To remove fire stain, I was told you need to heat the metal many
times- not to the point of annealing. Pickle in between each
heating to pull the fine silver to the surface. This is one trick
to "cover" the stain.
The above process is indeed one solution to fire stain problems. But
do note that it does not actually remove fire stain. If anything, it
can make the fire stain layer within the metal thicker. But as you
note, it also makes the fine silver layer at the surface thicker, so
if this is then preserved in finishing, the result is the desired
uniform surface. Also note that the phrase “pull the fire silver to
the surface” is incorrect. What happens isn’t moving or pulling
silver anywhere. Rather, it is removing the copper from the surface.
The copper atoms do actually move around via diffusion during this
process. But the silver doesn’t need to. It’s already there.
Plating is a common solution in mass production. Finish the piece
completely stain and all, then plate. I always disliked getting
plated jewelry for repair, so I do not like this solution.
Well, yes and no. It’s got extra costs involved. But silver plating
over a fire stained piece is simply doing via plating, the exact same
thing your first process of repeated heating is doing, which is to
build up a surface layer depleted in copper. Silver plating puts down
a fine silver surface, so the result is the same (and can be thicker,
if you wish). It does have to be a thick enough plated layer to be
able to hold up to some wear and tear, but the same holds true of the
depletion gilded surface.
After reading James Binnion's first post, I am wondering, if
ferric acid would remove because it will etch copper, but not
silver. Maybe someday I will have to test.
Ferric Chloride, not ferric acid. (no such thing as ferric acid).
And no, that won’t affect fire stain because acid etching of any sort
only acts upon the surface. Fire stain penetrates. Acid bright dip
treatments for fire stain are etching off the entire surface layer
till they are below the fire stain. You need an acid that will also
remove the silver within which the copper oxides are suspended.
Usually, bright dip formulas are based on nitric acid or nitric acid
salts, not hydrochloric acid or it’s salts (like ferric chloride).
Cyanides are sometimes also used for this.
I use a combo of boric acid and denatured alcohol as well as
battens flux on the seam as a preventative. Rarely encounter fire
scale in my career before switching to argentium for most of my
stock silver materials.
Now that’s got to be a mistake. Argentium was specifically designed
so that it does not form fire stain. Fire scale, to a degree, but not
fire stain. Since the alloy is less prone to oxidation, I can’t see
why your normal protections wouldn’t work as well with argentium
against fire scale.
However, I should also note that in general, boric acid by itself is
not known to be a good fire scale or fire stain preventative with
silver. The reason is that upon heating, the surface tensions of the
silver and boric acid tend to keep the boric acid from remaining as a
good surface coating. It balls up, leaving most of the surface
unprotected. In Prips flux, the two protective elements are borax and
boric acid, long time traditional means of protecting precious
metals from oxidation. But the TSP is there as a surfactant, a
wetting agent. it’s what allows the boric acid and borax to
sucessfully cover the metal surface while heating, to provide
protection. Borax or boric acid do offer some protection, but it’s
far from perfect. The classic way silver was protected was called
"burning in", which amounted to multiple coats of borax, lightly
heated and recoated until it formed a decent surface, before
prolonged heating to useful temps was done. See the description in
Seitz and Fiengolds “Silversmithing”. But in short, I’m surprised not
only that you’ve had more trouble with argentium, since it’s purpose
is to limit oxidation problems, but as well, that you had few
problems using just boric acid and alcohol. That’s traditional with
goldsmiths for use with gold, where it IS effective. But on silver?
Most people find that it’s marginal at best.
Peter in the different processes you describe, I have always heard
fire stain referred to as firescale. Thanks for clarifying. Also,
if I remember correct, the pink copper film that pickles of was
called "copper flashing".
As good a term as any. I don’t know that it has a formally correct
name. I’ve heard “flashing”, plating, coating, or just a strung of
unprintable words… I assume you’re talking about copper that plates
out on the silver when iron or another reactive metal is accidentally
in contact with the silver when put into the pickle…
This might be informative, too:
As always, our good friend Charle’s writings are always erudite and
informative. I do note that he, too, seems to be using the terms
fire scale and fire stain slightly interchangeably in this particular
20 year old article. Do remember too, that usage of particular names
is not always universal to everyone or every metals traditions.
Obviously, names differ in different languages. But they can find
different usages even among various english speakers. The main point
is that in general, they’re talking about the same two distinct
types of copper oxide that forms on sterling silver (and as Charles
points out, a few other alloys as well. But the gold alloys where a
true fire stain appears, rather than surface only oxide films, are
unusual, not the norm at all.)