Keum-boo (also Kum-Boo), translated as "attached gold", is an ancient Korean gilding technique used to apply 24K gold foil to metal (silver, iron, copper, platinum).
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 roughly 650 to 850 degrees Fahrenheit increases the movement of the atoms.
Applying pressure during the heating causes an electron exchange at the surface between the two metals, creating a permanent diffusion bond.
To create the foil, I melt 24K gold grains into an ingot. With the help of a rolling mill, the thinning process begins. After a few passes through the mill, the gold sheet is annealed, pickled, cleaned, and dried before it is rolled again. This process is repeated about 18 times until the 24K gold sheet reaches a thickness of about 0.0079 inches (0.02 mm).
I prefer to roll my own foil since the commercially available foil is not as pure (23.5K instead of 24K) is much thinner, and therefore won't wear as well.
Designs are sawn out and prepared for the Keum-Boo process by heating, pickling, and brass-brushing multiple times until heating does not cause their surface to discolor.
I cut 24K gold foil sections with metal cutters, knifes, scissors, and homemade tools, anneal (soften) and straighten curled sections, then carefully place them onto the Argentium.
The silver is heated either with a torch or a hotplate, depending on the shape and size of the project. Once the metal reaches 750 to 800 degrees, the fun part begins!
With the help of a steel or agate burnisher and gentle pressure, the 24K gold foil almost magically adheres to the silver.
After the pieces have cooled down, excess gold foil is removed, and the edges are cleaned and sanded.
Three dimensional shapes are created with the help of a hydraulic press, bending pliers, and metal or wooden dapping tools.
Parts are then ready for further steps (assembly, stone setting, soldering, applying a patina, lacquer or e-coating).