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About heat hardening silver


#1

Hi Folks,

There’s been a little discussion recently about heat hardening
silver, and I have a few questions:

  1. I don’t have access to a kiln, and my kitchen oven goes up to 550
    degrees F. Is this close enough to the desired temperature to afford
    me some degree of hardening?

  2. Any need for boric acid dip to prevent “oxidation”?

  3. Any concerns about color change in stones like amethyst, topaz,
    etc.? Should heat hardening be done before setting stones?

Thanks,

Dave
Dave Sebaste
Sebaste Studio and
Carolina Artisans’ Gallery
Charlotte, NC (USA)
dave@sebaste.com


#2

I tried the oven method with a couple of cuff bracelets recently,
first burning on a coat of prips flux. I needed to clean the oven
anyway, so I put it through the self-cleaning cycle. It actually
worked reasonably well, but would have been a lot better in a kiln.

Janet Kofoed


#3
   1. I don't have access to a kiln, and my kitchen oven goes up to
550 degrees F. Is this close enough to the desired temperature to
afford me some degree of hardening? 

Possibly. Precipitation hardening is not a process that absolutely
requires a specific temp. The usually given temps are where it’s
fastest and most efficient. However, 550 F is a quite considerable
distance from the desired range. I suspect that you’ll not get full
hardening, and may not get all that much. If you do get hardening, it
certainly will require more time than is normally used. I’d imagine
you’ll have to experiment a bit to see whether it works and how long
to leave it in for. And one other thought. Your oven is calibrated
to 550. But what about, if there is one, the self cleaning cycle that
many ovens have? Usually, these go hotter. You’d need some "tempil"
tablets to calibrate how hot the oven gets on it’s clean cycle, to be
sure it’s not getting TOO hot.

   2. Any need for boric acid dip to prevent "oxidation"? 

Yes. But I’d suggest prips flux, or another more complex true flux
mixture, rather than just boric acid. Sterling silver is less
efficiently protected by simple boric acid, since it tends to “ball
up” and pull away from the surface. Though 550 isn’t all THAT hot,
it’s still hot enough to build up a pretty oxidized surface if you
leave your metal in there for a couple hours, which may well be what’s
required…

   3. Any concerns about color change in stones like amethyst,
topaz, etc.? Should heat hardening be done before setting stones? 

As a general rule, yes. For amethyst or topaz, absolutely. This
depends a lot on what types of stones are being used. Since the
heating cycle can be done slowly, placing the work in the oven,
turning it on to heat treat and then turning it off and waiting till
it’s cool before opening, some stones will be able to withstand the
heat where they might not do so with, for example, torch heating in
normal work. Again, you’d have to experiment. But in general, heat
treat your work before setting the stones. Most of the semiprecious
and softer stones won’t like spending an hour or two at 550 or more.
But corundum, (sapphire, ruby), especially the synthetic ones, will
withstand these temps just fine.

Hope this helps.

Peter Rowe


#4

Peter, What is the “desired range” for heat hardening? I have
experimented with using the cleaning cycle on my oven to do a couple
of different things. Most ovens hit 800 to 850 degrees during the
cycle.


#5
   Peter, What is the "desired range" for heat hardening?  I have
experimented with using the cleaning cycle on my oven to do a
couple of different things.  Most ovens hit 800 to 850 degrees
during the cycle.  

Morris, The Handy and Harmon “handy book” states a temperature of 600
F, for 30-50 minutes. I’ll confess that in my prior post, I had it in
my head that it was 700, not 600. If your oven reaches 550, I’d bet
that this will do the trick, though slower than 600 would.

If the cleaning cycle is reaching 850, though, then I think that at
that point you’re maybe getting uncomfortably close to the annealing
range. Now, I’m not a metallurgist, so I can only go on the temps I
see published. I don’t know for sure at what point precipitation
hardening stops and annealing starts instead. But I guess I’d opt for
the lower temperature range instead, or at least test em both and see
which works better.

Also, the handy book suggests that for the highest hardness, you want
to initially anneal the silver at a much hotter than normal
temperature, like up to 1375, for 15 minutes, followed by a water
quench. what that does is to both dissolve the copper rich phase
(sterling is a mix of copper rich, and silver rich crystals, ie copper
dissolved in silver, and silver dissolved in copper), and allow
crystal grain growth to occur. Both will increase the effectiveness
of the age hardening procedure. But I’ve a problem with these
instructions. Many of the times I’ve quenched sterling silver from
that high a temperature, I get major cracks forming. In a number of
pieces I did in grad school, I used that propensity of overheated
silver to shatter when quenched to get some interesting effects of
broken and shattered forms. So the handy book’s specification for
annealing that high then quenching makes me wonder just how they wish
to do this without destroying the piece… Still, I mention it for
what its worth.

Hope this helps.

Peter Rowe