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Temperature and Firescale on Sterling


#1

Greetings all,

Does anyone out there know when - specifically at what temperature -
firescale starts to form on silver? I have looked up Brehpol,
grimwade and oppi but no-one seems to state a temperature. the reason
for the question is that I have been looking at age hardening silver
in the kiln. If your silver has to sit in a kiln at 300 degrees C for
two hours, will this cause firescale? So, any takers?

thanks
Eileen


#2

Eileen,

Sorry, I can’t tell you at exactly what temperature firescale forms,
but, I think I can tell you how to prevent it in a long heat
treatment. You need to try heat treat wrap. This is a very thick
foil of stainless steel available from machine shop suppliers. I have
used many different types (I do in house heat treatment of tool
steels) and like Sentry brand. They make laser welded little bags
that are very convenient. You’ll have to google them. If you try any
heat treat wrap, remember that it is absolutely required to roll down
the edges (sheet or bags) and roll them over and roll again, several
times to keep out the air. Adding a small piece of charcoal
soldering block can further help by reducing any oxygen already in
the bag when you seal it.

Dan
Daniel Culver


#3

I don’t know at what temperature the fire stain starts to form but I
suspect it is a time/temperature not just temperature variable. I
know that copper will oxidize at this temperature so I assume that
fire stain will from at this temperature. Here is what I have done
in a standard kiln. Place the sterling to be hardened into a steel
box and filling the box with charcoal granules. The sterling is
coated with prips flux and is placed in the middle of the charcoal.
When the work comes out and the flux is removed the work is bright
and clean.

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#4
Sorry, I can't tell you at exactly what temperature firescale
forms, but, I think I can tell you how to prevent it in a long heat
treatment. You need to try heat treat wrap. 

Much easier, and cheaper, would be to coat the item with a decent
coating of Prips flux to prevent fire scale formation, the same way
you might do to prevent it’s formation when initially soldering or
annealing the silver in normal fabrication work. Other fluxes that
prevent fire scale formation can also be used (I seem to recall
people have suggested cupronil, as one such). I happen to prefer
Prips, or Fred Fenster’s variation, “Frips”, just because being home
made, they’re really cheap. And they completely prevent fire scale or
fire stain, when correctly applied. That heat treat foil wrap
material does still leave a small amount of air inside (Unless you
could figure out how to vacuum seal it or something), so it wouldn’t
totally prevent all oxidation. Prips or Frips, will. And the foil is
quite costly by comparison. It’s needed when one is heat treating
steels, or other things where the heat treatment is at similar
temperatures. Those high temps will generally be too high for Prips
or other fluxes to fully protect the metal. The temps you need for
age hardening silver are fairly low.

By the way, you might also try thinking about this from the other
direction. fire scale/fire stain is objectionable because it looks
uneven when it forms in working, and is then unevently exposed during
the polishing/finishing process. If it’s totally even, a fire stained
surface may have a slightly different color from clean silver, but it
can still look fine (I’m referring here to fire stain, not the dark
black oxide that’s easily pickled off). Consider the classic
silversmithing firm of Georg Jensen. Most of their work in the early
20th century was simply given a fire stain finish. The items would be
made and complely polished and finished, out to a rouge polish,
without regard to fire stain. Then the item went back into a kiln for
a heat treat that gave it a uniform fire scaled and fire stained
finish. Pickled, and then repolished just lightly enough to shine up
again, without cutting through the fire stain, the finish looked just
fine.

Normally, I prefer a surface without fire stain. But if you’re heat
treating it anyway, if you fully polish the thing and clean it first,
you might find the easiest solution is just to be careful after heat
treating, not to disturb the fire stain surface if one forms at that
temp. That would mean pickling to get off the black oxide, and then
just very light polishing after. just a thought…

Peter