I have posted on Keum Boo before. However, it is such an impressive technique that I thought I would do it again.
Keum Boo is Korean for ‘attached gold’. That is just what it is — a way of making a small amount of gold go further by attaching it to silver in such a way that the gold actually fuses. You can do Keum Boo on fine silver (some comments about doing it on sterling silver later on).
Start with gold foil made for Keum Boo. Gold leaf will not work — in fact, gold leaf is so thin that it will simply vanish into the silver. Gold foil made for enameling will work, but the gold and silver will alloy in such a way that the gold takes on a greenish tinge. While that can be attractive, I’m going to assume you want your gold to look like gold.
Using sharp scissors, a craft knife, or a punch, cut your pieces of gold foil. I say ‘pieces’ plural because, while one layer of gold certainly looks nice, layering several sheets of gold foil will look all the better.
Make your object of choice from fine silver clay. Keum Boo works best if you do not brush the object at this stage. The image below is the object that I am working with now. (My apologies for the blurry photo — I did not notice that it was blurry until it was too late to take another.) This piece has been made from silver clay and fired, but not brushed.
While it is possible to do Keum Boo on silver that has been bushed, it is easier if you save the brushing for later. The white color that comes out of the kiln is caused by tiny pieces of silver standing up. That gives the gold something to grab onto!
The next step is to heat the silver to about 600 degrees F (500-800 degrees will generally do). A ‘rough and ready’ temperature measure, if you don’t have a measuring device, is a toothpick. If the toothpick’s end chars at a touch, the silver is hot enough. A beehive kiln is good for reaching the appropriate temperature, although one can do it with a torch.
Using tweezers, apply a section of gold foil. It is generally best to apply the gold foil to a smooth section of the silver. Burnish the gold with an agate burnishing tool (available at, for example, https://www.amazon.com/NIUPIKA-Burnisher-Bamboo-Handle-Polishing/dp/B01KZFF54U/ref=sr_1_1_sspa?ie=UTF8&qid=1506356505&sr=8-1-spons&keywords=agate+burnisher&psc=1). You might want to wear gloves for this. The silver must be kept hot, and it will take a great deal of burnishing to get the gold to adhere. Additional layers of gold foil can be applied in the same manner.
Once you have applied all the gold you are going to, you can either let the piece air cool or you can crash cool it. Then you can brush the piece. It is unlikely that you would brush off your gold, but, to be sure, you can cover the gold with your thumb while you are brushing. After that, you can tumble like any other piece made of silver clay — if your gold has been appropriately applied, it will not come off.
Here is what the piece pictured above looked like after applying gold and tumbling.
The next step is patina. Gold will not take patinas. This allows some nice design possibilities, because you can have shiny gold next to darkened silver. I chose to simply patina the tree, leaving the circle shiny.
Here is the piece after patinaing, and with a pinch bail and chain.
Generally, you are going to want to attach gold to fine silver. Sometimes, you might want to use sterling. This is particularly true if, instead of metal clay, you are working with sheet silver. Gold foil will not stick to sterling, a copper/silver alloy. To apply gold foil to sterling silver, you must deplete the sterling. That is, you must remove the copper from the top layer of the sterling. This is tedious, but can be done. Heat the sterling until a faint purple haze appears on its surface. Quench the metal, and then pickle until the purple haze is gone. Repeat this process until the heating causes no color change in the metal. You might want to do it one or two times after you think the copper is gone, because you don’t want your gold foil to not stick.
I encourage you to give Keum Boo a try. You can produce some truly striking pieces by adding just a touch of gold to your silver.