Technique focus: Keum-Boo
Keum-Boo is an Ancient Korean technique that refers to applying 24ct gold foil to silver by using heat and pressure. This technique has become widely used on Silver Clay pieces because of the limited amount of preparation needed.
Pure gold and silver have a similar atomic structure therefore once the metal is heated to the temperature of 500-700°F; the movement of the atoms is increased. Once pressure is added, this causes an electron exchange at the surface and creates a permanent diffusion bond.
*Note that Keum-Boo gold foil is different to gold leaf; gold leaf is thinner and will be absorbed by the silver, not fused together. (0.1 micron compared to Keum-Boo, which is 25 times thicker – 2.5 microns)
To cut the foil you will need sharp scissors, craft knife, paper punches (which are available in different shapes and sizes) and templates. Tweezers and damp brush are used to move the foil into place. Using an Agate stone burnisher is recommended for metal clay work, Keum-Boo and gold and silver applications.
It is recommend that Keum-Boo is used on pure fine silver, which is why most jewellers will use it on PMC. If used on sterling silver, you will need to apply the depletion process beforehand: this involves a repeated heating and pickling process until a layer of pure metal is left behind.
Before applying Keum-Boo to PMC pieces, fire them in a kiln; this removes traces of oil and patina. When the temperature reaches about 500°F-700°F, use a burnisher to tack the foil in place and then go back and burnish each area. When using Keum-Boo, the foil needs to be in position on the metal before turning on the burner. You can use table top kilns, electric/ gas stoves and a jeweller’s torch; however use thicker foil when firing with a torch.
Tip: Some people find it easier to use a dampened paintbrush to pick up the gold foil and apply it to the metal. You will need to keep a pin ready for poking the gold foil in case an air pocket forms.
It is also recommended that you save your scrap Keum-Boo foil; you can use this later to fill in areas that have torn in the foil or moved. For a matte finish, brush the piece with a stainless steel brush or a soft brash brush dipped in soapy water or a dampened pumice stone. For a high shine, use a barrel polisher in a mixed steel shot solution and soapy water.
Problems and advice
When working with Keum-Boo there are two main problems that might arise.
- The gold does not bond: this might happen because the burner is not hot enough, the silver was not cleaned properly, or the gold foil didn’t make surface contact with the silver.
- An item needs to be attached using solder: this is a problem because solder needs to be used on fully burnished pieces whilst Keum-Boo works best on pieces before they have been burnished. To solve this you can try either using a stainless steel burnisher to burnish areas for solder, solder these areas and then apply Keum-Boo; or you can apply a thicker layer of gold foil, then burnish, tumble and solder as normal. The thicker foil layer is necessary as some of the gold is likely to diffuse during the soldering process.
Keum Boo is a technique accessible to all levels of craftspeople, jewellers and metal clay artists. If you’d like to read a book about this technique before getting started why not try “Keum Boo On Silver: Techniques for applying 24k Gold to Silver” by Celie Fago, which will take you through every step of the process, from preparation, to application, to finishing?