Keum-boo (金鈇) is an ancient Korean gilding technique used to apply thin sheets of gold to silver, to make silver-gilt. Keum-boo refers to a technique that applies moderate heat to gold and silver (about 500-700F Fahrenheit) to make it permanently attached by rubbing the surface with a very clean, well-rubbed wood or animal horn. Used for cutlery, ornaments and jewelry. When using this technique, use it in the final finishing step. This is because there is room for gold and silver to melt in the middle of the production process. Gold parts can be attached when the melting temperature is lower than the manufacturing temperature.
Pure precious metals such as gold and silver have a very similar atomic structure and therefore have a good potential for bonding. Heating these metals to a temperature between 260–370°C increases the movement of the atoms. When pressure is added, this causes an electron exchange at the surface between the two metals, creating a permanent diffusion bond. This diffusion bond occurs far below the soldering temperature for either metal (Dhein, 2004).
Keum-boo 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.
Generally Keum-boo only use 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.
Examples of this technique have probably been observed, but not positively identified on pieces from the second half of the first millennium B.C. and from the early first millennium A.D. (Oddy, 1981). Traditionally, this technique is accomplished by first depleting a surface of sterling silver to bring up a thin layer of fine silver. Then 24 carat gold foil is applied with heat and pressure—mechanical gilding—to produce a permanent diffusion bond.
This technique is used in many cultures, including Chinese, Japanese and in the West to bond gold to other metals, including iron, copper, aluminum, gold alloys, white gold, palladium and platinum. Foil made from gold alloys can be applied to silver and other metals by first depletion gilding the surface of the foil (Lewton-Brain, 1987–1993).
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.
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. 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.
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.
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.