hey all, in rereading what I wrote in annealing the other night,
I realize that I’ve mixed up some info here. I was talking about
heat treatment to age harden sterling silver, as well as
annealing it, and mixed things up a bit.
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).
You’ll note that I’ve here suggested that a normal annealing
temp requires 1375-1400 degrees.
I got that pretty much from paraphrasing the section in the
Handy and harmon “handy book” where it’s talking about age
hardening, figures that corrolate as well with those given by
Alan Revere for the age hardening process, in his fine book (I
surmise that Alan probably got the figures from handy and harmon
too, though I don’t know).
Typing with the book in front of me, I just copied figures
without quite thinking about them and what those temps actually
were. Late nite messages… Later, thinking about it, I realize
that this was an error on my part, and that this was not, in
fact, the right way to anneal. Checking, I found that it was
correct for heat treating, however.
Anyway, in the above paragraph, I describe that initial
annealing temp as the normal best anneal… THAT’S NOT THE CASE.
The above sequence is way too hot for a normal anneal, and in
fact may be too hot for many fabricated items as well, since it’s
well above the melt point for most solders. It’s the optimal
preheat temp before age hardening, since what’s desired here is
to both maximize grain growth as well as initially redissolving
the copper uniformly prior to precipitation hardening. the above
sequence does NOT produce the best anneal. metal heated this hot,
though now soft, will produce an orange peel surface when bent,
due to the resulting large crystal size. Note that for items
that cannot take this pretreatment, the later low temp heat soak
will also increase hardness almost as much, though without the
large grain size, which increases hardness even more. Also note,
that for this sequence to work, it’s almost essential to have an
atmosphere controlled furnace. 15 minutes at that high temp
will defeat pripps, and just about any other flux used to control
fire scale. Without an atmosphere controlled furnace, you’ll
have a mess on your hand.
So then. The CORRECT procedure for normal annealing of sterling
silver is this:
THE BEST ANNEALING TEMPERATURE FOR NORMAL SOFTENING OF STERLING
SILVER IS BETWEEN 1100 F, AND 1200 F. (593C-649C) Temperatures
above 1200 tend to dissolve the copper rich phase in sterling,
and maximum softness will not be reached. In practice, this
means you heat the silver until just a trace of glow/cherry red
is seen. Sterling anneals very quickly, so a “soak” at this
temperature is NOT needed at all. The silver may be air cooled
or quenched. The handy book suggests there is little difference
between air cooling and quenching, but in practice, most
silversmiths will tell you that quenched silver is slightly
softer. too slow an air cool will start to age harden the metal
a little. However, if quenching, be sure to let that cherry red
glow dissappear before quenching. You can crack the silver if
you quench it from too hot a temp. I’ve actually used this on
purpose a few times, intentionally overheating the silver to a
fairly bright red, then quenching from that temp, to produce a
series of shatter fractures and cracks that were quite
spectacular. this was in sheet metal that I’d abused in
misaligned rolls and then bead blasted until it was quite
distarted and warped, and then it got shattered like that. The
cracks are things you could never do with a sawblade, and the
result was a piece of highly distressed looking metal that was
perfect for the piece I was making… But that’s another story.
Hope this helps. And I hope I didn’t mess anyone’s metal up too