Keum boo is ancient metalworking technique which gets it's name from Korean. Similar technique was although used in Japan, China and also in western countries.
In this technique, thin 24K gold foil is fused usually to silver using heat alone. Like mokume gane and damascus steel, this also is a diffusion welding technique. Differing thing is that temperatures necessary to make the weld are substantially lower. In 300-400 celsius there are changes happening in electron shells of pure gold, which makes it possible for thin gold foil to let oxygen travel through. This feature is also used industrially in specialised filtering solutions.
Once again I don't take any responsibility on anyone using this as a tutorial. This is simply to shed light on techniques that I use in my jewelry.
Silver is ideal base material for gold, because their atomic structure is very similar, which allows the metals to dissolve together easily. Usually sterling silver is used in jewelry for it's better working properties compared to fine silver. The problem is copper that is present in sterling silver alloy (7,5% of it), which oxidises very strongly when exposed to heat. Many precious metal smiths are familiar with the problem of firescale when working with sterling. To prevent this copper oxide, or firescale from forming during the keum boo-process it is necessary to 'depletion gild' the surface of sterling silver. This means heating sterling carefully until it oxidises and then dipping it in a bath of mild sulfuric acid (note noxious fumes and splatter!). Sulfuric acid dissolves copper oxide rapidly and when this procedure is repeated a few times sterling is left with a nice layer of pure silver on the surface. This was used in past times in gilding of sterling silver products, before electrolytic gilding was introduced.
Thin gold foil is easily cut for example with a pen knife or a pair of scissors. It's also easy to create patterns with commercially available paper cutters.
The process in itself is quite simple after the preparations are done properly. Sterling silver item, or plate, is heated up with a torch or on a hot plate and previously cut shape of gold foil is placed on it. When the temperature is right, gold foil is simply rubbed to make it stick using a burnisher.
Although gold foil needs to be very thin, only few hundreds of millimeter thick, it still is substantially thicker than leaf gold or any layer achieved with electrolytic gilding. This feature gives keum boo- surface a good wear resistance and the item can be worked in many ways without the fear of loosing the patterning, for example hammering, rolling or press forming are possible along with annealing and soldering.
Many times sterling silver surface is patinated to create a strong contrast with gold.