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Burning silver when polishing


#1

Is it possible to heat sterling so much on a polishing wheel that it gets
oxidized and looks like “firescale”? I have some pieces that look "ok"
before polishing, but then have those awful gray places after polishing.
Maybe I should quench in water more frequently?

Ideas? Suggestions?
Cheers
Virginia Lyons

#2
 I have some pieces that look "ok" before polishing, but then have those
awful gray places after polishing. 

G’day Virginia; join the club! I think that what happens here is that you
do get some firestain when you solder - I find it hard not to, despite
using anti firestain flux heavily. My theory is that having done the
soldering OK, you put the job in pickle and presently remove it looking
quite nice, if a little dull, due to the copper in the surface of the
sterling oxidising to copper oxide, which dissolves in the pickle leaving
the nice white surface of pure silver. Then you buff the work, removing
parts of the very thin coat of pure silver, which exposes more of the
copper oxide firestain which was hidden. And that is hard, and is
difficult to polish off. If it is very bad, the only remedy I know is to go
back to a fine abrasive and remove that patchy layer completely. I have
even had to put it in a cyanide bath and carefully plate off the patchy
grey surface to start again with the polishing.

Maybe I should quench in water more frequently?

Sorry, but I don’t think that would work at all.

Cheer up, and sing along with the boys in the Company Anthem


#3
 I have some pieces that look "ok" before polishing, but then have those
awful gray places after polishing. 

G’day Virginia; join the club! I think that what happens here is that you
do get some firestain when you solder - I find it hard not to, despite
using anti firestain flux heavily. My theory is that having done the
soldering OK, you put the job in pickle and presently remove it looking
quite nice, if a little dull, due to the copper in the surface of the
sterling oxidising to copper oxide, which dissolves in the pickle leaving
the nice white surface of pure silver. Then you buff the work, removing
parts of the very thin coat of pure silver, which exposes more of the
copper oxide firestain which was hidden. And that is hard, and is
difficult to polish off. If it is very bad, the only remedy I know is to go
back to a fine abrasive and remove that patchy layer completely. I have
even had to put it in a cyanide bath and carefully plate off the patchy
grey surface to start again with the polishing.

    Maybe I should quench in water more frequently? 

Sorry, but I don’t think that would work at all. Cheer up, and sing along
with the boys in the Company Anthem


#4

One other solution might be to plate a silver coating over the piece once
the polishing is complete. Nina


#5

I agree with what John wrote, and wanted to add one thing. With my work I
seem to get less fire scale when I work with a hot torch with as large a
head as reasonable for the piece. Heating the piece quickly and getting
the soldering over with quickly seems to produce less firescale than
longer, slower heatings.


#6

To get firescale off of my pieces, I start with White Diamond to get the
larger scratches and then go to Fabulustre to remove the firescale. It
leaves a nice finish, bright enough for my production work. Fabulustre is
amazing stuff…it isn’t really aggressive, but somehow it takes that
firescale off like magic! (…of course, I am pretty aggessive myself at
the wheel…pushing my work into the buff quite hard…bad form. But, it
works for me…!

Marlo M.
Seattle, WA


#7
 "I have some pieces that look "ok" before polishing, but then have those
awful gray places after polishing" 

Although I too use anti-firescale flux and/or a boric acid denaturalized
alcohol solution, I still get some firescale. The only thing that I have
found to work quickly (relatively) in removing firescale (from silver) is
to:

  1. paint the piece with a little bit of nitric on a q-tip. THe nitric
    shows the exact location of the firescale. (use gloves unless you want your
    hands to look as though you never bathe- the stain stays on for days!)

  2. Neutralize the acid on the piece with baking soda and water and rinse.

  3. Then I take a stiff muslim wheel and bobbing compound and keep adding
    bobbing to the wheel on a very frequent basis until the stain is gone.
    Tripoli will work too, but much more slowly. As a cautionary note, bobbing
    compound is even more abrasive than tripoli and will wear at your silver.
    I go overboard with the boric acid/alcohol solution on the piece during
    soldering to try and minimize the firescale on the piece and try also to
    not overkill with the torch. As John Burgess explained so well, the torch
    is the “demon” that causes the alloys to rise to the surface, revealing the
    firescale stain. Good luck virginia! Hope this helps.
    Shael


#8

Virginia, If these are fabricated pieces, there is a chance that you have
firescale hidden by layers of fine silver built up through repeated heating
and pickling. When you polish, you might be removing this skin of fine
silver and exposing the firescale.

You can’t polish silver by hand to the point of heating it to create
firescale. You would have to achieve near soldering temperatures.

Donna


#9

What you have experienced is not firescale, but what we in Danish would
call something like “firespot”. It is oxidised copper, which has been
formed just underneath the upper layer of silver.

To avoid it, flux all your pieces well.

To get rid of it, heat to a cherry red, pickle in 10 per cent sulphuric
acid, and repeat this proces three or more times. This works for me, but
let us hear from other Orchidians. There must be a whole lot of you, who
have also experienced these gray spots.

Kind regards
Niels


#10

Virginia, You have polished through the layer of fine silver on the
surface to the layer of oxidized silver (called fire scale. You either
have to not polish as much so that you leave the fine silver on the
surface or you have to get thru the layer of fire scale by polishing or
electro stripping down to the clean sterling silver. Another possibility
is to electro plate silver over the entire piece to cover all the subtle
differences in color with a fresh layer of silver. Probably the best
solution is to avoid the oxidation of the silver through a layer flux on
the piece or the piece could be soldered in an innert atmosphere.

Good Luck,
Etienne Perret
http://www.etienneperret.com


#11

For what it’s worth, Virginia, I have had the same problem and it seems
that the polishing agent is sometimes responsible. Green rouge has brought
up firescale every time I have tried it.

Lee


#12

John B’s analysis was right on. My findings aside from abraiding the
firescale off is to use Bobbing compound on a muslin buff. This compound
is essentially pumice held in a vehicle and does a good job of removal of
fire. Of course the more you try to prevent firescale the better it is.
Each time you apply the torch to the piece spray with Prip’s flux-but it
must be done EVERY time the flame hits the piece.

My 2 cents JZ Dule


#13

Dr Dule is correct., Pripps flux is the single most effective solution to
preventing firescale. Works everytime, in my experience. Gary in
Redding, Ca.


#14

JZ… you are the first to bring up Pripps anti-fire scale. It is a must
in my shop. There was a long thread on this a few months back. Check the
archives. If you prevent fire scale the polishing is so much easier.
Janine in Redding, Ca.

http://www.janinesjewelry.com


#15

I had this problem until I started using a tumbling process instead of
buffing. I fabricate the piece to completion, pickling as needed during
the process. I use cratex wheels and needle files to smooth rough edges
where needed. Once the piece is finished I start my tumbling process using
4 steps. I use this for both my gold and silver work. I haven’t tried it
from a cast to finish, but I understand it can be done with success. I
hope this helps.

I purchased my supplies from Rio Grande 800-545-6566 or www.riogrande.com.
I called them about a year ago and talked with a tech, explained what I
wanted to accomplish and they made suggestions. I took them and order the
products.

Here are the steps I use and the materials used during the process:

I use a Lortone rotary tumbler, each step takes from 2-4hrs. Each of the
tumbling media comes in cones and triangle/pyramid, I use a mix of both.
The item numbers are for the smallest boxes available (5lbs)

Blue Medium Cut.... cone #339-345 and triangle #339-209.
Clean Cut Pink... cone #339-408 and pyramid #339-441
Clean Cut Blue... cone #339-410 and pyramid #339-443

They are used with Sunsheen Gold & Silver Deburring Compound #339-307 (1
qt size makes up to 12 gallons)

My last step is the burnishing process. I use mixed steel shot with Super
Sunsheen Burnishing Compound #339-394 (1 qt makes up to 12 gallons). I
simply followed the instructions on the containers and made adjustments as
needed for my preference.


#16

Geez. 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 permiable to oxygen, and the oxygen that penatrates 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 distincly 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 depleated 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 useing 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 agressive
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 sulphuric acid pickle, but this is little different, chemically,
from the sparex most commonly used in the U.S. (Sparex is simply a
sulphuric acid salt, and has almost the same effect on the copper oxide as
actual sulphuric 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 prepolished, 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. Betcha 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
gold, 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 impermiable 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


#17

When I went to school at Rhode Island School of Design back in the 70s I
had the good fortune to have Jack Pripp as my metalsmithing teacher, and
yes he did teach us to use Pripp’s flux.

Etienne Perret
http://www.etienneperret.com


#18

Firescale! Both in cast and fabbed pieces…what a drag. Yes, Pripps flux
is really good. I am very fond of soldering silver in charcoal whenever
possible. I will carve a block to fit the piece as tight as possible.
Usually the soldering occurs on the back of the piece and therefore the
front is surrounded with a reduced atmosphere. Coupled with pripps flux
it’s difficult to go wrong. I save broken charcoal pieces to build small
structures arround larger things, having the added benefit of offering an
insulated area for the piece and lessening the need for a larger torch.
J.A.


#19

John, I guess you want to avoid the Match Lite variety of charcoal when
you use it to create a reducing atmosphere for soldering. Ha! Save it for
the barbicue. Good luck with the new gallery.

Etienne Perret
http://www.etienneperret.com