metal with no hidden traps to find when polishing.