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Tempering metal in regular oven


#1

I am having a hard time getting 20ga wire earrings in sterling and
gold-filled to be hard enough to resist bending. I have tried
tempering them with a Fretz hammer with a plastic tip. I hammer the
rounded end to give it a "hammered’ look thereby flattening and
tempering it, but the “legs” of the earring remain soft and often
bend when plastic guards are used to secure them to the earring card
or to keep them in the ear.

My jewelry instructor said she thought that the heat could be low
enough that a conventional home oven could do the tempering, thereby
bypassing the need to buy a kiln. Does anyone have any information
about this? If so, at what temperature and for how long would I
temper them. I’m assuming, as with air-cooling after annealing to
harden the metal, I would air cool them rather than quenching them.

Carol McKay
www.white-rabbitdesigns.com


#2

Ok, first off lets get the terminology right you are not trying to
temper them you are trying to harden them. Tempering is a softening
process done on steel to reduce hardness and increase toughness.

What you are referring to is a heat treating process called aging,
age hardening or precipitation hardening. There are several metal
alloys that you can do this to and sterling is one of them but gold
fill is not so you will have to try something else (work hardening)
to harden your gold fill work like the twisting process that has
been mentioned repeatedly on this list.

Sterling can be age hardened by first heating to the solution
annealing temperature (about 1382 F) then quenching. Followed with
the aging treatment at 550 F to 600 F for 1 hr. This is likely
beyond your ovens temperature range. No need to air cool afterwards.

You can skip the solution annealing step and get some improvement in
hardness but how much will greatly depend on how you metal has been
treated since its last annealing.

James Binnion
James Binnion Metal Arts


#3
I am having a hard time getting 20ga wire earrings in sterling and
gold-filled to be hard enough to resist bending. I have tried
tempering them with a Fretz hammer with a plastic tip. 

First off, lets correct the terms. Tempering is what you do to
soften steel after it’s been fully hardened to (usually) glass hard
status. Tempering is reheating it again to a lower temperature which
softens it a bit to the desired end result. Tempering is NOT
increasing the hardness, it’s reducing it. When you harden steel, you
first quench from high temps, making it generally much too hard.
Tempering then softens it a bit to the level of hardness you
actually wanted. Tempering is an operation unique to steel, not
precious metals.

Second. What do you think hammering with a plastic mallet will do?
What you need to do is work harden the metal. That only happens with
actual deformation of the metal and changing / distorting the shape
of the crystals. Merely banging on it a bit to surprise it and wake
it up won’t do anything unless you’re actually deforming the metal,
and a little plastic mallet generally won’t do enough of that to help
your ear wires. There is a misconception out there that a mallet like
this will actually harden your metal. That usually isn’t true enough
to matter, especially with wire, where you need substantial increase
in hardness and stiffness to make a difference. Now, if you’re mallet
is bending the wire back and forth, that hardens it. But simply
shocking it a bit? Nope.

I hammer the rounded end to give it a "hammered' look thereby
flattening and tempering it, but the "legs" of the earring remain
soft and often bend when plastic guards are used to secure them to
the earring card or to keep them in the ear. 

My suggestion would be to twist the wire after all soldering, but
before final shaping. Twisting does not greatly change the overall
shape of the wire, but it does distort the crystals and nicely work
harden the wire. Be careful, as you can over do it, breaking the
wire. Especially, one can snap off the solder joint where the wire is
soldered to something else, since often this is fully annealed more
than the rest of the wire, and ends up being twisted more than the
rest, just where it’s also weakest. But with care, twisting can do
what you need. It’s also a good way to take wire that it’s quite as
straight as one might wish, and make it so. With longer wire, hold
one end in a vise, or pliers, and the other in your flex shaft chuck
and hit the pedal briefly while putting a bit of tension on the wire
to keep it straight. Works like a charm. With shorter wires, like ear
posts, simply hold the very end with pliers or a pin vise and giving
the wire a turn or two or three, as desired, of twist, should
suffice. If the wire connects at a vulnerable solder joint, hold the
wire just ahead of that part with pliers while twisting the main
length, so you don’t break it off at the solder joint.

My jewelry instructor said she thought that the heat could be low
enough that a conventional home oven could do the tempering,

Again, this is not called tempering. In this case, the process
you’re referring to is called “age hardening” or “precipitation
hardening”. It’s important to note that this only works with some
metals, not all. Fine silver, for example, does not harden this way,
and I’m not sure about gold filled, but I doubt it would work. But
sterling silver does work. The temperature you need may be too much
for a kitchen oven. You also need accurate temperature control. The
needed temps are in the 650 to 700 F range, and too high or too low
by more than a very little, will not then work. Your home oven likely
cannot reach these temps except perhaps on a “self clean” cycle, and
even then, you wouldn’t have the needed precise temperature control.

It’s also important to note that what this does is increase the
hardness by a different means than work hardening. Instead of
distorting crystals in the metal, it causes the copper to literally
migrate to the boundaries of the silver crystals and crystalize out
there, making those boundaries less able to deform, thus the whole
metal is stiffer. But age hardened silver, while harder and stiffer
than fully annealed, is also slightly more prone to cracking than is
silver work hardened to the same degree (roughly half hard…).
Also, for best results, you first need to really thoroughly anneal
the silver, to a higher temperature than is the norm. Without that
step, the results of age hardening are less effective. Also, the
prolonged low temp age hardening heating, as well as the initial
anneal, can both increase problems with fire stain and oxidation.
Again, if you can physically harden the metal mechanically, you’re
better off, and it’s usually easier. As I said, twisting is a good
way to do it with round wire.

Hope that helps
Peter Rowe


#4

Carol; Forget the oven take a couple feet of wire put one end in your
foredom or electric drill if you don’t have one. Hold the other end
with a pair of pliers. make a 90 degree bend in the end to make it
easier to grasp with the pliers. give it a spin you can draw a line
down the metal with a sharpie at first to get the right amount of
turns it should look like a barber pole. You can experiment for the
hardness you want the more turns the harder the metal. Dave Owen

David Owen


#5

Have you tried half hard wire.

Vince LaRochelle


#6

Dear Peter (W. Rowe) and James (Binnion), I am so glad that you set
the record straight with people getting the terms correct for working
with precious metals. As usual both of you are a joy to read and both
of your explanations were so informative. Thank you for contributing
so much knowledge and being so willing to share.

Beth Katz
myuniquesolutions.com