In addition, most firescale does require a good bit of abrasion to
get rid of. Pickles tend to help by leaving a purer coating
(firescale-free) on the surface of the piece, but since most
firescale goes deeper than the top layer of metal, the firescale
seems to reappear during polishing. What's really happened is that
the coating created during pickling has been polished off,
revealing firescale that was not removed.
Yes, but It’s a little bit more complex. What we’re talking about
here are in essence, copper oxides. Copper forms two types of oxide
on sterling silver, depending on the number of oxygen atoms attached
to each copper atom. The surface of the silver forms a black oxide
of copper. The “less oxygen” red oxide of copper is what forms within
the silver surface. That happens because both copper and oxygen are
somewhat mobile within the silver when heated to soldering or
annealing temps (oxygen much more so). Oxygen can penetrate a bit
into the silver, where it can bond to copper. The red oxide of
copper can remain mixed with the silver, forming that penetrating
layer. Copper that migrates to the surface itself, is exposed to more
oxygen, so it forms that black oxide. It stays on the surface. One
effect of this is that under that black oxide, there’s a thin layer
where there’s actually little copper left. What was there migrated
to the surface and formed the black oxide, then couldn’t go back down
in, depleting, slightly, the surface layer of it’s copper. This
effect is capitalized upon by people doing reticulation, since
repeated annealings bring the copper to the surface where it
oxidizes and is pickled off, eventually leaving a thick enough layer
of copper free (fine) silver as to allow reticulation. Under that
fine silver layer will still be the zone where copper didn’t make it
out of the sheet, but did oxidize somewhat, forming the penetrating
oxide layer. Because it shows up as a “stain” on what one might have
thought was clean silver when it gets polished, this penetrating
layer is not called fire scale, but rather, fire stain. Fire scale
refers to the original black oxide layer that is removed by pickling.
Anything that helps to avoid oxygen coming in contact with the
metal helps to reduce firescale. Both fast torch work and a coating
of flux prior to heating will help. I was taught to cover my entire
piece (usually sterling) with flux before soldering to prevent
firescale. Some schools I have been in are horrified by this
practice, since it does result in very messy fire bricks; they
teach students only to flux areas requiring solder. Much neater,
but more firescale.
It amazes me to see this ignorance in schools. Good silversmithing
is the goal, not clean fire bricks. When metals first got it’s big
kick into the art schools after world war 2 (the GI bill was
responsible for much of the impetus), only a small number of teachers
originally started programs that eventually became important bigger
programs which in turn taught those who’d go on to themselves teach
at other schools. That original cadre of teachers included people
like John Prip, who adapted a european method of coating silver with
borax prior to heating, to come up with, via a bit of “kitchen
chemistry”, what’s become known as Prips flux. In the 60s and 70s,
you’d have had a hard time finding any art school metals program in
the U.S. where this method wasn’t known and taught, since all those
folks knew each other, shared the knowledge, and knew it’s
importance. Fred Fenster taught me about Prips when I studied at
Madison in the 70s, and I saw it equally at other schools as well.
Richard Thomas, at Cranbrook (which trained an inordinate number of
people who went on to teach at various places) not only taught us
Prips, but himself came up with his own innovation, a mix he jokingly
called “Ring-a-ding”, after the bell that goes off in your head when
you have a good idea. It didn’t require spraying, but was a dipped
flux mix. Unfortunately, it’s only good for protecting the metal
during annealing, since it tends to act as an antiflux for soldering,
unlike prips, which both protects the metal and acts as a passable
soldering flux too. “Ring-a-Ding” never seems to have caught on,
though I know a couple schools that still have it around. But for a
while at least, it seemed almost anyone would know about Prips. When
I went to grad school at Tyler in the 80s, one of the standard shop
monitor tasks was mixing up Prips as needed for the undergrads. We
made it by the gallon, and all those kids learned it’s use. But a
funny thing seems to have happened, mostly in the last 20 years. A
new generation of jewelers and smiths, who either were too lazy to
learn how to use prips, missed that lesson or weren’t taught it, or
just never quite got the idea, or who learned from books rather than
good teachers and never read the appendixes where Prips was often
mentioned, apparently seem to have forgotton the stuff. And just as
the original knowledge proliferated, now apparently the ignorance of
it seems to be doing so as well. So now we see schools or workshops,
and as evidenced here in Orchid, a raft of beginning and not so
beginning silver workers who apparently don’t know how to prevent
fire stain. those who’re aware of the possibility, often think they
need fancy commercial products to do it, even including more
expensive silver alloys instead of sterling. In aesthetic senses, and
in terms of the growth in the knowledge of advanced techniques
including Keum Bo, mokume, granulation, and whatall else, the field
has advanced a lot since the 60s. But in terms of many of these
simple basic principals and hand operations, we as a group sometimes
have gone backwards. We need power tools and fancy products because
we’ve forgotton the simple direct (and less lucrative to tools
suppliers) original ways. Same thing sometimes with how people want
to learn the craft. Often afraid to just try things and work out the
problems, but instead people want step by step “hold my hand”
instructions for even the simplest and most obvious tasks. Just
amazing to me. And a little puzzling. And a little depressing
sometimes… the knowledge is certainly not lost. it’s out there.
Published. And in people’s head ready to be shared when asked. Yet
still it often goes overlooked.
As for copper, flux will help redusce firescale, although I don't
always use it for annealing.
With copper itself, of course, the use of flux of some sort to
prevent fire scale may not be so critical. Unlike silver, where fire
scale also means deep fire stain causing a lot of work to abrade
away, potentially damaging the work in the process, copper itself
doesn’t form any sort of deep penetrating oxides. It’s all on the
surface. Simple Pickling after heating gets you back to pure copper.
of course, if you can’t tolerate the loss of polish or surface
finish, or the loss of that oxidized surface copper, then you’re free
to use flux to prevent it. Cupronil was developed for just this
requirement. But not preventing scale on copper doesn’t result in
such an automatic level of real damage to the metal.
