...allow it to cool to room temperature without hastening
the cooling. ...
What happens in this process, from my understanding, is
that you allow more cooling time, so molecules have more time
to align themselves loosely. If the metal needs to be even
softer, you can do this process several times, each time
getting the metal a little softer. This method also works well
for repousse' work.
Um. Not quite. this is true for steels, which harden due to a
phase shift on heating that you “trap” with quick cooling. but
with silver, something else occurs. Copper is not completely
soluable in silver, especially at room temp. When you heat to
annealing temps, the copper goes completely into solution. The
annealing temperature cause distored crystals from cold working,
or other stresses, to be relived. Distorted crystals actually
reform into more but smaller crystals. This smaller, more even
crystal structure is not only softer, but better working from a
strength standpoint as well. If you prolong heating to much, or
heat too hot, the recrystalization proceeds to actual crystal
growth, with the new crystals growing together again to form
fewer and larger crystals. The metal will still be soft, but not
as strong. And, if you cool it too slowly, or hold it at an
elevated temp below the annealing temperature, then the copper
which dissolved uniformly in the silver when you annealed it can
differentiate into two distinct compositions: Pure silver
crystals, and crystals which are the eutectic composition of 28.1
percent copper. With extended somewhat eleveated temperatures,
Some of the copper will come out of this solution with the
silver, forming seperate copper crystals at the boundaries
between silver crystals. These differentiations cause a
considerable INCREASE in hardness. The phenomenon is called age
hardening, or precipitation hardening. You can take annealed
sterling silver and more than double the hardness with this heat
treating, to about a “half hard” state. All without any working
of the metal.
When you anneal sterling silver, you want to bring it to a
sufficient temperature to effect the recrystalization that gives
that uniform dense small crystal size, and then you want to
quench cool it quickly enough so the copper does not have a
chance to differentiate. This will give you maximum softness, as
well as the best working metal.
In normal shop practice, we anneal sterling silver by torch
heating just till we see some “glow”, and then waiting just till
the glow is gone, and quenching in water. For maximum softness,
use a kiln set at 1375-1400 F (745-760C). Hold at that temp for
15 minutes, and then quench in cold water. (allow to air cool
in cool air till the bright glow is almost gone, to avoid
cracking the metal) This results in a hardness of about 56
(vickers). cold worked, such as rolling to a 60 percent
reduction, will produce a hardness of 140-180 V. If you take
the annealed metal, and hold it at a temp of 600 F (316 C), and
holding it there from 30-50 minutes, then allowing it to air cool
(no quench), you’ll raise the hardness back up to about 110-120V,
the same as about a 50 percent reduction in the mill, (half hard,
which is defined as reducing the thickness by two B&S guages).
Hope this helps.
Peter Rowe