Keum-boo, written with the Korean characters meaning attached gold, is a traditional technique for fusing pure gold foil onto a fine silver substrate by means of heat and pressure rather than soldering. The technique allows a jeweller to introduce small areas of bright gold colour onto a silver surface without the difficulties of soldering or plating, and it has had a quiet revival in contemporary studio jewellery since the late twentieth century.

Process

The silver substrate is first depletion-gilded to bring a layer of pure silver to the surface, then heated to between 260 and 370 degrees Celsius, well below the melting point of either metal. A piece of 24-carat gold foil, hammered to roughly 30 microns or thinner, is laid on the heated silver and burnished with an agate or steel tool. The gold bonds to the silver by solid-state diffusion, producing a permanent metallurgical join. The technique requires high-purity gold and silver because alloying elements interfere with the diffusion bond.

Use in contemporary jewellery

Keum-boo gives a richer surface than gold plating because the gold layer is thicker and more securely bonded, and it allows precise placement of decorative areas. Designers including Komelia Hongja Okim, who has been instrumental in introducing the technique to American studio jewellers, have demonstrated its range. Buyers should be aware that keum-boo is not interchangeable with vermeil or with gold-filled wire; it is its own technique, and pieces should be described accurately rather than collapsed into a generic gold-on-silver category.