What is Fire Scale?
|
Answer:
Here is a great answer from Peter Rowe:
Jeez. Every few years, or is it months, we go through this same thread
about firescale from (usually) newcomers to the craft who've not yet found
the answers to the problem.... Always amazes me, since learning to prevent
fire scale was one of the first things Fred Fenster taught us back in that
beginning sophomore class in metals, back in '72. It's become such second
nature now that I'm surprised when someone has trouble with fire scale on
fabricated silver (castings are another problem, without such an easy
answer)
Niels, the two effects you mention are the just different aspects of the
same underlying problem, differing mostly in degree, or which layer of the
resulting surface one is talking about. Firescale is a deep oxide of
copper, formed below the surface of the silver, on prolonged heating.
When you first heat sterling, the copper on the surface immediately
oxidizes to a black oxide. This oxide, being on the surface, is easily
removed by the pickle when you are done, leaving a copper depleted surface
on the metal of mostly fine silver. But at elevated temperatures, silver
is quite permeable to oxygen, and the oxygen that penetrates into the metal
reacts with the copper in the sterling to a considerable depth. Closer to
the surface, with more oxygen, it forms that black oxide, which tends to
migrate to the surface of the metal. The deeper you go, however, more of it
does not form the black oxide of copper, it forms the red one, which is
less oxygen bonded to the copper. This oxide, which remains where it
forms, protected from the pickle or other chemical attack after soldering,
remains below the initial copper depleted surface of the silver after
you've pickled the item. To a certain depth, there is only a little of
this in the surface metal (enough to faintly change the color of the metal
to a slightly dustier/whiter color), as most of the copper oxide that
formed was the black oxide, which is now gone. At the depth where the
black oxide no longer formed and migrated to the surface, you then get a
heavy layer of the red oxide, which is distinctly redder in color. Repeated
or prolonged heatings increase the depth of penetration of this red oxide
(It's reddish color may be why it's called firestain, in addition to the
process that gives rise to it). It also increases the depth to which the
black oxide forms, and after pickling, thus the depth of the resulting
copper depleted layer, which is the matte white surface you see after
pickling. When you finish fabricating such an item, it will initially take
a nice looking polish, as you're polishing just the fine silver surface,
all the copper that used to be there having been oxidized to the black
oxide and removed. As you cut deeper into that surface which is still
mostly fine silver, there are increasing traces of the red oxide But when
you break through this layer, you then discover the heavy layer of red
oxide, which will appear as a thin creamy white with reddish tinge layer,
and below that, you break into clean sterling silver, which will have a
better, deeper polish and color than the lighter and slightly dusty looking
color of the copper depleted layer above the red oxide layer. Generally,
until you cut through that initial surface, you can be unaware that you've
got a problem. And it usually only shows up with rouge, since initial
polishing steps with Tripoli or white diamond compound don't produce a high
enough polish to easily show the faint color differences that become so
apparent and disturbing against a high rouge polish.
There are a number of ways of dealing with firescale, fire coat, fire,
firestain, or whatever you wish to call it.
One can solder without an oxygen atmosphere. In industry, conveyor belt
soldering furnaces using controlled atmospheres eliminate oxidation, and
thus fire scale problems. Those of us without such furnaces can only drool
at the ease of this solution.
One can remove fire scale by mechanical means, which means polishing it
off. This has the disadvantage of being time consuming and bothersome. But
it works, if your metal had the initial thickness to allow such aggressive
polishing.
One can cover over the fire scale. Put a final polish on the piece
completely ignoring the fire scale that shows. Just make it a good high
polish. Then electroplate with fine silver. This covers over the fire
scale, at least until some end user wears through the plated coating.
One can chemically remove the fire scale, by acid etches or
electrostripping. this, to me, has little to recommend it, as clean metal
is being removed just as much as the fire scale. Bright dips and pickles
are NOT selective. Unless you've already polished through the fine silver
surface, and now are looking only at the red oxide actual fire stain layer,
pickle cannot reach it. Bright dips usually must be acids or agents
capable of also etching the silver itself, so as to remove that initial
fine silver layer in order to get to and remove the fire scaled underneath.
While these methods have some advantages in being able to reach recesses,
they are not very selective, not leave a fine polished surface, and can
also remove a lot of metal in a hurry.
One can use the fire scale, or more commonly, the copper depleted fine
silver surface, as the final finish. In this approach, you do most of your
polishing, getting the surface at least ready to use rouge. Then you heat
the silver till it discolors, or as Niels does, to red heat, and pickle.
he uses a sulfuric acid pickle, but this is little different, chemically,
from the "Sparex©" most commonly used in the U.S. (Sparex© is simply a
sulfuric acid salt, and has almost the same effect on the copper oxide as
actual sulfuric acid does, only a bit slower, and it's safer to use near
things like human skin...) This can be repeated until the silver shows a
uniform white matte color. Because it was carefully pre-polished, this will
now still need only a very light polishing operation to bring up a fine
polish. This polish, however, is of the copper depleted surface, and the
metal is not quite the same color as the darker and higher polish that
sterling silver takes. this can be especially effective with things like
scratch brush or other matte finishes, since these are less different from
their appearance with sterling. Care must be taken in finishing not to cut
through to the actual fire stain layer, or worse, to the clean silver
underneath. This approach, by the way, is a highly traditional one,
especially in Europe. The famous Danish firm of George Jenson % co. used
this method on most of their wares, to the point where some people refer to
this as a George Jenson finish. Note that this does not actually remove
the fire scale. It only buries its disturbing aspect under a thicker
depleted surface.
And Finally, my preferred approach. DON'T GET FIRESCALE IN THE FIRST
PLACE!!!
Whew. Betch'a thought I'd never get to that part, didn't you. For me, and
for a number of others, it's by FAR the best solution. This simply means
to solder in a way that oxygen cannot reach the surface of the metal. With
lime, we'd do that with a simple dip in a slurry of boric acid and alcohol.
But with silver, this is not sufficient.
What does work are many fluxes, but most soldering fluxes are so active
that they tend to deplete themselves before you're done soldering, and the
result is that fire scale is formed anyway. What is needed is a fairly
neutral flux that forms a good gas impermeable glaze on the metal at lower
temperatures, which then survives prolonged heating during soldering or
annealing without breaking down too.
The best, by far, flux to use to prevent fire scale is a HOME MADE cheap
and easy formulation called Prips flux, after John Prip, a teacher for many
years who introduced the formula and it's been popularized for several
decades at most American art school metals programs. A surprise to me is
always that so many metalsmiths have not yet heard of it. Perhaps it's
simply that most of the books, though they mention, it, don't give it
enough emphasis, and the tools/equipment suppliers obviously don't tout a
formula you make yourself.
I've discussed Prips flux at length several times, here in Orchid, or on
rec.crafts.jewelry (the newsgroup I moderate), in an article I wrote about
4 years ago detailing it's preparation and use, so I won't go into quite
all the details again (it should be in the archives of either orchid,
rec.crafts.jewelry (found on the deja.com site) or on Andrew Werby's web
site, where he's deemed it valuable for some reason to publish a few of my
windier articles (grin) The URL of that article is
http://users.lanminds.com/~drewid/PWR_Pripps.html .
Please note that for some reason, in that article, I've given John Prip a
second "P" at the end of his name that he wasn't born with. Dunno why.
Just happened. Sorry John.
For you readers to busy to go find it on the web, the essence is: 120
grams boric acid, 80 grams borax, and 80 grams TSP (the real stuff, not the
common substitutes). Added to a quart of boiling water, with enough
additional water to keep it in solution as it cools. The result is Prips
flux. Use it by preheating the silver hot enough so that the flux can be
sprayed on the metal, so it dries instantly on contact, forming a uniform
white thin crust. Coat all parts of the silver before any soldering or
annealing operation. The best sprayer type is the little mouth atomizers
used in ceramics to apply glazes. Just two little tubes on a hinge. Unlike
spray bottles, they don't clog, they're cheap, and the spray is very fine
and even.
Please note that Prips flux is only marginal as a soldering flux. It will
work if the metal and solder is clean already, but it is not very active at
dissolving oxides that have already formed or that form during heating.
(this is why it does not deplete itself rapidly). It will completely
prevent fire scale from forming if used properly, but it will not remove
fire scale that has already formed.
If anyone needs additional help with this, please feel free to contact me.
I'm only able to read Orchid sporadically at the moment, so feel free to
ask on rec.crafts.jewelry or by email, since I may not see the replies
here.
Hope this helps.
Peter Rowe
|