I am not as familiar with the alloying specifics of the metals
nearly as well as James and Peter but I do have a suggestion. I too
LOVE the look of Fine Silver. My best suggestion to get that "look"
and still use the harder & stronger sterling for your vessel would be
to use a depletion gilding process to “raise the fine silver” as they
call it. (You aren’t actually raising the fine silver, you are
actually burning/pickling off the copper on the very surface of the
piece, leaving a very thin fine silver layer at the surface of the
metal.)
I use this method all the time on my work, as I really love the
bright white of fine silver, too. There at LOTS of posts in the
forums about this method if you aren’t familiar with it. It is a
simple repetitive act of lightly heating and then pickling your
finished piece until the copper in the surface “burns” off. It’s used
quite a lot by enamellers to get the fine silver surface needed for
enameling, as well as by many metalsmiths who just prefer the fine
silver look. You may want to consider it.
My understanding of the need to anneal at that unusually high
temperature prior to heat treating, is a bit different. Normally
handled, cooled, annealed, etc, sterling silver is a mix of crystals
with two distinct compositions. One is a silver rich composition
with very little copper, while the other is the eutectic alloy of
silver and copper.
Yes there are two different crystal structures in sterling silver
but unless you are looking at as cast sterling you will not see much
if any of the eutectic alloy. What you will see is the majority
crystal is mostly silver with a small amount of copper this is the
alpha phase. The second crystal is mostly copper with a small amount
of silver which is designated the beta phase. If you look at the
eutectic if it is present with a sufficiently powerful microscope you
will actually see it is not a third crystal type but a lamelar
structure of alternating layers of very tiny alpha and beta phase
crystals. In wrought material that eutectic structure will disappear
during the working of the wrought material from ingot or continuous
cast bar down to sheet or wire. The only way you will get the
eutectic back is by grossly overheating the sterling to the point
where some of it is actually molten.
That means the bulk of the copper in sterling silver is in those
eutectic crystals. My understanding of the pre- hardening anneal is
that at that high temperature, with subsequent quench, the
composition becomes one of all uniform crystals having roughly the
same percentage of copper, rather than those two distinct forms.
Then, when age hardening, because the copper starts out uniformly
distributed, the end result (caused by copper migrating to the
crystal boundaries) is more uniform, and more of the copper will
have migrated to the crystal boundaries.
The bulk crystal structure in the sterling is the alpha (mostly
silver with some copper) phase with a small amount of the beta phase
basically scattered in little clumps here and there. When the metal
is heated hot enough the beta phase crystals will dissolve and become
alpha phase crystals. If you then immediately quench you will freeze
it in the alpha phase. When you subsequently do the age treatment
tiny beta phase crystals begin to precipitate out of solution and
form in the grain boundary areas of the alpha structure. These tiny
beta crystals act like throwing sand in to ball bearings. They make
it so the larger alpha crystals cannot slide across each other. This
is sometimes called pinning because the beta precipitate “pins” the
crystal matrix together.
Age hardening the original formula moves less of the copper,
because the eutectic crystals are not as unstable as the uniform
"sterling" mix, plus the resulting distribution of copper at grain
boundaries is less uniform, giving inferior results.
See above
That's what I believe is happening, but this may not be fully
correct. Perhaps Jim Binnion would care to verify or correct my
notions on this. I think he's the one whose Orchid posts originally
planted those thoughts in my poor ol brain. What I'm not sure of is
whether those seeds grew straight or not... (grin) So, Jim, how
far off am I?
The only real confusion is the bit about the eutectic, it is just
not really present in wrought material. But if you substitute alpha
phase for eutectic in your description it is conceptually correct.
As far as I remember you are correct. I just meant to say that
soldering at medium or hard solder accomplishes this annealing, and
unless you do more than work that will substantially work-harden
some part of the piece I don’t think you’d have to anneal again.
Being perfect is of course another matter. A lot of the work people
do on Sterling, would be to solder up already formed pieces, such
that soldering is the last step before heat-treatment. I do feel that
the annealing isn’t needed here (I too, hope JAmes chimes in on
this). If the work involves additional forming after soldering than
annealing would be called for, but I was trying to convey that there
is a matter of subjective judgement about this. If the crystalline
structure isn’t completely uniform before heat-treatment the results
will be slightly un-uniform. If the differences are great enough
cracking would result.
How much is too much?
I took the original question to ask about balancing the possible
damage from the annealing to the benefits of the heat-treatment. If
a perfect heat-treatment is desired, then annealing is required. I
really didn’t mean to question directly your just
wanted to add a point of view about the subjectivity of the results.
I always value your opinion, Peter.
As James tells us in his other post I am completely wrong. After
reading his post I am reminded of my own metallurgy studies and also
why I quit heat-treating sterling 20 years ago and just used thicker
cross-sections. That is a lot of work to do it right, although I will
start following his guidelines next year when we reintroduce flatware
into our product line. Sorry Peter to have taken things in the wrong
direction.
One is a silver rich composition with very little copper, while the
other is the eutectic alloy of silver and copper. That means the
bulk of the copper in sterling silver is in those eutectic
crystals.
Hi Peter, you are mixing as cast state and wrought state crystal
structure.
As cast silver is composed of dendritic crystals that have a non
homogenous structure due to the changing alloy composition as it
cools, this is called coring. The center (core) of the crystals are
very silver rich and as it cools the layers of atoms change their
ratio of silver/ copper till the eutectic ratio is reached. The
eutectic then freezes filling all the voids between the dendrites.
In wrought or cast sterling that has been soldered or annealed there
is little to no presence of eutectic as the alloy wants to shift
into it’s two phase state.
After casting wrought material is cold or hot worked to change it
from ingot to sheet or wire. During the work and annealing cycles
the crystal structure changes to one where at room temperature the
majority of the crystals are silver rich alpha phase material with
very little copper in them and a second phase (beta) of copper rich
crystals that have very little silver in them. As you heat the
material the silver is able to hold more copper in the alpha phase
crystals. If you heat it hot enough 1382 F (750 C) you will get all
the copper into the alpha phase crystal structure. If the metal is
then quenched without any delay it will lock all the crystals in the
alpha state. This is called solution annealing in reference to all
the copper being in homogenous solution with the silver. When you
are annealing it takes a bit of time for this change in structure to
occur this is where the hold time at an elevated temperature
requirement comes from.
In age the hardening process once the alloy is solution annealed and
quenched it is hardened by heating to 572 F (300 C) when the metal
is held at this temperature for about 30 min. the copper begins to
precipitate out of the alpha phase crystals. It forms tiny beta
phase crystals in the area between the existing alpha phase
crystals. These small crystals act like pins to keep the alpha
crystals from sliding or moving easily This is what causes the
increase in hardness by reducing the ductility of the alloy. Kind of
like throwing sand into a ball bearing. If you continue to heat at
572 for too long or heat to a greater temperature the beta crystals
continue to grow to the point where they are now of similar size to
the alpha and they no longer act as pins in the matrix.
In work hardening the reduction in ductility comes from the
distorting of the crystals by cold work. The greater the amount of
cold work the more distorted the crystals become and the more stress
is locked into the crystal matrix and the more energy is required to
further distort it. Eventually you will react a point where the
energy required to distort the crystals is greater than the tensile
strength of the material and instead of further distortion the
crystals simply fracture. However if you stop before you fracture
the matrix you have a very hard and strong material.
Work hardened sterling can be quite a bit harder than age hardened
sterling but only if it has received enough cold work. To be
“fullhard” you must reduce the sterling by 70% in cross sectional
area. It is not easy to get this much reduction without a power mill
or good draw bench or a lot of hammer work. Everyone should try it
some time just to get a feeling for how hard full hard silver can
be.
Age hardened sterling can be in the range of 130-140 HV. Work
hardened sterling can be in the range of 170-180 HV. Those numbers
assume you have done everything right. The numbers for Argentium are
very close to those of standard sterling but tend to be a little
lower.
One is a silver rich composition with very little copper, while
the other is the eutectic alloy of silver and copper. That means
the bulk of the copper in sterling silver is in those eutectic
crystals.
Hi Peter, you are mixing as cast state and wrought state crystal
structure.
Interesting quirk of the internet here. The post you’re commenting
on, Jim, is from almost exactly five years ago (2010), similar topic
title. I didn’t totally read through it, but I think the discussion
was along slightly different lines.
Not sure. What I do recall, though, is that to be sure of what I was
writing, I pretty much paraphrased that description from the Handy
and Harmon handy book’s description, which as I recall, mostly talked
about wrought metal, I think. Don’t recall much about cast silver at
all. Not sure. (I’d check if I remember where that little “book”
is…)
I haven’t actually weighed in on this current thread at all…
That is very strange. I routinely delete anything over 6 months old
in my Orchid folder so I have no idea how a 5 year old message is in
the queue. What is even stranger is the date being so close to this
current thread. I saw them grouped together and did not catch the
slight difference in subject line.
I am certain about the eutectic not being there after several
work/anneal cycles. The extremely tiny crystals that make up the
eutectic structure are absorbed by the neighboring larger crystals
by diffusion.
I am certain about the eutectic not being there after several
work/anneal cycles. The extremely tiny crystals that make up the
eutectic structure are absorbed by the neighboring larger crystals
by diffusion.
Any literature or resources that will help me gain a level of
knowledge so that I can understand and appreciate this
I feel a both overwhelmed and that I am missing out on being able to
apply some useful Kind of like seeing a very useful tool
for the first time with no guidance on how to use it.