Here is a copy/paste of a reply post by Peter Rowe.
This is a great post and he explains alot about firescale/ firestain and fluxes
This is the post where I learned about what the different properties of the ingredients in the Pripps flux. (heat range, surface tension, etc)
(question from poster)
I have questions about firescale. Is it possible to prevent it? Is
it really possible to get rid of it once it happens? this also is
probably something at the molecular level.
(reply from Peter Rowe)
Sally, go through the Orchid archives looking for posts on the
subject of Prips flux (sometimes misspelled Pripps).
Fire scale is fairly simple to deal with, since it’s a surface oxide
that comes right off in the pickle. The real problem is fire STAIN,
which is that pinkish/greyish “shadow” that shows up on the silver
just as you’re getting to a final polish.
The conjecture that it’s “something at the molecular level” suggests
you don’t fully understand how this happens.
On heating sterling silver, oxygen in the air becomes able to
diffuse somewhat into the silver surface, and to oxidize the copper
in the silver. The copper within the silver is also somewhat mobile
when the metal is hot. Copper oxides too can migrate some, and that
which reaches the surface oxidizes further (the black oxide of copper
rather than the red). It’s then trapped at the surface. That means
there’s something of a one way street going on, and with some
sustained heating and continued exposure to oxygen, you end up with
a four layered surface. At the top is the black copper oxide. Then is
a layer from which the copper is mostly gone, since it was close
enough to the surface that it reached the surface and did not return,
so this zone is then almost pure silver. Below that is a zone where
oxygen reached, forming the red oxide of silver, but here, it did not
manage to migrate (diffuse) out of that layer, being too deep to make
it out. This is the layer that gives you the fire stain problem.
Below it, of course, is the depth to which oxygen did not reach, so
there are no more oxides, and it’s clean silver. When you take this
metal after heating and pickle it, the surface black oxide layer is
removed, leaving you with a matte white surface. it’s the fine silver
layer. If, when you polish the end piece, you don’t cut through this
layer, then you’ll not have a problem with the finish, but it can be
thin, so this is difficult. If you polish into the fire stained
layer, then it contrasts with the fine silver layer, and even more,
with the un oxidized sterling silver below it.
The solution to all this trouble is simple in theory. You simply
have to prevent oxygen from reaching the surface of the metal while
you’re heating it to solder or anneal. The answer is the right type
of fluxing agent.
With gold, a simple coating of boric acid, left by a slurry of
alcohol and boric acid powder into which the metal is dipped, is
enough. But gold doesn’t form the fire stain layer, so it’s less of a
problem anyway, since simple pickling gets you back to clean metal.
With silver, it’s a bit more complex. plain boric acid is also the
fluxing agent most effective at blocking atmospheric oxygen from
reaching the surface, but though some jewelers will use the above
simple boric acid/alcohol dip by itself, boric acid alone is not
fully effective since the surface tension of the silver can prevent
the boric acid from properly coating the silver. And the boric acid
doesn’t, by itself, fully cover the whole temperature range you wish
to protect the silver through. Mixing borax with the boric acid
solves that temperature range proglem. Mixing in other wetting agents
allows the flux to properly cover the metal. Prips flux uses
trisodium phosphate (TSP) to do this. There are other commercial
fluxes, such as cupronil, or firescoff, that also are effective at
protecting the silver during heating. Note that many soldering
fluxes by themselves, are not totally effective. The highly active
white paste fluxes, such as Handy flux or Dandix, for example, do not
seem to properly block oxygen from the metal, even though they are
dissolving oxides from the surface. So with them, even with the flux,
fire stain can sometimes form. The less active Prips flux or
similarly acting fluxes, do not do this.
If you learn to properly apply and use Prips flux (which you make
yourself. Its very cheap. Using it takes a little practice, as it’s
best used by spraying it on, not just brushing), your problems with
fire stain, and fire scale, will be gone. When I first learned silver
work back in the 70s, in an undergraduate metals course, Fred Fenster
taught us to use Prips flux, with the same statement, that it would
never be a problem. And he’s been right, at least for me. The only
times I get fire stain in fabrication, is when I’ve been too lazy to
use the Prips. Casting is another issue, though. Here, fluxing isn’t
something you can do to prevent fire scale or fire stain on the
castings. There are other ways that help, but the simplest is to use
one of the newer fire stain free casting silver alloys.