I was privileged to have metallurgist Andrew Nyce in the keum-boo
workshop I taught at Wolf Designs in Portland,
Maine,<www.katewolfdesigns.com>. He very kindly wrote up this
explanation of diffusion bonding for me today and gave me permission
to pass it on to you.
Regards, Jayne Redman
THE METALLURGY OF KEUM-BOO
Keum-Boo in metallurgical terms is pressure-assisted diffusion
bonding with the burnisher used to transmit the pressure. What is
meant by diffusion? First we need to construct a three dimensional
lattice of atoms (Au, Ag or alloys of Au or Ag). First draw a three
dimensional cube and place circles (to represent atoms) on each
corner and each face. You now have a face centered cubic lattice
structure. The atoms of Gold and silver and their alloys arrange
themselves in a face centered cubic array. To understand diffusion
we must accept that as a metal is heated to temperature above .5 of
its melting point, vacant lattice sites are created. As a result,
the atoms are free to move randomly throughout the three dimensional
lattice. If we were to move a silver lattice against a gold lattice
and heat both above the temperature where vacancies are created, the
gold and silver atoms would interdiffuse and bond the lattices
together. In Keum-Boo of 18, 22 or 24 karat onto silver or
conversely fine silver or sterling silver to gold or gold alloys,
the atoms interdiffuse creating a metallurgical bond which is very
strong.
If 24 karat sheet (.01to.05 mm) is simply laid flat against a silver
sheet, only the high points would be diffusion bonded. By using a
burnisher, the entire surface can be diffusion bonded if the
burnishing action covers the entire surface. Trapped air or organic
matter trapped during diffusion bonding will result in bubbles
forming if the bonded surfaces are heated to a temperature higher
than the diffusion bonding temperature. Oxygen can diffuse through
silver but not readily through gold. Nitrogen will not diffuse
through.
I found Andrew Nyce’s explanation of diffusion bonding to be…well,
very nice. But in it he says that the metal is heated above its
melting point, which creates spaces in the lattice so that diffusion
takes place. Very nice.
But, in Keum Boo, the metals are not raised to, or even near, their
melting points. It is done on a hot plate. Does the same phenomenon
occur? I do not think so. Can anyone clarify this? Mr. Nyce?
Curious in SF. Alan
Revere Academy of Jewelry Arts
760 Market Street - Suite 900
San Francisco, CA 94102
tel: 415-391-4179 fax: 415-391-7570
clip . . . To understand diffusion we must accept that as a
metal is heated to temperature above .5 of its melting point,
vacant lattice sites are created.
Without that figure in the sentence, just as you probably surmised,
those vacant lattice sites would do little to redeem the puddle of
alloy we’d be making.
But, in Keum Boo, the metals are not raised to, or even
near, their melting points. It is done on a hot plate. Does the
same phenomenon occur? I do not think so. Can anyone clarify this?
Mr. Nyce? Curious in SF. Alan Revere Academy of Jewelry Arts
Alan , Keum boo works because of the property of mutuel disolvability
of pure gold and silver. When the gold is burnished down the
molecules migrate from one to another causing a bond that is very
durable indeed. Heat aids this by exciting the molecules thus aiding
penetration. This is also the reason that a heavier foil is needed.
If the foil is too thin, (leaf) the result is that enough
co-migration takes place to wash out the color, leaving that pale or
even greenish area. The co-migration of molecules can actually happen
at room temp. This is why when plating gold over silver a nickel layer
is suggested as a substrate (or barrier to co-migration). The
alternative is a very thick plate. It has been shown that vermiel
work without the nickel (or other metal) coat, will eventually show
fade in the gold color even if it is not worn. Keum boo does not have
the fade problem if the foil used is thick enough. HTH Mark
Mark Thomas Ruby
SunSpirit Designs
Loveland, CO
970 622-9500 studio
970 622-9510 fax
The key to what he said is “metal is heated to temperature above .5
of its melting point” this should be written for the rest of us as
50% of the melting point. In Kuem-boo the metal is not quite that
hot I think typically you only reach about 500 which is about 25% of
fine golds melting point and about 28% of Fine silvers but you have
several things that contribute to the process that allow the bond to
occur. The first is the lack of oxides both fine gold and very high
carat gold are basically oxide free surfaces the oxides on the
surface of other metals and alloys are what keep the surfaces from
bonding. If you were to try Keum-boo with say copper instead of gold
you would find you cant do it this is because of the oxide layer that
is in/on the surface of the metal . Fine silver at low temperatures
(in the range of temperatures that Keum-boo is done at) does not form
stable oxides so there is no oxide present on the fine silver surface
that is formed by the depletion of copper from the surface of the
sterling by repeated heating and pickling. So you have two metal
surfaces that are brought in contact with each other rather than two
oxide layers. This allows the surfaces to even attempt to bond to
each other. The second thing that aids the bonding process is
pressure from burnishing. This pressure adds energy (just like heat
adds energy to the metals, by adding enough energy to metal it can
no longer remain in a solid form and melts!) to the the interface
between the gold and silver and it provides more intimate contact
between the layers which increases the number of points that can
bond together. So by combining the elevated temperature and the
oxide free surfaces and the pressure and intimate contact you create
a environment where the rest of his description of the crystal
matrices occurs.
You can create diffusion bonds in metals at room temperature if the
surfaces are flat enough and clean enough and they are left in
contact for a long enough time. Gold plated onto copper will diffuse
into the copper over time unless there is a barrier layer of nickel
in between them. Diffusion is just speeded up with the application of
heat and pressure. Jim
Jayne and others. I found Andrew Nyce's explanation of
diffusion bonding to be...well, very nice. But in it he says that
the metal is heated above its melting point, which creates spaces
in the lattice so that diffusion takes place. Very nice. But, in
Keum Boo, the metals are not raised to, or even near, their melting
points. It is done on a hot plate. Does the same phenomenon occur?
I do not think so. Can anyone clarify this? Mr. Nyce? Curious in
SF. Alan
Alan, you misread my post. Dr. Nyce states that as a metal is heated
to a temperature above .5 (one half) of its melting point, vacant
lattice sites are created.
But in it he says that the metal is heated above its melting
point, which creates spaces in the lattice so that diffusion takes
place. Very nice.
Dear Alan, that statement bothered me also and I’m afraid the
explanation does not explain what is happening. I have always looked
upon the process in a rather simple way. Since molecules are always
in motion, the addition of heat,however small, will increase the
velocity of the molecules in both sheets of metal. At a certain
point (tempature) the velocity of these molecules will permit some
to escape from the surface and if there is no barrier between the
two surfaces, such as air or an oxide of a metal, these molecules
can intermingle with each other and thereby create a bond. The
burnishing creates that intimacy of contact which permits this
intermingling of enough molecules to effect this attachment. This is
not like the fusion metals which takes place in creating a billet for
mokume gane technique.
I know this may be a simple explanation and may not be entirely
scientific, but it helps me understand the process. Perhaps our
resident scientist John can offer an explanation. Joe Dule
This is not like the fusion metals which takes place in creating a
billet for mokume gane technique.
Joe I must disagree, this is exactly the process that is used in
diffusion mokume gane the only difference is the time is longer and
temperature is higher to create a stronger bond. The pressure is
provided by bolts or wires instead of a burnisher. The higher temp
and the longer time help to overcome the presence of oxides that are
present on lower karat alloys and base metals but it is the same
process on an molecular scale.
Joe I must disagree, this is exactly the process that is used in
diffusion mokume gane the only difference is the time is longer
and temperature is higher to create a stronger bond.
Dear Jim, Thank you for the explanation and I accept your
correction. My feeling was that an oxide barrier does prevent the
diffusion of atoms between the metals in the Keum Boo technique and
therefore requires the use of pure metals, or alloys without the
inclusion of base metals. I have keum booed 18K, 14K and 12K alloys
quite readily as long as no base metal was a part of the alloy. But
I have not been able to bond commercially prepared gold alloys by
the low heat and burnishing technique. Though C.L. Brain has
indicated that the attachment of Gold sheet to a billet of silver is
different in the Doublee technique is different from that which
occurs in Keum Boo, I have always felt it to be quite similar.
Thanks for the explanation of why we are able to overcome the oxide
barrier in doing Mokume gane but not Keum Boo. Joe Dule
Thanks to Jim, Peter, Jayne, Joe, Mark, Pam, and the others who took
the time to clarify the fascinating process of diffusion bonding
which we have come to call Keum-Boo. No matter how much I learn
about precious metals, I am still awed at what they can do.
