Age hardening metals

Jewelry Discussion
1

Dear Mark & All, If you would like to read more about age
hardening of Sterling silver and gold the only place I’ve ever
seen it documented is in two sources. The frst edition of Metal
Techniques For Craftsmen by Oppi Untracht talks about it a
little bit on page 13. The other source is The Handy Book of
Precious Metal, Which is a book you can’t buy. They used to give
it away for free. Call Handy & Harmon at 212-752-3400 and see if
it is still available. The Handy book is great. It was out of
print for many years and released again in 1988.

Regards,
TR the Teacher

Jewelry Dept.
Minneapolis Community & Technical College
www.mctc.mnscu.edu/acad/academic/programs/tech/jewelman/index.html

2

Age hardening or heat treating is a vastly under utilized
characteristic of precious metals. It becomes important when you
are making something that needs springiness after you have
assembled it, as when making a tension setting, money clip, pin
mechanism, or even hardening a prong setting to increase its
wear resistence. In essence, what you are doing when age
hardening is allowing the grains to grow in size so that the
metal becomes less malleable.

When researching precious metallurgy for my book, Professional
Goldsmithing, I compiled from the Alloy Data Sheets
supplied by the World Gold Council in 1990. Up to that point,
the WGC had issued this as part of the individual
data on each alloy, but not as a table comparing them, which is
what I did. The tables on page 206 and 207 in my book are as
complete as possible and offer procedures for age hardening 13
alloys of 14k and 18k gold in a range of colors, plus sterling
silver.

For example: The most common 14k gold alloy is yellow gold. It
is composed of 585 parts gold, 205 parts silver and 210 parts
copper and has a Vickers hardness of 190 when fully annealed.
After completely work hardening (by hammering, rolling, drawing,
forming, etc), it reaches 260 Vickers which we know from
experience is much harder. However it can be age hardened even
further, to 270 Vickers by heating to 360 degrees Centigrade for
60 mintues after annealing. That means you can make it harder by
heating than by working!

The same holds true for most 18k gold alloys, although the exact
procedures vary according to the specific composition of the
alloy. Depending on the application, you might want to select an
alloy that can be worked easily and then age hardened to a very
resistent state.

For example: If you wanted to make a tension setting in 18k
gold, the best alloy would probably be a rose color (pink) with
a composition of 750 gold, 45 silver, 205 copper. Fully annealed
it has a Vickers hardness of 165 so that you can move the metal
in place during setting. After maximum working (which is not
achieved during setting) it can reach a maximum of 240 Vickers.
However after all the work is done and the stone is set, you can
increase the hardness significantly, to 325 Vickers by annealing
and then heating at 280 degrees Centigrade for 60 minutes.

This is not as complicated as it might sound at first, and the
knowledge of how precious metals behave can be very helpful when
applied by the bench jeweler.

Alan
Revere Academy of Jewelry Arts
San Francisco

3

Alan…Ysesterdays discussion of age hardening was a truly
great one after such a long thread. I thank you for it. Do you
have any info on how precise the temperature and time must be to
achieve maximum hardness ?

Sol K.

4

Alan, It’s great reading your comments. My view about age
hardening is this. Most times I do not anneal it before oven
hardening. When I’m all done with a delicate piece with multiple
parts and multiple soldered joints I don’t want to risk damage
to stones or joints by annealing. It seems to work just as
well, but again I don’t want to damage the detail of the piece
with the probe that measures hardness. So the question remains,
do you always anneal first or is that not necessary?

All the Best,
TR the Teacher

Jewelry Dept.
Minneapolis Community & Technical College
www.mctc.mnscu.edu/acad/academic/programs/tech/jewelman/index.html

5 
 How precise the temperature and time must be to achieve
maximum hardness?

I can only guess on this. Since the came from a
laboratory environment, I think that the maximum effect will be
acheived by precisely following their procedures. However, it
will probably work to some extent, with a looser interpretation.
If anybody is able to answer this question better, please chime
in.

Alan Revere
Revere Academy of Jewelry Arts
San Francisco
http://www.revereacademy.com

6 
   I can only guess on this. Since the came from a
laboratory environment, I think that the maximum effect will
be acheived by precisely following their procedures. However,
it will probably work to some extent, with a looser
interpretation. If anybody is able to answer this question
better, please chime in.

The ideal temperature specifications are fairly narrow. You are
heating the metal up enough to allow precipitation hardening to
take place, but you don’t want it hotter than needed for this, or
instead of maximizing precipitation of the copper out of it’s
solid solution state, you’ll be allowing some of it to stay in
those solid solutions… To an extent, cooler will also work,
but will take longer. In this respect, the process is similar to
annealing. Metal can often be annealed at much lower temps than
we normally specify, but the time required for annealing can be
very long as a result. The bottom line is the closer you can
stick to the specs, the better your results will likely be. But
don’t allow less than perfect lab equipment to keep you from
trying this. Normal kilns with normal pyrometers should let you
do it just fine.

Peter Rowe

7

Sol, Alan, TR I just returned from the Santa Fe Symposium put on
by Eddie Bell of Rio Grande. There were several papers that
talked about precipitation hardening otherwise called age
hardening. The temperatures are critical but not in the way you
might think. It is possible to harden the metal alloys that
exhibit this characteristic both upon heating and holding at the
precipitation point or upon cooling down slowly from a higher
temperature through the precipatation point. This is why some
alloys need to be quenched to reach maximum softness. To reach
full hardness the alloy needs to be held at the point where the
precipitation of the desired phase occurs for some time . The
times that were mentioned at the conference were on the order of
10-20 min. The temperature will vary with the alloy and size of
the piece. Not all alloys will precipitation harden. There was a
paper at the conference about a platinum alloy that is more than
95% platinum that will precipitation harden and can be cast,
rolled and drawn. There was a lot of excitement about it as it
can be made as hard as some tool steels just by heat treating,
and the process can be reversed by annealing or solution treating
as the metallurgists called it.

Jim

Jim

@jbin
James Binnion Metal Arts
4701 San Leandro St #18
Oakland, CA 94601
510-436-3552

8

I’m not sure if the situation is different in 14K or sterling
age hardening, but aircraft aluminum alloys (6061 series for
instance) age harden because copper diffuses into the grain
boundaries locking up the slip planes that allow the metal to
bend, twist and stretch. The idea is to find a temperature that
gets this copper mobile but doesn’t facilitate too much grain
growth and recrystalization. A similar thing happens with copper
diffusing from the contact zone in gold (and silver) granulation
to form a locally eutectic zone causing the new bond. j

9 
There was a paper at the conference about a platinum alloy that
is more than 95% platinum that will precipitation harden and
can be cast, rolled and drawn. There was a lot of excitement
about it as it can be made as hard as some tool steels just by
heat treating, and the process can be reversed by annealing or
solution treating as the metallurgists called it.

Jim, I think you’re referring to the alloys Steven Kretchmer
developed, and is now being produced and distributed by Hoover
and Strong as platinum SK. It’s an interesting alloy, to be
sure. But it does present a couple drawbacks. Among other
things, I’m told it must be cast in a vacuum. Not a vac caster,
I mean melting and casting occurs in the absence of air. While
high end platinum casting machines like the Yasui and a number of
others will do this, it puts all those working with the old Torit
machines kinda outta luck. Plus, it melts something like 500
degrees cooler than iridio platinum. That makes it easier to
melt and cast, sure. But note that as a 950 platinum, it is
only required to be marked “PT”, with no indication of it’s
unusual alloy. Pity the poor benchworker who gets a ring of this
stuff to be sized. If he/she tries to fuse it, it won’t work at
all. Worse, try to flow 1700 solder on the joint as you’d
normally do with others that can’t easily be welded, such as
cobalt platinum. The platinum SK melts before that 1700 solder.
This stuff needs nothing higher than a 1500 solder, and I’d
predict that some folks are gonna not like the resulting visible
seams. The bottom line is that items made with this stuff need
to be sent back to the manufacturer for sizing, or to someone
with a laser who can weld a seam in vacuum. It’s a fascinating
alloy. But I hope that users of the alloy will see fit to mark
their wares in such a manner that unsuspecting jewelers won’t
accidentally do major damage in attempting to do routine work…

Peter Rowe

10

hi alan, it is my understanding that age hardening takes place
through certain alloys exsolving to the grain boundaries, not
thru grain enlargement. since i’m not really a metalurgical
maven, i too would like to hear more. best regards,

geo fox

11

Peter,

This alloy is not Steven Kretchmers.  It is a new alloy that is

being made by Imperial Smelting and Refining Co. of Canada. It
can be cast using standard equipment it melts at 1650 C and is
composed of 95.2% Pt-4.8% (Ga,In,Cu) It is reported to be
somewhat more sluggish than 95%Pt-5%Co but they say that it can
be cast with a 200 C superheat and this will overcome the
sluggishness. It has a as cast hardness of 280 Vickers it is
over twice as hard as 95Pt-5Co at 135 HV. When the cast alloy age
hardened it reaches 318 HV. This is supposed to make the
polishing much easier. As a wrought alloy it can be annealed in
air and softens to 175-to 185 HV compared to 120-130 HV for
95.2Pt-4.8Ru. It can be hardened to 420-430 HV with a tensile
strength of 183,000 psi by cold work and heat treating. This
compares favorably to tool steels which are in the neighborhood
of 170000 psi! 95.2Pt-4.8Ru only will reach 250-260 HV and 116000
psi. It has some drawbacks it requires frequent annealing at a
very high temp. of 1000-1100 C and it must be held there for 10
min. It i’s also prone to edge cracking when being rolled. Also
rolling and drawing with hand tools will be tough. They suggest
purchasing it close to the required dimensions to reduce the
effort required to work it. It certainly is not a perfect
material but it has some interesting properties. The wear
resistance should make it a long lasting material and it can be
fabricated in thinner sections and still be stronger than Pt
95.2-4.8Ru. It was also suggested that it could be used as a
plumb solder for Pt 95.2-4.8Ru due to its 200 C lower melting
temp… Any how it may be worth checking out. This is not a
commercial for the mfr. it is just an example of the kind of
that was available at the Santa Fe Symposium.

Jim

@jbin
James Binnion Metal Arts
4701 San Leandro St #18
Oakland, CA 94601
510-436-3552

12

For those who have been following this thread, please be advised
that Steven Kretchmer holds a patent for the use of age
hardening in compression (tension) setting. This means that he
has the sole legal right to use that technique and that he will
pursue anybody else who employs it.

Alan Revere
Revere Academy of Jewelry Arts
San Francisco

13

Jim,

Thanks for that detailed report. Wow. Sounds like a workable
material, though I still note it’s lower melting point than
iridium/platinum alloys, which suggests that some poor sap
running into this stuff might try to solder it with 1700 (or even
1600 would be dicey) solder. However, it’s workability in air
makes it doable. Yeah, it sounds a bit hard to work. But it
can’t be any nastier or more frustrating to work with than, say,
some 18K white casting gold alloys. Now THERE is a shitty metal
to have to fabricate stuff in. At least the stuff I get daily,
which is really a casting alloy, and we have to fabricate stuff
using the used buttons. Just a great source of workable
metal… Can’t convince the boss that he’s save our time, and
therefor money, if he’d get a different 18K white alloy designed
for fabrication for those jobs…

Anyway. I’ll look forward to working with this stuff. Sounds
REALLY strong, when properly treated… You didn’t mention the
heat treat temps. Are they low enough to allow treating after
diamond setting?

Peter
(gotta get back to packing for SNAG). gonna be there?

14

Hi James,

How can I get these writings about precipitation hardening. Do I
have to wait until the 1999 Santa Fe symposium book will be
edited or maybe it is already on the net. I tried to follow the
thread on the forum and maybe I missed something. Does anyone
gives any cues about infos or writings about this subject. I
already have Mark Grimwade’s book. As I am starting to write a
new lesson about precious metals (of course in French) these
fresh infos are really interesting for students. Thanks

Vincent Guy Audette

15

I too would like to learn more about the age hardening process.
Does any one know of any postings on the web about age
hardening?

Thanks,