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Trivia, real trival: Fossil finger prints and making alloys


#1

Hi all,

Yesterday I had a chance to play “metalsmith” and was annealing some
copper. The copper was relatively clean but my fingers had the
typical oils on them as one would expect. So when I placed a small
sheet on a firebrick I could see a finger print.

I then began to heat the metal, and as I was heating it so that metal
began to glow I could still see the print. I did not plunge the piece
into water and just let it cool. Much of the cupric oxide firescale
flew off, but enough remained so that I could see finger print. I find
it astonishing that such a small disturbance such as finger print
could persist with all that heat. Perhaps the oils facilitated the
formation of the fire scale before it formed elsewhere.

Normally I plunge the copper into a bucket of water because that is
what I have been lead to believe is the best way to soften it. But I
found the copper just as soft and pliable when it cooled relatively
slowly. In addition, how do I tell just how hot is “hot”? And more to
the point, does it make a difference, and if so how?

On many of our beaches we have large “granite” rocks. I have been
told that one can tell how quickly the parent rock cooled by looking
at the dark crystals in the matrix; if the rock cooled quickly the
crystals are quite small and vice versa. This got me thinking about
sterling silver. I take it for granted that when I buy a sheet of
sterling it will be just fine. But how do the mixers know that the
copper has been well dispersed in the silver and how do they know how
quickly to cool it. In other words, if one is making alloys at home
how does one maintain consistency. This is an implied acknowledge of
the skills those who make those sheets.

This is what happens when I play. Hope it happens to you also.

David


#2

dave,

as i read you letter on annealing copper it struck me that you
weren’t familiar with the molecular process that annealing brings
about. when the material is work hardened (to you stiff and
non-pliable ) the molecules in the metal have been flattened out to
an oval shape. by heating the metal up you release the tension
created by working tha metal and the molecules return to a more
naturally rounded state. this holds for all non - feruos metals .
material that is ferrous (steel etc. etc.) requires a slow cool
down to maintain the malability cooling in liqiud would make that
brittle! a rule of thumb i’ve always used as was taught to me is that
when copper silver brass etc. etc. reaches a dull red it is annealed
. the only advantage to plunging the piece into liquid would be if
you were pickling it in acid . this can be dangerous because of
boiling and splatters but it tends to make the pickle take a fast
bite and clean the piece that much quicker. of course there is a side
issue of flex and distortion but that is usually minor. as for even
alloying the materiel so long as it is completely melted mixes evenly
only in a partial melt will you get unmixed allos not to easy to do.

Talk to you later Dave


#3

Annealing

Some alloys have a property of precipitation hardening and without

trying to go into the details the end result is that heating at a
temperature below annealing but above room will actually harden the
alloy. There is also a time element involved so the faster you cool
it the less hardening you will see. To avoid this hardening you
should quench rapidly after annealing. In general with most non
ferrous alloys if you quench from annealing temperature you will get
the softest form. If you air cool it will still be soft but not as
soft as when quenched. With sterling you will see more difference
between air cooled and quenched material. Also with sterling if you
overheat it when annealing and then quench it can crack. There are
some alloys that must be quenched like 18k red gold and some white
golds.

Jim

James Binnion Metal Arts
4701 San Leandro St #18
Oakland, CA 94601
Phone (510) 533-5108
Toll Free (877) 408 7287
Fax (510) 533-5439


@James_Binnion
Member of the Better Business Bureau


#4
when the material is work hardened (to you stiff and
non-pliable ) the molecules in the metal have been flattened out to
an oval shape.

To begin what is a molecule? According to the Handbook of Chemistry
and Physics it is, “The smallest unit quantity of matter which can
exist by itself and retain all the properties of the original
substance”. Further, the Handbook defines copper by the notation Cu,
meaning that it can exist as a single atom and retain all of its
properties. In addition, the Handbook states that copper can exist as
a crystal, specifically it forms a cubic system. We are talking about
very small units if matter here. So how does work hardening "round"
molecules of copper?

A more common explanation is that an annealed sheet of copper
consists of a relatively small number (we are still talking about a
very large number of crystals) of relatively large crystals that
become transformed into a sheet of many more crystals of much smaller
size. Further, there is evidence to suggest that these crystals are
deformed in a manner such that they have very high surface to volume
ratios (not spherical but instead say, saucer shaped -prolate spheroid
of revolution?). This phenomenon can be observed under a polarizing
microscope (somewhere on the web there are pictures of copper in the
annealed state and the work-hardened state; I don’t recall the URL).
However, it seems to me that the explanation of crystal deformation
confuses “correlation” with “causation”. That is, when copper is
worked small crystals form. How does the formation of small crystals
"cause" the previously soft metal to become stiff and no longer
pliable? What is the physical explanation?

I raise this question because we are all familiar with quenching
copper and it staying soft. In contrast if we quench iron it becomes
brittle. Why the difference, even though both are metals have cubic
systems?

by heating the metal up you release the tension
created by working tha metal and the molecules return to a more
naturally rounded state. 

How? What tension are we talking about here? Please cite your
reference.

this holds for all non - feruos metals .

And what exactly are the ferous metals? Are they Iron, Ruthenium and
Osmium. It is my understanding, and I could be wrong, but if one
quenches gold, or at least its alloys, it also becomes brittle.

material that is ferrous (steel  etc.  etc.) requires a slow cool
down to maintain the malability cooling in liqiud would make that
brittle! a rule of thumb i've always used as was taught to me is that
when copper silver brass etc. etc. reaches a dull red it is annealed

That is what I have read also. But it raises a question. What if we
heated a piece of copper very slowly for a long period of time, just
before it reaches a dull red colour versus what if we could very
rapidly heat the copper to dull red for an extremely short period of
time. Would both sheets be equally annealed? And what happens when the
copper is heated to bright orange red or close to its melting point?
Can the sheet be “over annealed”?

. the only advantage to plunging the piece into liquid would be if
you were pickling it in acid . this can be dangerous because of
boiling and splatters 

Agree. However, that will unlikely happen if one plunges the piece
deeply into a large volume of liquid.

as for even
alloying the materiel so long as it is completely melted mixes evenly
only in a partial melt will you get unmixed allos not to easy to do.

I disagree, my basis being empirical observation. As I wrote before,
we can observe that when solutions cool slowly their components can
precipitate out at different rates and thus create crystals of
different sizes in the eventually frozen mixture.

So how does the metallurgist who makes the sterling know that there
is a perfect mix and how does he or she know how slowly or quickly to
freeze it? How do they maintain quality assurance?

One final question. Does it matter whether we know this stuff or not?
I believe it does, not from a techincal perspective from the point of
view of respect for the materials with which I am working. I don’t
"make" the piece. My tools, the metal and I together make the piece.
It is my belief that by understanding what we are working with we
transform the activity from craft to art.

David


#5
  ... That is, when copper is worked small crystals form. How does
the formation of small crystals "cause" the previously soft metal to
become stiff and no longer pliable? What is the physical
explanation? 

It doesn’t. Work hardening deforms crystals, and stresses the
crystal boundaries. The stress on the crystal boundaries accounts for
more of the hardening effect than does the actual deformation of the
crystals themselves. With pure copper, the degree to which a single
crystal can be deformed is truely remarkable… It’s the crystal
boundaries which eventually start to fail. The material transforms to
small crystals with increased boundary area, and again undeformed, but
smaller crystals, when you anneal the metal. This increase in the
area of the crystal boundaries when the crystal size (usually
referred to as grain size) is decreased accounts also for the fact
that worked and annealed metal becomes better able to be worked
without cracking than is metal with large crystals (such as castings,
etc).

   I raise this question because we are all familiar with quenching
copper and it staying soft. In contrast if we quench iron it
becomes brittle. Why the difference, even though both are metals
have cubic systems? 

Pure iron does not become hard. It’s iron with sufficient carbon
incorporated in it which does this, due to different types of crystal
structure that can form with high or low temperature phases of the
alloy. Quenching the metal from higher temperatures “traps” the high
temperature structure, which by chance, happens to be hard and
brittle.

   And what exactly are the ferous metals? Are they Iron, Ruthenium
and Osmium. It is my understanding, and I could be wrong, but if one
quenches gold, or at least its alloys, it also becomes brittle. 

Ferrous metals are those containing iron. Ruthenium and Osmium are
two of the platinum group metals. Platinum group metals share a few
characteristics with iron (they are adjacent in the periodic table of
the elements), but iron is not a platinum group metal, nor are the
platinum group metals considered ferrous.

While you can crack gold alloys by quenching from too high a
temperature, due simply to thermal shock, non of the gold alloys I’ve
ever seen will harden upon quenching. Many of those which include
sufficient copper, however, can be age hardened by a sufficiently slow
cooling, or better, a heat treat cycle at below the annealing
temperature. This is the opposite of the treatment processes used
with steels, where that “below annealing” heat treat, or tempering
process, is used to reduce excess hardness to the desired level,
rather than increasing it.

   That is what I have read also. But it raises a question. What if
we heated a piece of copper very slowly for a long period of time,
just before it reaches a dull red colour versus what if we could
very rapidly heat the copper to dull red for an extremely short
period of time. Would both sheets be equally annealed? And what
happens when the copper is heated to bright orange red or close to
its melting point? Can the sheet be "over annealed"? 

With pure copper, no heat treatment will increase the hardness. Only
annealing is possible. Slow heating vs. rapid heating will make
little difference, other than perhaps the amount of oxidation that
takes place. However, any heating, especially to temps considerably
higher than needed for annealing, allows the atomic mobility that
allows deformed crystals to recrystalize/reform into smaller
undeformed crystals, to proceed even more. At elevated temperatures,
in addition to just recrystalizing, the then reformed small crystals
start to grow together. The recrystalization becomes a process where
some grains grow in size, as smaller adjacent grains become
incorporated with their neighbors. The result is again large
crystals. While this metal then is still fully annealed, the increase
in grain size will reduce the malleability and ductility of the
resulting metals, since the amount of deformation then possible
before crystal boundaries start to rupture is reduced. So extended
high temperatures are usually to be avoided. In some alloys, such as
golds or sterling silver, the effect can be quite pronounced.
Over-annealed sterling silver may be soft, but simple bending of a
sheet that’s been overannealed can cause an obvious "orange peel"
texture for show up on the sheet, as the crystals deform differently
from the crystal boundaries.

   So how does the metallurgist who makes the sterling know that
there is a perfect mix and how does he or she know how slowly or
quickly to freeze it? How do they maintain quality assurance? 

quality assurance is maintained by testing/observation of the
finished product. Normally, the faster one can solidify the ingot, the
more uniform the ingot will be. The processes used can also have a
big effect. The modern continuous cast methods of actually extruding
a continuous ingot from a hole in the bottom of an induction melt
crucible, for example, produces a more uniform ingot structure than
does a more traditional poured ingot. Among the concerns is that
poured ingots freeze from the outside in. This does, as you’ve
mentioned, lead to ingot structures which are not always the same from
the outside to the inside. Usually, in rolling and annealing, over
multiple passes through both processes, the grain refinement that
takes place can allow the finished sheet to have a pretty uniform end
structure. But it’s not totally ensured. Thus the need for a refiner
to take care with the processes used to produce the finest possible
end product.

   One final question. Does it matter whether we know this stuff or
not? I believe it does, not from a techincal perspective from the
point of view of respect for the materials with which I am working.
I don't "make" the piece. My tools, the metal and I together make
the piece. It is my belief that by understanding what we are working
with we transform the activity from craft to art. 

I pretty much agree. The more you understand your materials, the
better your control over them, and the greater your ability to make
them do the things you wish. These are aspects of the craft of
metalwork, not the making of art. But to be able, as an artist, to
have the greatest freedom of expression, and the widest possible
ability to create in whatever means/forms you wish, the better you can
expand your skills and understanding of your craft, then the better
able you will be to express yourself as an artist.

Peter Rowe