Working with 24K Keum-Boo Gold Foil

Keum-Boo is a traditional Korean technique in which 24K gold foil is bonded to fine silver using heat and pressure. During heating, a diffusion bond is created where gold and silver atoms connect at the surface. This causes the gold to permanently bond to the silver.

This technique can be applied to fine silver, fired silver clay and prepared sterling silver.

Preparing the Silver

For proper bonding, the silver must be clean, grease-free and free from oxidation.

With fine silver or fired silver clay, the gold foil can usually be applied directly. When the piece comes out of the kiln it is generally already clean and grease-free.

With sterling silver, the copper must first be removed from the surface layer through depletion gilding.

• Heat the piece several times and immediately quench it in cold water, then place it in a pickle solution so the oxides dissolve.
  
  
• In between heating cycles, the silver can optionally be gently brushed with a brass brush and some soapy water to remove oxidation and bring more fine silver to the surface.
  
  
• Once the surface contains enough fine silver and no longer discolors, the piece is ready for Keum-Boo.

Preparing the Gold Foil

Cut the desired shape from the gold foil using sharp scissors. Because the foil is delicate, it is recommended to place the gold between a folded piece of paper while cutting. This helps reduce the risk of tearing or deforming the foil.

Paper punches can also be used to create decorative shapes.

Gold foil is much thicker and sturdier than traditional gold leaf, making it easier to handle.

Placing the Gold Foil

Place the gold foil onto the silver surface in the desired position.

Because the foil is extremely light, it can easily shift or blow away. The foil can therefore temporarily be secured with a tiny amount of saliva. Saliva is grease-free and is commonly used by goldsmiths to temporarily hold the foil in place without leaving residue behind.

A small amount of glue stick (Pritt) can also be used if desired.

Burnishing the Gold Foil

During heating, the gold foil is pressed down from the center outward using an agate burnisher or steel burnisher with light, even pressure. This removes trapped air beneath the foil and creates a proper diffusion bond between the gold and silver.

Work slowly and carefully so the foil maintains good contact with the silver surface.

• If the agate stone becomes too hot, the gold may stick to the burnisher. For this reason, many artists prefer to work with two agate burnishers so they can alternate while the other cools down.
  
  
• With a steel burnisher, the gold can stick to the metal if the tool becomes too hot. Regularly cool the burnisher in water during the process. This does not negatively affect the bonding process.
  
  
• For deeper textures or relief patterns, a brass brush can also be used to gently press the gold foil into the surface texture.

Keum-Boo on a Hot Plate

Keum-Boo can easily be performed using a hot plate. This method offers excellent control over the process and is ideal for small jewellery pieces, fine details and applying multiple gold accents.

First place a metal plate on the hot plate and allow it to gradually heat up. A fairly thick piece of steel, copper or brass works very well for distributing the heat more evenly to the silver piece onto which the gold foil will be applied.

The ideal working temperature is usually around 400–450°C. At this temperature the 24K gold begins bonding well to the fine silver.

• Place the workpiece on the heated metal plate and carefully hold it in place with tweezers.
  
  
• Then rub over the gold foil from the center outward with an agate burnisher or steel burnisher until the foil is fully bonded.
  
  
• Once the gold foil is securely attached, remove the piece from the heat source.
  
  

IMPORTANT: Do not heat the workpiece longer than necessary. Prolonged heating causes the gold to diffuse further into the silver surface, which may soften or reduce the intensity of the gold color.

However, this effect can also be used creatively. By applying multiple layers of gold foil or varying the heating times, subtle color variations and depth effects can be achieved.

Keum-Boo in a Kiln

Keum-Boo can also be applied using a kiln. This method is mainly used for already fired silver clay jewellery and is a higher-temperature fusion technique in which the gold bonds more strongly with the silver surface than with traditional hot plate Keum-Boo.

For Fine Silver or Fired Silver Clay

• Fully preheat the kiln to approximately 700°C. Wait until the kiln has actually reached this temperature before placing the workpiece inside.
• Place the workpiece in the kiln for approximately 2 minutes.
• Carefully remove the workpiece from the kiln and immediately place it onto a heat-resistant work surface.
• Hold the piece with tweezers and rub over the gold foil from the center outward using an agate burnisher.

The combination of heat and light pressure creates a strong diffusion bond between the gold and the fine silver.

For larger surfaces or if certain edges are not fully attached yet, the process may need to be repeated several times.