Technical question about firescale

I have viewed the many postings about fire scale but would like to
know if fire scale gets ‘deeper’ or more ingrained with each
subsequent soldering. I make architectural based brooches (sterling)
that sometimes have up to 20 separate solder joints (most of them
hard). I have been polishing but tend to lose crispness, have tried
electroplating but sometimes get a quick ugly tarnish on the pieces
not long after plating (do the pieces need to be neutralised or
similar after being commercially plated)? but I am also using some
gold in the pieces and the plater says he has no way of covering the
gold to avoid silver plating, and most recently I have been trying to
wire brush the brooches after dipping in a 50/50 nitric-water
solution - but several dips later find I still have fire scale - any
suggestions, encouragement or more in depth

PS - I have also tried a boracic acid and methylated spirits mix but
it ‘pocked’ the surface with quite deep little holes??? and more or
less ruined many hours of work.

thank you for your assistance

Marilyn

Fire scale without question gets deeper and thicker with each
heating. Hard solder increases the problem but I find that once you
have gained the soldering skills well worth the added risk of fire
scale…I rarely use even medium solder. I suggest that you read the
archives about prips flux (pripps ?), it really does work with care
and practice; boracic acid mix is a much more delicate barrier, best
just for gold.

Most silver plating gives you a VERY thin coat (a few atoms thick)
of soft fine silver which is porous, the bad stuff underneath will
still react with the atmosphere and show through the plating given
time. If you insist on a plating cover-up talk to your plater about
suitable resists which YOU can apply to the gold parts. I’ve used
nail polish in the past as a plating resist but not for hiding fire
scale, just not worth the time considering the pripps solution.

Another option is to use ‘argentium’ silver. No personal experience
but an archive search will provide lots of Still legally
sterling, but different working properties/techniques without fire
scale.

Persevere, what you are going through is a a common experience and
plague of every one who has ever pointed a torch a sterling.

Jeff
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand

I still have fire scale - any suggestions, encouragement or more
in depth 

This sounds like a situation in which it would make sense to
consider switching to Argentium sterling-- if I understand right, no
firescale! Right, Trevor?

Noel

Marylin,

Yes firestain can travel quite deep into the sterling with repeated
heating. Oxygen is a very small atom compared to silver atoms and
this allows it to diffuse into the spaces between the atoms in the
silver matrix. Silver can absorb huge amounts of oxygen when heated,
when molten it can hold 20 times its volume in oxygen. The silver
will release the oxygen when cooled but the copper in the sterling
will not as it forms a stable compound with oxygen. This red oxide
of copper is the firestain you see.

Try using Prips Flux (look up recipe in archives). It will last much
longer under the heat of the torch than the boric acid alcohol fire
coat. I find the boric acid alcohol mixture to be almost useless in
controlling fire stain for silver soldering of any but the smallest
of pieces. But even Prip’s Flux has its limits if too much oxygen is
allowed to get at the work while hot. So to help with this try a
fuel gas/compressed air torch rather than an oxygen/fuel gas torch.
This type of torch gives you the ability to create a large bushy
flame that is much cooler than a oxy/gas torch and you can keep most
if not all the work covered in this flame while heating. By using a
neutral to slightly reducing flame that is totally covering the work
you will greatly reduce the amount of oxygen present and therefore
reduce the fire stain. I use the National 3A-B torch
(http://www.nationaltorch.com/Ntorch.html) along with a “N” series
tip with propane and compressed air when I solder silver where I am
concerned about fire stain. One word of warning, the torch can be
used with either oxygen or compressed air but if it is ever used with
compressed air then it must not ever be used with oxygen or it could
cause a fire or even an explosion due to the oil and dirt present in
compressed air that is deposited on the internal surfaces of the
torch.

The other thing that can help if the firestain is relatively minor
is to bring up a layer of fine silver by repeated heating and
pickling of the work and brushing the work with a soft bristle brass
brush and soapy water between cycles. This coating will not stand up
to much polishing so is best if done after polishing and leave the
work with a brass brush finish which is a fine, soft matte finish

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts

360-756-6550

Mary,

I looked at your picture. If you have been doing this type of work
before, and you are experienced, it seems to me that there might be
something wrong with your castings. I cast and fabricate, and when I
solder, just using paste flux, My silver never looks like your
picture. Flux, solder, pickle, even repeated soldering should not do
what your pictures shows. If that is not what it is, seems like it
is some contaminated or poorly cast pieces. The random pattern is
not what fire stain looks like. I used to do production casting, and
I once had contaminated metal. The pieces were brittle, like like dry
pasta. You could just snap the pieces in two. I believe it was lead
contamination. You might have to scrap those castings, start with
better pieces.

Richard Hart

Marilyn

Yes, I’m afraid that firescale can ‘grow’ as the the metal is heated
over and over without the proper protection and/or use of the wrong
flame. Twenty joins are not unusual when constructing complicated and
detailed pieces of sterling jewelry.

Once you do get forescale, there are only a few usually destructive
ways to remove it…filing, sanding, acid etching, depleting and, as
you have tried, plating. None of these are satisfactory when
producing high quality pieces. I tell my students, the best way to
get rid of firescale is to not get it in the first place, and I say
that again here. Prip’s flux, in whatever iteration you use
it…whether you make your own or purchase the commercial variety, is
essential and, if it is used properly and regularly, your fire scale
should be reduced (if not eliminated) to a small blotch here and
there or to fire stain which is much easier to deal with. The one
thing you must do when using Prip’s flux is establish a complete
covering over all parts of the item…but especially those parts that
will constitute the visible surface when completed.

After you have established a regular protocol in applying the
Prip’s, you must use the appropriate flame for the size of the piece
being soldered. A strong oxidizing flame is just what it says it
is…it is supplying a high amount of oxygen into the surface of the
metal. Furthermore, if the flame is a small one, it will take longer
to bring the sink portion (the largest part being soldered) up to
critical temperature… all the while pumping an equivelent amount of
02 into the depths of the metal. Rather, use a larger reducing flame
when possible as this will lower the amount of 02 impacting the metal
yet bring the sink to critical temp as quickly as possible.
Now…don’t overdo this or you will end up with a puddle, but
carefully consider the size of the torch tip and the kind of flame
each time you heat a piece.

After finishing soldering, give more thought to how to finish the
surface. Consider, for example, using a vibra tumbler. By careful
selection of the tumbler medium you can debur, smooth, burnish and
polish delicate pieces with little or no impact on the crispness of
the design. I work part time in a factory where we make highly
accurate machined parts that must retain their exact shapes and
dimensions. We tumble hundreds of parts without losing either. Many
fine jewelry makers do the same thing with their castings…that’s
how they get that perfect finish yet retain sharp corners and design.

Hope this give you something to think about and is useful in solving
your problem.

Cheers from Don at The Charles Belle Studio in SOFL where simple
elegance IS fine jewelry! @coralnut2

This sounds like a situation in which it would make sense to
consider switching to Argentium sterling-- if I understand right,
no firescale! Right, Trevor? 

Sorry for the late response, still catching up from the xmas backlog.

Needless to say yes, I would strongly recommend Argentium Sterling
(AS) in an application like this. Avoiding firescale (and tarnish) is
what AS is all about and that feature of it would certainly go a long
way to assisting Marilyn in the type of work she is doing.

Imagine being able to assemble your work piece by piece as you like
with no need to use the anti-ox stuff EVER, just ignore it
completely. In fact the more times you heat “unprotected” AS the
better because that way it builds up it’s tarnish resistance over the
repeated heating cycles.

Sounds to me like her kind of work is one place where AS would be
ideal.

Cheers,
Trevor F.
in The City of Light
Visit TouchMetal.com at http://www.touchmetal.com