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Keum-Boo
Article by: Charles Lewton-Brain 1987-1993
http://www.ganoksin.com/borisat/nenam/keumboo.htm
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This Korean technique for applying 24k gold to silver is in fact
widely used in various cultures; Japanese, Chinese and in the
west historically primarily to adhere gold to iron, steel and
copper. I found few historical mentions in the west of
application of gold to silver using the same methods used in
Asia, though there are plenty of Roman and Greek artifacts which
upon reexamination in recent years seem to have been gilded in
this manner. The method can also be used to attach 24k gold to
itself, to apply gold foil to other standard and colored gold
alloys, palladium, white gold and platinum. The Korean method
is also spelled kum-bu. Several Koreans have given me slightly
different versions of the procedure. The version I personally
prefer is to take the finished object made in sterling,
depletion silver it (bring up the fine silver) by repeated
heating, quenching in water and pickling until it is completely
white and then heat with a hot-plate or a flame; whichever
provides the most even and constant type of heating for the
particular object. One may choose to brass brush with soapy
water in between picklings. Thin gold foil is placed on the
object and a polished steel burnisher tacks it down and then
presses it over the surface fixing it permanently in place. The
gold will not stick until the correct temperature is reached. If
a hot plate is used generally a thickish piece of steel, copper
or brass is used to transfer the heat more smoothly to the sheet
silver being applied with gold foil.
Koreans generally use Keum-boo only on the finished object but
if adhesion is good sheet metal with applied gold patterns can
be prepared and rolled for later use in fabrication. This is the
manner I usually use it in. Any solderings or heatings that are
done do not affect the keum-boo. If the gold is very thin
(enamelling foil) or the silver is heated very high there is the
possibility of gold diffusion and absorption by the silver and
everything from an increase in paleness and greenness to a
fading out due to total absorption. If small bubbles appear one
burnishes them down flat with the fingernail at the end of
construction and they disappear. If they are large then one
pricks their center with a pin and reheats the metal to repeat
the keum-boo procedure of burnishing thus fixing the gold foil
in place. The gold foil may be made by rolling a piece of 24k
gold as thin as one can go on the mill and then continue to roll
it with in between annealings. An alcohol lamp or even a
cigarette lighter may be used to anneal the gold when it is this
thin. Some people continue to roll with a piece of paper or
metal on each side of the gold to increase the pressure on it. I
usually use a piece of sheet metal at the end to increase the
rolling pressure. When the micrometer barely measures it it is
quite thin (.001 mm). Most failures in adhesion with keum-boo in
my opinion come from too thick a gold foil being used.
The only original contribution of this paper lies in theorizing
the mechanism by which Keum-boo works. A metallurgist observing
a keum-boo demonstration informed me that above a certain
temperature thin gold foil begins to pass oxygen atoms through
itself and is actually used as a filter material for gases in
some industrial applications. Theoretically then the gold when
thin enough passes oxygen through and with pressure (burnishing)
produces oxygen-free conditions in contact with the silver (or
other metal) below it -- allowing pressure welding to occur.
Western sources describing applying gold to steel and copper
using this procedure mention as a colour/temperature indicator
that the metals oxidize bright blue before the gold will stick
(Diebeners, p. 72, Wilson, p. 472). Experimentation with a
cleaned piece of steel heated over a low flame as a heat
transfer to the silver showed this to be true; blue appeared
when the gold stuck. This temperature lies between 650-950oF or
350-510oC (Andrews, p. 50). In support of this idea it is noted
that at about 350oC (650oF) gold shows changes in it's electron
rings. It has been postulated that this corresponds with the
dissolution of a gold oxide present on the metal surface
(Gmelin, p. 670). This is the temperature range where steel is
bright blue and gold foil will stick to the base metal.
While I had not had much luck burnishing gold foil to copper
except under a cover of molten flux Richard Mafong in Atlanta
reported no such difficulty. I tried again with thinner (0.002 -
0.004 mm) sheet and this proved very successful. If gold
possessed this filtering ability it might dissolve oxides by
removing available oxygen and allowing mechanical, pressure
adhesion to occur. This seems in fact to be the case as I have
placed thin gold foil easily onto copper and aluminum, polished
and unpolished. Of interest is that the thin gold foil works
well on aluminum and this seems to offer some possibilities of
combining gold and aluminum. Success with steel has so far
mostly eluded me though it should be noted that western sources
mention roughening the area to receive gold with a dilute
solution of hydrochloric acid (Diebeners, p. 72) or nitric
(Wilson, p. 472) before applying it. While in the west the
historical point has often been to place the gold onto steel
(armorers) in Korea goldsmiths complain about the gold sticking
to the steel burnisher if it gets too hot. It is in this manner
that I have easily attached gold to steel; onto the polished
burnisher while working. I usually have a small cup of water
handy and repeatedly quench the burnisher to cool it while
working. Water on the burnisher does not affect the keum-boo
process. Enamelling gold foil may be used for keum-boo, though
it is so thin it has a green tint from the silver beneath. Once
applied however it is easy to place more gold foil on top and
bond it to itself to thicken the covering. If the silver base is
in sheet form it can be rolled and the thin enamelling foils
resemble green watercolor washes. Where they overlap each other
the gold color is intensified so that one has a palette of
greenish tones and golds to work with if one plans to roll and
uses very thin gold foils. Thin gold foil can be applied in
this manner to platinum, palladium, white gold and other gold
alloys thus offering color and pattern options for gold jewelry
and objects. Richard Mafong reports using a 14k gold thin sheet
as a keum-boo material. He heats and pickles it repeatedly to
depletion gild the surface and treats it in the same manner as
pure gold in applying it to the silver. Because of the ease
with which keum-boo may be done it offers a very controllable
method of pattern development using gold on other metals.
Mafong's use of 14k offers a choice of gold color as well if the
pure gold on the top surface of the 14k is removed by polishing
after the keum-boo procedure. Thin colored golds such as reds
and greens could be applied this way, the tops emeried off to
reveal the core color. Dr. Joe Dule from New York City has made
a 12 Karat Au/Ag alloy for keum-boo work; a 50/50 mix of gold
and silver which appears very white, like a white gold. This can
rolled out extremely thin and be applied to a sterling object
like 24k gold foil. If the object is then darkened with
potassium sulfide solution any 24k material remains bright gold
against the black ground and the 12 karat alloy shows up white
and bright allowing one to have white, gold-yellow and black to
work with as a compositional system.
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BIBLIOGRAPHY
Andrews, Jack, Edge of the Anvil, Rodale Press, Emmaus, PA,
1977.
Diebeners, Wilhelm, Wekstattrezepte fur Graveure, Gurtler,
Galvaniseure und Stempelhersteller, Wilhelm Diebener, Leipzig 0
5, Druck; Glass und Tuscher, M 135-305.
Ganzenmuller, Wilheim, Gmelins Handbuch der Anorganischen
Chemie, System Nummer 62, GOLD, Lieferung 1 und 2, Verlag Chemie
GMSH, Weinbaum, 1950.
Wilson, H., Silverwork and Jewellery, John Hogg Pub., London,
1912.
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