Keum-boo is an ancient Korean gilding technique used to apply thin sheets of gold to silver. Traditionally, it is accomplished by first depleting the 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. Examples of Keum-boo have been found dating back to 500 BCE.

Source: Wikipedia