Kum-boo (also spelled keum-boo, keumbu, geumbu, geumboo; Hangul: 금부, attached gold) is a Korean metalsmithing technique in which a thin sheet of high-purity gold foil is bonded to a silver substrate by the application of moderate heat and burnishing pressure. The result is a permanent diffusion bond that produces a layer of gold visually indistinguishable from solid gold and significantly more economical for decorative work.

Process

The silver substrate is first depletion-gilded: heated and pickled repeatedly to remove copper from the surface and leave a microscopic layer of fine silver. The depletion produces a clean, oxide-free silver surface that is essential to the bonding. A piece of high-purity gold foil, typically 24-karat (999 fine) and rolled to a thickness of about 0.025 to 0.075 millimetres, is then placed on the prepared surface.

The work is heated to approximately 260 to 370 degrees Celsius (500 to 700 degrees Fahrenheit), generally on a hotplate or pan, and the foil is burnished against the silver with an agate or steel burnisher. Below the eutectic point of the gold-silver system, the two metals diffuse into each other in the contact zone and form a metallurgical bond. The bond is strong enough to withstand subsequent forging, doming, and chasing, although care is required at the foil edges.

Lower-purity golds (18-karat, 14-karat) do not bond reliably because the alloying metals interfere with diffusion. The use of fine silver (or silver depletion-gilded to fine silver at the surface) is similarly essential.

Historical context

Kum-boo appears in Korean metalwork from at least the Three Kingdoms period (first century BCE through seventh century CE), with surviving examples in the National Museum of Korea and other Korean and East Asian collections. The technique flourished during the Silla and Goryeo dynasties and is documented continuously through the Joseon period. It is conceptually related to (though distinct from) the Japanese technique of kinmokuyo and to various depletion-gilding methods used in pre-Columbian South America (tumbaga) and ancient Egypt.

Modern revival

The technique was largely unfamiliar to Western metalsmiths until the 1980s, when the Korean-American jeweller Komelia Hongja Okim taught it widely in the United States and in Society of North American Goldsmiths workshops. Her teaching brought kum-boo into the standard repertoire of contemporary studio jewellers, particularly in North America, the United Kingdom, and Australia. It is now routinely taught at art-jewellery programmes including those at Cranbrook, Penland, the Royal College of Art and the Glasgow School of Art.

Practical considerations

The principal advantages are the convincing appearance of solid gold at a fraction of the metal cost, the design freedom to bond gold to specific areas of a piece (rather than gilding the whole), and the absence of plating chemistry. The principal limitations are the fragility of the foil at edges, the requirement for a clean fine-silver surface, and the inability to reliably bond lower-karat gold or to bond gold to base metals. The technique is well-suited to chased and pierced work, signet design, and pieces where a localised gold accent is desired without electroplating.