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Precipitation hardening sterling


#1

Dear Orchid folk,

Having learned the limitations of my tumbler and steel shot, I’m
looking for a method of actually hardening my pieces to their best
possible hardness, after all soldering operations have been
accomplished. Work hardening by sawing, hammering and filing, etc is
not always appropriate or sufficient, as often the final soldering
operation has to be accomplished as the last fabrication step and so
it negates any benefits such work hardening offers.

To that end, I have been reading about precipitation or age
hardening. Most of what I’ve read so far has talked about the last
stage needing temperatures of about 300 degrees Celcius and above. I
have also read that a domestic oven (which is all I have access to
at the moment) can be used as long as you can heat to a temperature
of at least 220 degrees Celcius. My oven has a maximum temperature of
245 degrees Celcius, and so it is suitable if such is
correct. However, it does say that if using such temperatures, you
need to increase the time it’s kept at that temperature, but it
doesn’t tell you how long to heat the pieces for.

Therefore, do any Orchid members routinely use precipitation
hardening in a domestic oven, for regular sterling silver? If so,
what time scale do you use if using a temperature of 245 degrees
Celcius? Also, how long would Argentium need to be heated at such a
temperature? If anyone has a time for 18K yellow gold (standard
alloy), that would be useful too, remembering that I only have
access to a domestic oven with a maximum temperature of 245 degrees
Celcius.

Thanks very much.

Helen
UK
http://www.hillsgems.co.uk
http://helensgems.ganoksin.com/blogs/


#2

I was just thinking…

Wouldnt work hardening pieces potentially make solder joints brittle
and more likely to fracture therefor be left for cast pieces?

Any ideas?

Cheers,
Jon Horton


#3

For best results you need to heat the sterling to about 750 C (1382F)
and quench it before precipitation hardening, this is called solution
annealing. The problem is that all silver solders are molten at that
temp so for work that is soldered you need to do the best you can to
heat it as hot as you can and not melt your solder. Then quench in
water followed by a aging treatment of 2.5 hours at 280 C for (536F)
not 500C sounds like someone mixed up C and F temperatures. Anyway
even on soldered work that is treated by the partial solution
treatment followed by aging it will make for a significant
improvement in hardness.

Jim
James Binnion
James Binnion Metal Arts


#4

I can only talk about Argentium because that’s the only kind I’ve
learned about. Argentium can be heat hardened to almost twice the
hardness of standard sterling in a 400F oven for 30 minutes or so. It
does depend upon the size of your piece. For more info on Argentium
heat hardening and it’s technical properties, you can check

http://tinyurl.com/mx5y9a

Here’s another excellent article on Argentium…

http://www.ganoksin.com/borisat/nenam/argentium-testing.htm.

From what I understand, precipitation hardening involves rapidly
cooling (quenching) the metal from red heat and is not suitable for
soldered or delicate items.

Michele
MikiCat Designs
http://www.mikicatdesigns.com


#5
I can only talk about Argentium because that's the only kind I've
learned about. Argentium can be heat hardened to almost twice the
hardness of standard sterling in a 400F oven for 30 minutes or so. 

This is not correct.

The Argentium website has the following statement on it “Argentium
930 Silver can be heat treated to almost twice the hardness of
ANNEALED standard sterling silver.”, note the annealed part of this
statement.

So to compare apples to apples

Argentium annealed 60-70 HV
Standard Sterling annealed 56-66 HV

Argentium aged (precipitation hardened) 110-120 HV
Standard Sterling aged (precipitation hardened) 120-140 HV

Argentium cold worked 150-160 HV
Standard Sterling cold worked 140-180 HV

These numbers come from Mark Grimwade’s “Introduction to Precious
Metals”

So as you can see there is very little difference between Argentium
and standard sterling hardness when comparing the same conditions.

Jim

James Binnion
James Binnion Metal Arts


#6
for (536F) not 500C sounds like someone mixed up C and F
temperatures. 

Yea, like me. I got it in my head you said 500 C don’t ask me where
that came from. Anyhow your oven is likely not going to be hot
enough for the precipitation of the second phase. I did a little more
digging and found a paper where the author treated the sterling at
700C (1292F) for 1/2 hr and quenched followed by an aging treatment
at 300C (572 F) for 1 hour and hardness tests of the resulting
sterling items showed 120 HV as compared to 140HV for sterling
treated at 745C for a half hour and a 300C aging treatment of 1 hour.
So the bad news is you need a kiln to do this but if you get one you
should be able to get very good hardness results when compared to
annealed or as soldered sterling.

Jim

James Binnion
James Binnion Metal Arts


#7

Hi James,

But isn’t it also true that argentium becomes brittle in its
hardened state as well, more so than sterling?

Thanks in advance for your thoughts on this,
Christine


#8
Anyway even on soldered work that is treated by the partial
solution treatment followed by aging it will make for a significant
improvement in hardness. 

Brilliant Jim, thanks. If it’ll give some degree of improvement in
hardness, then it’s worth me giving it a go. Shame there’s no real
way for me to experiment and test the relative hardnesses at home.

Helen
UK


#9
From what I understand, precipitation hardening involves rapidly
cooling (quenching) the metal from red heat and is not suitable
for soldered or delicate items. 

That’s true for most solders, but the solder I use has a liquidus
temperature of 810 degrees Celcius, which is higher than many
solders, so I’m not worried about that.

Thanks for the info re Argentium - I’m thinking of giving that a go
too.

Helen
UK


#10

Hi,

Here is an excerpt from one of my articles about Argentium Silver:

Argentium Sterling hardens well after a slow air-cool, and quenching
is not essential to the hardening process.

Heat the Argentium sterling in an oven or kiln to 580 F (304 C).
Begin timing after the oven or kiln and the support for the Argentium
sterling have reached the required temperature. Heat for at least 45
minutes (a very large or thick piece may require longer), then
air-cool the piece to room temperature. There is no negative effect
on the alloy if the metal is left in the oven longer than the minimum
time.

Lower temperatures can be used if the heating time is increased. At
428 F (220 C), the minimum temperature required, Argentium sterling
needs to be heat-treated for approximately 2 hours. I usually
heat-treat my Argentium sterling pieces at the maximum temperature of
my kitchen oven, 550 F (288 C), for an hour and a half.

To avoid contamination and to minimize discoloration while heating,
make sure the oven is clean to prevent smoke caused by burning food
drippings, and place the Argentium sterling on a clean Pyrex dish.
Using a metal rack or a metal pan is not recommended.

Any discoloration from the hardening process can be removed easily
with pickle. I think that it is good to pickle even if it is not
discolored, to remove any copper on the surface, thus reducing future
tarnish. For further anti-tarnish insurance, or if you want to
refresh the shine which probably was dulled by the hardening process,
I suggest using a Goddard’s Long Shine Silver Cloth or Liquid.

Don’t enclose the Argentium sterling when heat-hardening it –
insulation slows down the heating process and prevents oxygen from
reaching the metal and creating germanium oxide, which prevents
tarnish.

The hardening process won’t harm fine silver, sterling silver, gold,
or copper alloys used in combination with Argentium sterling.
Argentium sterling can be annealed and then rehardened if needed.

Cynthia Eid
http://www.cynthiaeid.com/


#11
From what I understand, precipitation hardening involves rapidly
cooling (quenching) the metal from red heat and is not suitable
for soldered or delicate items. 

Precipitation hardening is also referred to aging or artificial
aging. It is done on alloys that have at least two phases or crystal
structures at room temperature. In standard sterling silver the
silver rich alpha phase can only hold about 2 % copper in solution at
room temperature. So the rest of the of the copper is in the form of
a copper rich beta phase crystal structure. So sterling at room
temperature has two distinct crystal structures in it. These two
crystals are slightly different in basic unit size so there is a
stiffening of the matrix as it tries to accommodate both crystals.
This is what makes sterling harder and less ductile than either fine
silver or pure copper.

In precipitation hardening we take advantage of this two crystal
structure and control how it develops to make it even harder than
normal. The hardening process has two steps the first is the
solution heat treatment where the alloy is heated to a temperature
high enough to get all components into solution otherwise known as a
single phase. While the alloy remains in the solid state this
solution treatment disperses all the alloy components uniformly
throughout the crystal matrix. So in the case of sterling the silver
and copper are now in a uniformly distributed mixture throughout the
crystal matrix, no more silver rich or copper rich areas. Then the
alloy is quenched to lock the components into a single phase. If you
cool it rapidly enough the atoms don’t have time to segregate and
remain as a single phase crystal. After solution treatment we want to
allow the beta phase to precipitate out of solution in a controlled
fashion, this is called aging. To do this the alloy is heated to a
moderate temperature that allows for the copper rich phase to come
out as very tiny crystals at the grain boundaries of the existing
matrix. These small beta phase crystals lock or pin the larger alpha
crystals in place and make it harder for them to move. This
resistance to movement is what is measured by hardness tests. So by
this controlled precipitation of the beta phase we are able to take
the annealed sterling from an annealed hardness of around 60HV to
around 130HV when aged.

This process is the same for standard sterling or Argentium the
recommended solution temperatures are different (565C/1050F for
Argentium and 748C/1378F for standard sterling) for 30 min. Both
alloys are aged at 300C/580F for an hour. These temperatures and
times are the optimum, you can deviate from them and achieve some
decent results. The Argentium website quotes a process where you just
allow air cooling after soldering and then age at the 300C/580F temp
for 1hr and cites a hardness of 100-110HV You can get similar results
from standard sterling as long as you quench rather than air cool and
then age at the normal temperature and time.

You can get the controlled precipitation or aging at lower
temperatures but the times rapidly get to be rather excessive as the
temperature decreases. The beta phase will eventually precipitate out
at room temperature but it may take many decades to occur. The
Argentium website quotes that 220C/365F will do the same hardening
with a two hour time at temperature. I have also seen reported times
of more like 6-8 hours but there is very little good data available
on these alternate times and temperatures. But in Helen’s case it
would certainly be worth experimenting with to gain at least some
improvement in hardness.

Jim

James Binnion
James Binnion Metal Arts


#12

Hi Cynthia

I know I have read your earlier threads on handling Argentium
Silver. This sounds to me like you are firing the finished piece of
jewelry - guess I don’t want to be confused.

Did you also allude to the fact that the sheet should be heated,
too? I had really understood that to be the case. I have two 6 x 6
sheets that I have never tried…think it is about time, but just
need some refresher.

Totally read every thread that has your name attached…you are such
a wealth of info. Thanks for all you submit. Of course, to everyone
else who enlightens my life!

Just recouping from having my right shoulder replaced. 6 weeks the
21st (old arthritis!)…am really getting eager to do some hammering,
can saw but that is about it! Did a little bit on the flat lap to
get some inlay straight - not mine - a students! My inlay items are
screaming at me! Briefly got to take in the Denver Gem Shows
Friday…woohoo…lots of planning going on - pretty stuff - didn’t
get to Tucson this past Feb so did my part helping these folks make
a living! HAHA

Rose Marie Christison


#13

Hi,

Re: brittleness

I do not find that Argentium Sterling is brittle after precipitation
hardening. I think that people who have found AS to be brittle have
over-heated the silver. When annealing or soldering AS, it is
important to remember that AS is unlike SS or copper in terms of
heat conductivity; it conducts heat in a manner more similar to gold.
When I teach a workshop on Argentium Silver, I find that people are
surprised at how quickly it is annealed. When I first began working
with AS, I, too, overheated when annealing it, until I spent time
annealing in the dark so that I could learn to know when it is
annealed in the light. For most people, the simplest way to avoid
over-heating when annealing is to scribble on the AS with Sharpie
marker, and heat until the marker fades and nearly disappears. I heat
thicker metals a little longer after this happens. It is also
important to let AS air cool a bit before picking it up or quenching
it, as the heat does not dissipate as quickly as with SS. AS is
fragile when red-hot, and does not cool as quickly as SS.

Re: Apples to Apples

In addition to the comparisons that have been made about
precipitation-hardening SS and AS, there are these differences:
Argentium Silver does not need a 30min long high temperature anneal,
nor does it need a quench. Full instructions for all heat treatment
methods of hardening Argentium Silver can be found at

Cynthia Eid
http://www.cynthiaeid.com/


#14

Hi Cindy,

In addition to the comparisons that have been made about
precipitation-hardening SS and AS, there are these differences:
Argentium Silver does not need a 30min long high temperature
anneal, nor does it need a quench. Full instructions for all heat
treatment methods of hardening Argentium Silver can be found at
www.argentiumsilver.com 

Yes I mentioned the process with the air cooling of AS in my last
post. But to gain the maximum hardness in either alloy you still need
to quench as published on the Argentium website data sheets. The
mechanism for precipitation hardening in AS is due to the lack of
solubility of copper in silver just like standard sterling. The
germanium lowers the temperatures of the melting point, annealing
temp and the temperature where the copper is completely in solution
but it is still the copper that is providing the hardening effect.
In Argentiun just like standard sterling there is more copper in the
alloy than can be held in solution at room temperature so if you
control the growth of the copper rich phase by artificial aging you
make it harder. As some on Orchid have mentioned and a recent paper
by Dr. Jorg Fischer-Buehner confirmed that lower solution
temperatures can still produce significant hardening in standard
sterling (120HV from 1292F solution treatment in Jorg’s paper). As
far as I can see there is really not a significant difference in the
behavior of standard sterling and Argentium in regards to
precipitation hardening. I think that the real reason we have not
been using the precipitation hardening process on standard sterling
is that we were told by metallurgists that the process needed the
very high temperature solution treatment but what was being
described was the optimum process not the only one. So I would expect
to see an improved hardness in the standard sterling alloy with air
cooling from annealing temperature as the solution step just like
Argentium. It is relatively easy to show with some time on a Vickers
hardness tester maybe we can get one of the metal suppliers to run
some tests comparing apples to apples as you say.

So between standard sterling and Argentium there are minor
differences in final hardness and temperatures required for
precipitation hardening but that is about it. The firestain
resistance and improved tarnish resistance are still Argentium’s real
significant advantages.

Regards,

Jim

James Binnion
James Binnion Metal Arts


#15
It is relatively easy to show with some time on a Vickers hardness
tester 

It is also relatively easy to show to your own satisfaction without a
tester other than your fingers. If you try heat hardening in a kiln
or your home oven, you will be able to tell the difference very
easily. And if you don’t want to try to bend a piece of finished
jewelry to see how much harder it is, just start with a few, say,
1cm x 5cm strips, subject them to various schedules and temps, and
bend them!

Noel


#16

Given the recent posts about the precipitation hardening of silver
alloys and some of the confusion which seems to have appeared over
what exactly precipitation hardening is I thought I would try to
answer some of the questions raised.

Let me declare my interest right at the start. I am a qualified
metallurgist who has worked in the silver manufacturing industry for
over 20 years. For the last 18 months I have been employed by
Argentium International Limited as their Quality Assurance Manager.

So first of all what is meant by precipitation hardening, what is it?

If we consider an everyday analogy to assist in our understanding of
what happens within the silver alloy; think of sugar being dissolved
in hot tea. At any given temperature there is only so much sugar that
can be dissolved into the tea. When no more sugar can be dissolved at
that temperature we have what is called a saturated solution. If the
temperature of the tea is increased then more sugar can be dissolved
into the tea until the saturation point is again reached. If you then
allow the temperature of the tea to fall, the tea becomes
super-saturated and the sugar is precipitated from the tea as small
crystals.

The same situation exists with metallic solid solutions. The
solubility of one metal in another metal increases as the temperature
increases. In the case of silver-copper alloys, molten silver and
copper are completely soluble in each other in all proportions. When
solidified, silver alloys having a copper content in the range from
about 2% through 27%, contain, two discrete constituents or phases
that can be seen when examined under a microscope. One is nearly 100%
silver; the other is a silver-copper eutectic* (71.9% silver; 28.1%
copper) which has a melting point of 780degC (1435 F ).

So if we go back to our analogy of tea and sugar, the silver rich
phase is the tea and the sugar is the silver-copper eutectic. All
silver-copper sterling silver alloys contain a mixture of these two
phases. The exact proportion of the phases depends on the cooling
rate (for investment castings) or the working and annealing schedule
that the piece has received for sheet and wire products. All we are
aiming to achieve with the precipitation hardening heat treatments
is to first take all the silver-copper eutectic phase into solution
in the silver phase. Then quickly cool the alloy to create a
supersaturated solution; finally to use a low temperature heat
treatment to encourage the growth of the silver-copper eutectic
precipitate, to harden the alloy.

The heat treatment schedule to achieve precipitation hardening in
sterling silver is well documented (1). It consists of the following
stages:

  1. Heat the alloy to 745-760degC (1375-1400 F). This takes all the
    silver-copper eutectic into solution in the silver - rich phase.

  2. Hold at temperature for 15 minutes. This allows the piece to be
    heated to temperature throughout its cross-section.

  3. Quench rapidly into cold water. This retains the silver-copper
    eutectic phase in the silver-rich phase as a supersaturated
    solution. The alloy is now in a softened condition.

  4. To re-harden the alloy it is now heated to 280-300degC (536 - 572
    F) for 30-60 minutes and then allowed to air cooled. The exact time
    to achieve the maximum hardness is dependent on the cross-sectional
    thickness of the piece being treated.

If we now look at how the hardness can change at each stage of this
process (these figures are from numerous databases, publications and
personal experience):

  1. Sterling silver, as-cast investment pieces and annealed sheet and
    wire (as supplied from the manufacturer) has a typical hardness of
    65-75VPN (DPN).

  2. Heating to 745-760degC and then quenching produces a soft
    malleable condition in the alloy. This has a typical hardness of
    55-60VPN (DPN).

  3. After precipitation hardening it is possible to achieve
    hardnesses of up to 120-140VPN (DPN).

There are two aspects involved in precipitation hardening to
consider, the first is that at 745-760degC, the solution treatment
temperature (red heat when torch annealing) the alloy is very soft
and malleable. Many pieces will distort at this temperature when
picked up and quenched. The second is that when heat treating at
280-300degC to increase the hardness, if the ideal heating time is
exceeded, it is possible that the alloy will then start softening
again. This is due to the silver grain size enlarging and coarsening.

If only the first heat treatment is carried out at 745-760degC then
you have a very soft piece. With time it is possible that some of the
super-saturated silver-copper eutectic may precipitate out of the
silver-rich phase and harden the piece. As this hardening effect
occurs at room temperature over a long time period this process is
also sometimes referred to as age hardening. This is a very slow
reaction and is the reason for the second stage of the precipitation
hardening heat treatment at the lower temperature (280-300degC). It
allows the hardness to be increased in a controlled way, in a
reasonable time.

If however only the second heat treatment, at 280-300degC, is
carried out then there is only a slight increase in hardness. This is
due to the grain coarsening effect described earlier and also the
precipitation of any silver-copper eutectic already retained in the
silver rich phase. This is not, however, a significant increase in
hardness, typically only being a 10VPN (DPN) increase on the original
hardness value.

One other fact to remember is that one of the reasons that this
process did not find a major commercial application was due to the
deoxidation of silver alloys with phosphorous. It is well known that
molten silver alloys readily absorb oxygen. To prevent this and also
to remove oxygen from the molten silver alloy, graphite crucibles and
charcoal melt covers are used. In addition, commercial manufacturers
use a deoxidant to assist in scavenging the oxygen from the molten
silver. Often phosphorous is used and any residual phosphorous can
create a low melting point constituent in a silver-copper alloy,
which might result in hot shortness. This may cause cracking when
standard sterling silver alloys are quenched from high temperatures.

It must also be remembered heating sterling silver at high
temperatures for long periods will produce deep firescale unless
protected. This is normally done with an inert gas, nitrogen or
argon.

That deals with the traditional silver-copper alloys and I hope
explains the theory of precipitation hardening. It is not my
intention to discuss the hardening of Argentium[tm] silvers as this
is detailed elsewhere on Orchid by others. I would just make the
point that Argentium is an alloy with three constituents, silver,
copper and germanium. As a consequence it has a different structure
compared to traditional sterling silver alloys and hardens in a
different way.

*Note: In a two-metal alloy system, the “eutectic” is a specific
ratio of the two metals that exhibits the lowest melting point.

  1. Butts and Coxe, Silver, Economics, Metallurgy and Use. (Chapter
    18: Alloying Behaviour of Silver and its Principal Binary Alloys.)

Charles Allenden.
Quality Assurance Manager


#17
 In the case of silver-copper alloys, molten silver and copper are
completely soluble in each other in all proportions. When
solidified, silver alloys having a copper content in the range from
about 2% through 27%, contain, two discrete constituents or phases
that can be seen when examined under a microscope. One is nearly
100% silver; the other is a silver-copper eutectic* (71.9% silver;
28.1% copper) which has a melting point of 780degC (1435 F).

First the statement that copper and silver completely soluble in
each other is absolutely wrong!

Silver can only dissolve 90 parts per 1000 and copper only 80 parts
per 1000. Excess of either metal creates and alloy consisting of
silver rich crystals ( alpha ) and copper rich ( beta ), which is
true of standard silver/copper alloys used.

I am also confused why we talking about silver/copper alloys when
argentium silver is silver/germanium alloy, but putting it aside for
a moment.

The range 2% to 27% is way off. Silver/ copper alloys behave this way
if content of silver is less than 950 parts per 1000. The low range
is correct though.

The statement should read greater then 5% but less less or equal to
27%.

The description of phases is wrong as well. The type of alloy
referred is known as hyper-eutectic and it is consist of eutectic
phase ( author is correct on this point ) and the second phase will
contain approximately 5% of copper. The exact percentage will depends
on temperature of the alloy when poured, how long it takes for
temperature to drop to 779 degrees C, and other factors.

The upper range of 776 degrees C, given for precipitation hardening,
is a very serious mistake for sterling silver which puts survival of
the piece been hardened in question.

All of the above is only applicable to silver/copper alloys. Silver/
germanium alloys is a different ball game and author may or may not
be right. I simply do not know.

Leonid Surpin
www.studioarete.com


#18
The same situation exists with metallic solid solutions. The
solubility of one metal in another metal increases as the
temperature increases. In the case of silver-copper alloys, molten
silver and copper are completely soluble in each other in all
proportions. When solidified, silver alloys having a copper content
in the range from about 2% through 27%, contain, two discrete
constituents or phases that can be seen when examined under a
microscope. One is nearly 100% silver; the other is a silver-copper
eutectic* (71.9% silver; 28.1% copper) which has a melting point of
780degC (1435 F ).

I am curious about your use of the term eutectic in your post, it
appears that in almost every place you use the term eutectic I would
expect you to say either the beta phase or copper rich phase. Not
being a metallurgist I don’t know for certain but in my years of
research and study of metallurgical papers and articles relating to
precious metals I have never heard of precipitating a eutectic phase
from a solid solution. I was under the impression that the eutectic
could only form upon cooling from a liquid into a two phase solid.

temperature are typically two phase alloys unless there is special
effort made to quench from a single phase solution achieved by
elevated temperature over some period of time. Otherwise you will
have the silver rich alpha phase and the copper rich beta phase
present in the alloy. The eutectic alloy (which has a very fine grain
alpha+beta phase microstructure) will show up during the final
solidification in castings but is not present in any appreciable
amount in wrought material as the heat treatment and
recrystallization that the wrought material undergoes during the
reduction from cast ingot to final mill product converts that very
fine grain eutectic structure into a more homogenous structure of
alpha and beta phase crystals.

From “Introduction to Precious Metals” by Grimwade the description
of typical as cast sterling is

"a non equilibrium structure of primary dendritic crystals of
alpha phase surrounded by eutectic structure of a
finely-dispersed mixture of small alpha + beta crystals is
obtained" 

whereas wrought material he describes as

"With wrought products, the as cast structure is modified by a
combination of working and annealing. The final annealed
structure is essentially a solid solution of alpha phase with
perhaps a very small amount of beta phase crystals present as
small sub microscopic particles." 

From the section on silver copper alloys the ASM Handbook Volume 2,

"Heat treatment. Sterling silver can be age hardened without
difficulty, and part of the merit of this composition in
providing acceptable properties after miscellaneous treatments
results from some hardening on cooling in air. The solubility of
copper in silver at 650 C (1200 F) is about 4%, and at 730 C
(1350 F) about 6%, so sterling silver processed at these
temperatures is duplex with small amounts of the copper-rich
phase scattered through the silver-rich matrix. Aging treatments
cause precipitation of the copper-rich phase, and if prolonged,
increase the electrical conductivity considerably. Coin silver
will remain duplex after any annealing treatment and ages in much
the same manner as the 7.5% Cu alloy. Both alloys respond to an
aging treatment of 2 h at about 280 C (535 F) or 1 h at 300 C
(575 F). The mechanical properties of coin silver are virtually
the same as those of sterling silver after the usual annealing
treatments at about 650 C (1200 F), because the composition of
the silver-rich phase will be the same. Alloys containing 20 to
30% Cu have much more of the copper-rich phase and show less age
hardening. In practice, relatively little deliberate use is made
of the precipitation- hardening phenomenon in the silver-copper
alloys. The solution temperature 705 to 730 C (1300 to 1350 F) is
rather close to the solidus temperature 780 C (1435 F) and
requires better temperature control than is available to many
artisans who work with these alloys. In alloys heavily deoxidized
with phosphorus, incipient melting in the grain boundaries may
occur at 705 to 730 C (1300 to 1350 F), and when this happens the
piece is likely to crack during quenching. Furthermore, these
alloys are extremely soft at the solution temperature and are
easily damaged. On the other hand, when soldering is done, the
metal surrounding the joint may be heated to the solution
temperature, and air cooling will cause some hardening. This
counteracts the softening that would otherwise result from the
soldering of work-hardened metal." 

There are some more recent references like Dr. Jorg Fischer-Buhner’s
paper at the 2003 Santa Fe Symposium that show that significant
hardening can be obtained with lower solution annealing temperatures,
for example 700C/1 Hr/water quench/300C/1 Hr yielding 110-120HV in
standard sterling alloy.

Jim
James Binnion
James Binnion Metal Arts


#19

Mr. Allenden, thank you for your thoughtful and detailed description
of precipitation hardening. It was very helpful. May I ask you what
I’m sure is a very pedestrian question? To help prevent the oxygen
absorbtion and firescale you mention, could one coat the piece in
some kind of flux prior to heating it? Would the flux last long
enough at such temperatures to be effective? Would it react itself
with the metal marring the surface? Thank you again for sharing your
knowledge with those of us who are new to this field.


#20

Hello Alice,

First my apologies for not responding sooner, I am a very bad
Orchadian in that I only read through the posts about once a week due
to time constraints. Thank you for your kind words, I was hoping to
describe precipitation hardening in a manner which would allow people
to understand what was occurring within the metal itself with the
different heat treatments, also please call me Charles, being Mr.
Allenden makes me feel old!

With regard to your question about the use of fluxes to protect the
silver during annealing you raise a very interesting point.

Most fluxes are designed to operate for limited life times at lower
temperatures than you would typically use for annealing. They work
by combining with oxides on the surface of the metal to form a glassy
layer which then acts as a barrier to further oxidation.

When you are soldering (more correctly called brazing for joining
operations above 450oC) then it is recommended that the flux you use
is active at about 50oC below the melting point (solidus) of the
solder you are using and also active to about 50oC above the upper
melting point (liquidus) of the solder. This ensures that the flux
acts to clean the surfaces being joined prior to the solder flowing
and also allows you to encourage the solder to flow towards the heat
source by capillary attraction by having a higher temperature at the
point you want the solder to flow too.

The problem is that these fluxes have limited life times as they are
only designed to give protection for the time required to form a
soldered joint. The glassy residue they leave behind can generally be
removed by pickling or polishing the piece. It is important to ensure
all flux has been removed as it can act as a site for subsequent
corrosion by trapping moisture beneath it.

As you move towards the higher temperature fluxes these are
typically based on disodium tetraborate (borax) and boric acid. These
are tenacious and nearly always require extended pickling to remove
all residues. In some cases a caustic (sodium hydroxide) solution is
recommended for cleaning rather than the more usual acid pickle.

I have limited knowledge of the ‘protective fluxes’ which are now
being marketed to protect during annealing operations. My
understanding is that they also are good for short time cycles (5-10
minutes) at temperatures up to 800oC but once your annealing times
start to exceed 15 minutes than the protection starts to break down.
You are also left with a very tenacious residue which can be very
hard to remove other than by polishing back the surface to clean
metal.

I hope this helps,
Charles Allenden
Quality Assurance Manager