Keum-Boo

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).

24K gold foil Keum-Boo.jpg

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). 

Melting a 24K gold ingot.jpg
gold ingot
gold ingot put through rolling mill
torch annealing 24 K gold
rolling 24k in rolling mill.jpg

I prefer to roll the foil myself since the commercially available foil is not as pure (23.5K instead of 24K), is much thinner, and therefore won't wear as well as mine.

Finished rolled gold foil

Designs are sawn out of Argentium silver sheet and prepared for the Keum-Boo process by heating, pickling, and brass-brushing multiple times until heating does not cause their surface to discolor.

Sawing out an Argentium Silver pendant.j
argentium silver depletion gilding.jpg

Then 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.

cutting 24K keum-boo foil.jpg
annealing 24k gold strips keum-boo.jpg
Placing 24 k gold foil pieces on silver
 cut out 24k gold foil disc on bench block

The metals are heated either with a torch or a hotplate, depending on the shape and size of the project. Once the silver reaches 650 to 800 degrees, the fun part begins!

With the help of a steel or agate burnisher and gentle pressure, 24K gold foil almost magically adheres and permanently bonds to silver. 

burnishing 24k gold foil on hotplate.jpg
Keum Boo with torch on tripod-Argentium
Mikado earrings, keum-boo, burnishing 24K gold foil strips.jp

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.

trimming 24k gold foil after Keum Boo.jpg
Pressing and forming a silver sheet in a 20 ton.jpg
shaping a silver disc in a dapping block.jpg

The jewelry is hardened by placing it in an oven set to 550 degrees for one hour. This process is called precipitation hardening. Argentium silver responds very well to it. This treatment ensures best possible wearability and durability.

Heat Treating Argentium silver in an Ove

Once the pieces are cooled off, they are ready for the last step, applying a patina, a lacquer, or an nano ceramic e-coating).