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Heat hardening sterling


#1

A few people have asked me about this off line so I will try to
clarify the info.

First the how to.

Standard sterling silver (Ag92.5 Cu7.5) can be heat treated to make
it harder. It will not be as hard as it is possible to work harden it
but it will be significantly harder than annealed sterling.

The first step to get the greatest hardness you need to heat the
sterling hot enough to get all the copper into solution (I will write
more about this below) and then quench it. This locks all the copper
in solution and makes the sterling as soft as possible. Contrary to
what many of us were told or had read somewhere there is not a
specific solution temperature but rater a range of temperatures where
you can get enough copper into solution for a significant increase in
hardness. Here are several points along that continuum.

1472F (800C) : This is way too hot for virtually any application but
will yield maximum hardness when aged.

1346F (730C) : This is typically too hot for soldered work but
wrought items like flatware can be treated to this level.

1292F (700C) : This is in the range of medium silver solder flow.

1202F (650C) : Work Solution treated at this temperature shows only
very minor hardness improvement.

Solution treatment is best done in a kiln for a period of time long
enough for the reaction to dissolve as much copper as possible.
Remember this is a solid so it takes a bit of time for the atoms to
move around in the crystal matrix. One hour should be sufficient,
then the work needs to be immediately quenched in water. My preferred
method for doing this is to place the material to be solution treated
in a stainless steel can with a loose fitting lid filled with
charcoal. The work is coated with prips flux prior to burying it in
the charcoal. Rio offers a can like this for bronze clay and copper
clay firing. They also offer granular charcoal but you can find it
much cheaper if you buy 5 lbs from McMaster-Car. I wrap a piece of
stainless steel binding wire around the items being treated and leave
a loop of it protruding above the surface of the charcoal. When the
treatment is done I grasp the wire with tweezers and pluck the work
from the charcoal and quench it.

The second step is to allow some of the copper to come out of
solution in a controlled manner this is referred to as aging or
precipitation.

To do this you heat the solution treated sterling to a moderate
temperature and hold it there for a period of time. Again the use of
pripps flux will help keep the work from discoloring during this
part of the process. Aging is best done for somewhere between 30 min
to an hour at a temperature of 536F-572F (280C-300C). You can quench
after aging but it is not necessary to the hardening process.

Hardness values 1hr solution treatment, 30 min aging at 572F

1472F (800C) : 160HV (quenching from this temperature is quite
likely to fracture the sterling so again this is too hot)

1346F (730C) : 140HV

1292F (700C) : 120HV

1202F (650C) : 60HV

For comparison annealed standard sterling is about 56-66HV and fully
work hardened is about 180HV. BTW it is almost impossible to make a
piece fully work hardened as you need more than 80% reduction in
thickness without any heating afterward to get there.

So how does one best use this info?

We often see the solution treatment listed as a final step before
finishing, while this will work best in the case of flatware or
other items without solder it can definitely be done with soldered
items if one uses a slightly lower solution treatment temperature of
1292F (700C) and hard solder. But are we only going to see
improvement with items that fit those limitations? No, if you
solution treat the stock material to 1346F (730C) before you begin
to do the soldering part of fabrication and are careful to quench
the work as soon as the solder is solid and it is safe to move the
work the vast majority of copper will stay in solution. Even easy
silver solder needs to be heated to 1260F (682C) to become fully
fluid. At this temperature and given the brief time of the soldering
operation very little of the copper will be able to come out of
solution. So it is reasonable to expect that you will get hardness
numbers in the 120HV-140HV range as long as you solution treat your
materials before soldering and quench rapidly after soldering
followed by an aging treatment prior to final finishing work.

If you cast sterling you can also take advantage of this phenomenon,
you will not get quite the same hardness but you can get up to 110HV
which is significantly better than the as cast state. If you quench
the casting within 4 min of pouring you can clip and rough finish
the casting and prior to final finish age at 572F (300C) for 30 min
and you will get a hardness of around 110HV. The critical part is the
rapid quenching after pouring the casting.

How does this compare with Argentium? The Argentium website lists a
solution treatment temperature of 580C / 1076F for 20 min and an
aging temperature of 300C / 580F(which is probably a typo as
300C572F) for 90-120 minutes. This yields a hardness of 120HV. It
further states you can air cool the Argentium after soldering or
annealing then age at 300C / 580F for 90-120 minutes and get a
hardness of 100HV. So if you follow the appropriate heat treatment
processes for either Argentium or standard sterling you can expect to
see similar hardness values. The Argentium has a lower solution
temperature and looks like it hods the copper in solution a little
better than standard sterling but is also a little softer in ultimate
hardness values. But either one can be substantially hardened by
solution and aging heat treatment after fabrication or casting.

The hardness and temperature data for this post come from:

Introduction to Precious Metals by Mark Grimwade

An Update on Hardening of Sterling Silver Alloys by Heat Treatment by
Dr. Jorg Fischer-Buhner http://tinyurl.com/25j8zse

The Argentium Silver website http://www.argentiumsilver.com

Jim

James Binnion
James Binnion Metal Arts