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.