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Heat hardening of silver


#1

I am not having success at heat-hardening sterling, following
directions taken from the forum & other sources. I cook fully
annealed sterling at about 600 degrees F for 30 minutes and get -
nearly fully annealed sterling. Perhaps my temp control isn’t good
enough. If the temp drifts up to 700 degrees will it undo the
process? Any other thoughts?

Many thanks-
Linda


#2

Rio sells these little heat button thingies that help you calibrate
your oven thermostat.

Sorry, “little heat button thingies” is as close as I can get to the
proper technical term…

I have my order form, but not the catalog, and they’re on page 102
in the 2004 Tools catalog, order # 703018.

Sojourner


#3
I am not having success at heat-hardening sterling, following
directions taken from the forum & other sources. I cook fully
annealed sterling at about 600 degrees F for 30 minutes and get -
nearly fully annealed sterling. 

Hello Linda,

The only silver alloy that I know of that will heat-harden (aka
precipitation hardening) at those temps is Argentium.

Regular sterling silver just begins to precip harden at around 765 C,
or approx 1400 F. (Brehpohl, p. 171).

Cheers,
Trevor F.
in The City of Light


#4

Linda, on page 7 of “The Complete Metalsmith”, by Tim McCreight, he
speaks of heat hardening and says it can be achieved by warming the
metal sufficiently to begin recrystalization and holding at this
temperature long enough to allow gradual ordered crystal growth. He
then says specifically for sterling: quote: “After all soldering is
done, heat to 280 degrees Centigrade (536 degrees Fahrenheit) and
hold for 2 1/2 hours. Quench in pickle and finish as usual.” End
quote

Perhaps you are not heating your metal long enough. I personally
have not tried this method of heat hardening so cannot speak from
personal experience. But you indicate you only held the metal at
600 degrees for 30 minutes which according to Tim McCreight’s
comments would not be long enough.


#5

Linda,

Heat hardening is a two step process first you must "solution treat"
the sterling by heating it to 1382F (750C) for 30 minutes, then a
rapid quench in water. The second step is the "ageing treatment"
which is 572F (300C) for one hour. You can only use this process on
items that have not been soldered, any silver solder will melt at
the solution treatment temperatures. If you are just trying the
ageing part of the process the results will not exhibit any
significant change in hardness.

Jim

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau


#6
    After all soldering is done, heat to 280 degrees Centigrade
(536 degrees Fahrenheit) and hold for 2 1/2 hours.  Quench in
pickle and finish as usual. 

While I have lots of respect for Tim he is just wrong on this . You
will not get any appreciable hardening without first doing a
solution treatment of the sterling at 1382 F for 30 minutes. At this
temperature all of the silver solders are molten. You cannot do this
on soldered work it is just a waste of time and energy as there is
just not enough gain in hardness to make it worth doing. Read up on
solution treating and age hardening and this will make sense.
Brephol is one good source on this there are many others that are
not sterling specific but the concept is the same.

Jim

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau


#7
 If you are just trying the ageing part of the process the results
will not exhibit any significant change in hardness. 

This is odd, because I have followed the instructions in the Rio
catalog (just heating to around 600 degrees for a half-hour or so)
and had it be very effective!

–Noel


#8

James:

I posted this process a few years ago here about tempering gold and
sterling silver.

I got the from Handy and Harmond Cheif Metalureist: He
stated: For Sterling Silver temporing put the item in a elect
furnace prefered cold run the temp up to 1,000*f and hold for 5
minutes , then shut it down and let the oven cool to room temp.

If your temp gague is with in a 2*f margin it will work. I temper my
Elk and Caribou game heads in Sterling silver and it has worked verry
good for me in the last 25 yrs.

You can do 14K gold the same way but go to 1,100*f and use the same
program. I don,t do the gold any more as I found my refiner has 24K
yellow # 251 in spring dold which has worked better.

Yours :
Billy S. Bates
royalminiatures.com


#9

Hi Linda

I have had some success from heat hardening Sterling sil . I have
only tried 300 deg Celsius for 1 hour . It does not turn it to stone
but it does provide a noticeably harder material for my needs which
is to provide a more durable and lasting metal for my customers who
are quite hard wearing on their jewellery. I imagine the temperature
range should not vary more than a few percent to attain best
results. If your thermostat is out i would recommend getting new
digital thermo controller.

Phil W


#10

OK, I feel a little better. First, thanks for the suggestions.
Second, you see that almost none of you agree on the process, nor do
the books. I’ve read Brephol & the article in AJM, the metals books
from all my metals suppliers, and still am not achieving success.

Jim - a question. Should the soaking process at 1,382 F occur after
annealing, or is it useless then? Brephol & others have said that
the process only works if the piece has been somewhat work hardened
first - but now that the tubing is bent and dead soft, I don’t know
how to work harden it without changing the surface or the shape.

I ask because I have tried the following: Solution treated annealed
sterling at approx 1,380 F for an hour. Waited for the kiln to cool
down (overnight, actually) and soaked at 550F for 45 minutes. My
sterling is still dead soft. Ridiculously soft. It is not fine
silver - it is definitely sterling. This is getting critical because
I am fabricating a necklace that requires sections of curved tubing -
I annealed the tubing to bend it but cannot cut it into sections
without it kinking or collapsing. That’s why I need to harden it. I
also want the neck ring to be quite springy.

So any further suggestions would be highly welcomed, even if I have
to start with fresh tubing. Right now I’m experimenting with the
process on scrap silver, to no avail.

Thanks again for your help on this -
Linda


#11

Noel,

First a question. This piece you aged how much work had been done on
it was it cast or wrought , had it been soldered on? All these
things will affect the amount of age hardening you get. New annealed
sheet or wire from the supplier may harden quite a bit. Material
that has been cast or soldered several times probably will show
little change.

Solution treatment and age hardening of sterling works best when all
the copper in the sterling alloy is completely dissolved in the
silver. To do this you have to get and hold the copper totally in
solution in the silver.

Sliver’s crystal matrix will not hold all the copper that is in
sterling in solution at lower temperatures. So you end up with two
crystal structures in sterling one that is mostly silver with a
little copper (alpha phase) and one that is mostly copper with a
little silver(beta phase). These are pretty much evenly distributed
throughout the sterling crystal matrix. So to get all the copper to
dissolve in the silver you heat it. This is very similar to the way
you can dissolve more sugar in hot water than cold. At 1382 F (750C)
silver will hold 7.5% copper in solution. Because this is a solid
you need to give it some time to dissolve all the copper rich beta
phase into a single solid solution of alpha phase crystals, hence
the thirty minute hold time at this temperature. If you then rapidly
quench the sterling alloy you will freeze it in a single phase
(alpha) crystal structure that is the softest and most workable form
of sterling.

If you take this single phase alloy and heat it up to 572F (300C)
tiny little beta phase crystals will precipitate out of the alpha
phase solution in a very uniform distribution throughout the matrix.
These little beta crystals have the effect of making any slipping or
rearrangement of the crystal matrix very difficult. Something like
throwing a handful of sand into a precision gear train it just locks
it up. This is called artificial ageing or ageing treatment. This
ageing will also occur at lower temperatures even room temperature
but it will take a long time (centuries) hence the term ageing.

If you try to age a mixed crystal solution like you will normally
have from a cast piece or wrought piece that has had cold work and
soldering operations done on it you will get very little
precipitation of the beta phase because it has already been
separated out from the heating, cooling and working it has
undergone. The amount of precipitation you get will vary a lot
depending on how the metal was treated. The amount of cold work you
put in it, the number of times you soldered it, how long you
annealed it, how rapidly you cooled it will all affect the change in
hardness. Sterling when it is fully annealed has a hardness of 56
on the Vickers scale. As cast it is about 63 Vickers. When it is
work hardened by reducing its thickness by 50%-70% it is about 140
to 180 Vickers and if properly age hardened it is in the 110-120
Vickers range. You may get a slight increase in hardness without
prior solution treatment but it is not going to be a 60 point
increase. For reference for what the Vickers numbers mean a typical
18k yellow gold is 150 Vickers annealed and 220 cold worked, 14k
yellow is 190 annealed and 260 cold worked and a 18k nickel white is
220 annealed and 320 when cold worked. 320 is at the low end of
tool steel hardness. My point is that sterling is soft, and unless
you can leave it in a work hardened state or can properly age harden
it even a 15 point gain that you may be able to feel a difference
when you try to bend it will still leave you with a soft material
and you will have spent the time and energy to do this without much
effect.

For a quick reference on hardness testing check out
http://www.calce.umd.edu/general/Facilities/Hardness_ad_.htm

Regards,
Jim
Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau


#12

Linda,

Thinking out loud. There have been discussions here before on
cutting tubes without them collapsing.

Have you considered filling the tubes with water and then freezing,
cutting quickly while they are rigid? I suppose filling with oiled
sand may also work.

Once your sections are as you want them to be, can they then be work
hardened in a tumbler with steel shot?

Terrie


#13
  First a question. This piece you aged how much work had been
done on it was it cast or wrought , had it been soldered on? 

Hi, Jim,

First, let me say that the pieces I’ve heat-hardened have been
constructed pieces. I can’t say how much they were hardened, but I
felt that it was more than I could get from the tumbler. Plus, of
course, tumbling will also harden fine silver to some degree, and I
didn’t want to stiffen my bezels.

But mainly, I want to thank you for the great explanation. What you
say is very clear, and makes sense to me. It also clarifies why some
pieces seem to benefit more from heat-treatment than others. But you
know how it is-- a lot of things work in practice that will never
work in theory ;>). It does seem to do some good most of the time,
even if it shouldn’t.

–Noel


#14
        Should the soaking process at 1,382 F occur after
annealing, or is it useless then?  Brephol & others have said that
the process only works if the piece has been somewhat work hardened
first - but now that the tubing is bent and dead soft, I don't know
how to work harden it without changing the surface or the shape.

You will still get some hardening but not as much as if you started
with work hardened material.

    I ask because I have tried the following:  Solution treated
annealed sterling at approx 1,380 F for an hour.  Waited for the
kiln to cool down (overnight, actually) and soaked at 550F for 45
minutes.  My sterling is still dead soft.  Ridiculously soft.  It
is not fine silver - it is definitely sterling. 

You MUST quench it directly from 1380F . By allowing it to cool
slowly you are allowing the sterling to return to a two phase
crystal structure. When you heat it to 1380F you dissolve all the
beta phase alloy in the sterling and only have alpha phase. By
immediately quenching you lock the alloy in the alpha phase. That is
the whole point of this process to get all the copper into a single
phase solution and hold it there till you precipitate it out in a
controlled manner by heating at 550F. By doing this in a controlled
fashion you cause the copper rich phase to come out as a widely
distributed tiny crystals that have a tendency to stiffen the
crystal matrix which causes the desired increase in hardness.

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau