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My keum boo won't "boo"


#1

Any Keum Boo experts out there? I have successfully done it around 6
or 7 times. But today, I can’t get the gold to bond. I have a nice
depletion layer, and a kiln with digital thermometer. I am trying to
work in the 700-800 degree range. I started with 22k gold bezel and
rolled it down as far as I could (using copper sheets to sandwich it
for the last couple of passes through the roller. Still, the gold is
way thicker than the foil normally used. I do this on purpose so I
can cut out an intricate design using my CNC mill. It worked before,
but not today. BTW, I scupulously cleaned the gold before trying to
apply it. Could the gold be too thick? Why would it matter. I also
noticed the gold is not perfectly covering the slightly curved silver
surface. I tried to curve the gold to match the silver, but it is not
perfect. Also noticed that the gold feels surprisingly springy for 22
k. I wonder if I inadvertanly got 18 k or something. I tried
oxidizing a small piece and putting it in my pickle solution to see
if it was 18 k and I could depletion guild it as well. nope, it must
be 22 k. any hints would be appreciated.


#2

Hello Todd,

I think your issue is the way you are rolling out your metal.

You may want to sandwich the gold in between brass and steel sheets
Basically, Brass, steel, gold, steel, brass.

This way your rolling mill only touches the brass and the steel
touches the gold. The problem is that the copper is softer then your
gold.

I bet your copper rolling plates come out curled and stretched with
an imprint of the gold sheet margins?

You need to pass the gold through with a metal harder then the gold,
which can be a pain as the metal becomes thinner. The brass backer
sheet will thin out as well but because the steel is touching the
gold it will cause much more movement in the gold.

If you were working in silver, I would suggest stacking the bezel
strips and soldering just the tips, which will thicken the metal and
allow it to be rolled thinner, however the tips will need to be
clipped off and this is costly “waste.”. The stacking trick is how
aluminum foil is made, this is why there is a shiny side and a matte
side, because the sheets are backed onto each other.

Also, think about this, you have to anneal the gold after you thin
it out by about 50% hopefully before 50% as this will cut your
fractures down, the thinner the metal, the faster 50% is reached and
the more annealing!!!.

You should quench your gold in 70%+ alcohol to limit the fracturing.

Check out my blog, I have post on quenching and annealing which may
help out

http://www.ganoksin.com/gnkurl/xs

The post is a bit jumbled right now, I have to re-write it and
re-post it so it makes more sense, any input you can give would be
great!!!

Take care and I hope the sandwich solves your issues.
Kenneth


#3

Wow, that’s certainly a lot more about rolling down the gold than I
ever knew before. You are right about the copper. I did notice the
imprint, and the way that if the gold moves out of the imprint, you
get more lines on the gold. Sounds like your method would work.
Strange that I got good (enough) results before with just the copper.
Cutting up brass and steel seems like a lot more work, but maybe
worth it. I think the gold I used before was 24 k and the gold I’m
using now is 22 k. Would that make a difference?


#4

Hi, Todd!

I don’t know that I am a keum boo expert, but I have done more than a
little and have learned a lot through trial and error. I found 24K to
work best. The reason the gold sticks to the silver (as best I
understand it) is that the oxygen molecules pass through the gold
creating a molecular bond between the gold and the silver. If your
sheet is too thick, the oxygen will not be able to pass through it
and the gold will not stick. In order to make the gold stick, there
has to be a combination of heat and pressure. The more heat, the
less pressure is needed and conversely, the less heat, the more
pressure is needed.

It is not necessary to depletion guild the silver, although it makes
the keum boo process a bit easier. You can get the gold to stick to
aluminum, copper, steel or just about any other metal if you work at
it hard enough.

I make my own keum boo foil by rolling it in the mill with no
protective sheet. I roll until the rollers are difficult to roll with
nothing in them and then make several more passes. You will need to
anneal the gold. 24K anneals at 575 F. The way I anneal it is to
throw it on a common hotplate when it on “high” and leave it for a
couple of minutes. There is no need to pickle.

To complete the keum boo, I usually work on a hotplate (did you know
that those things get up to 1100 F) although if the piece is large or
difficult to make contact with the coiled (a cuff bracelet) I will
use a torch to heat the piece.

Burnish as you heat and the gold will stick! I usually use a
"rolling" burnisher rather than a rubbing burnisher.

Here is a link to a paper I wrote on gold diffusion bonding with
metal clay - but it’s the same technique with sterling or fine
silver milled metal: http://www.ganoksin.com/gnkurl/xy

I have done a lot a keum boo and would be happy to help you with any
other questions you have.

Deb


#5
I think the gold I used before was 24 k and the gold I'm using now
is 22 k. Would that make a difference? 

Yes.

You’ve gone to some trouble, in preparing the silver, to eliminate
the copper in the silver surface, since and it’s oxides interfere
with the bonding of gold to silver. If there’s then copper in the
gold, you negate some of this, and will have more trouble, depending
on how much of the 22K alloy is copper. If the alloying metal is all
silver, it shouldn’t interfere, but that’s not common for 22K.

The reason Keum Boo usually works is that at elevated temperatures,
oxygen is quite mobile and permiable in pure gold (and in silver).
Silver forms only a weak oxide, if at all, and when there is a gold
layer on the silver, when it’s heated, it acts almost like a flux,
and the oxygen at the surface interface tends to migrate into and
through the gold, leaving a surface interface between pure metallic
gold and similar silver, without interfering oxides. Silver and gold
are completely and easily soluable in each other, and thus diffusion
bond easily when nothing is getting in the way, so then, with just a
bit of burnishing, the two diffusion bond. Copper, however, binds
much more strongly to oxygen, trapping it, so if there is copper in
the interface between the metals, the copper oxide then interferes
with the perfect contact and diffusion bond. Even if not oxidized,
copper does not diffuse easily into the gold as does silver (gold
and copper, like silver and copper, are only partially soluable in
each other, and the copper atom is a distincly different size from
either of the others), so copper at the surface, even if not
oxidized, tends to slow or block the diffusion process at least some.
It doesn’t take a lot of copper to get in the way. This is part of
why, when diffusion bonding metals for things like Mokume, the temps
are often not far below the melting point of the lower melting metal,
and as well, the stack of metal is set up so that at temperature, it
is clamped under considerable pressure, much more than the mild
burnishing used in Keum Boo. Done like that, diffusion bonding can
then occur, but even then, it takes much more time; it’s not the
almost instant bond one gets in keum boo. Now, consider that 22K
gold contains 2/24 alloy, which is 8.33 percent alloy, and that’s by
weight, so since the gold is generally much denser than the alloying
metals, the atomic ratios and ratios by volume of the alloy to gold
is much higher than that. If the alloy in your gold is all or mostly
copper, which is sometimes the case, then you could have copper
percentages at the surface considerably higher than in the sterling
silver you likely went to such pains to prepare by depletion gilding
in order to remove that copper. Having the copper oxides in/on the
gold layer is no different in it’s effect on the bond as if it’s in
the silver surface. If you could depletion gild the gold, removing
the copper at the bonding surface, you might get better results, but
depletion gilding thin gold foil without damaging it isn’t so easy
to do. You’ll be much better off simply using 24K.

Peter Rowe


#6

Thanks all for the tips. I need to work on my rolling techniques,
and/or stick to 24 k gold. I had hoped to use thick(er) gold to help
me when I use a milling machine to cut out very intricate patterns,
where a slighlty thicker foil helps. I have been using a technique
where I use double sided tape to hold the foil down while cutting
with a very fine tip. Works great so far.

Thanks again.