Kum-Boo 24K Overlay on Silver


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By Komelia OkimMore from this author

Kum-Boo (rhymes with "loom low") is a Korean appliqué technique of surface decoration in which pure gold foil is fused onto the surface of finished silver ornaments or objects. The historical background and source of technique are vague. For thousands of years, Koreans have been using brass, silver and gold wares and utensils for their daily eating and ceremonial purposes. Especially in winter, brass and silver wares are commonly used to keep food at the correct temperature. Since the Korean War ended in 1953, most brass and silver utensils have been replaced by aluminum and stainless steel wares for common use. Beginning in the late '70s, however, a revived interest in traditional customs has aroused a new interest in the use of finer metals, particularly for ceremonial, birthday, and wedding celebrations and for ancestral memorial services.

kum-boo
Mountainscape No. II, 925 silver, 24k gold, anodized titanium and brass screws, 8 x 11 x 10", 1989

Among upper-class people, silver wares are commonly used to display their wealth and to enjoy both their prosperity and status. These wares are also used to test defective, spoiled or poisoned food, and in this regard they echo a role finer metals have played among royal families for centuries. Koreans and some other East Asians believe that the ingestion of pure gold will improve health and well-being. Many herbal medicines are covered with very thin sheets of pure gold. Similarly, acupuncture needles are often made of a high-karat gold in the belief that this metal has properties that will enhance the effectiveness of the treatment.

Many Korean silver utensils are decorated with 24-karat gold overlays in the form of letters and patterns that convey wishes for good health, wealth and longevity. In most cases the ornament is set in the interior of a cup or bowl, or within the bowl of a spoon, so that the food will come in contact with the gold and therefore be able to assimilate its positive characteristics before being ingested.

Special Evening, Punch bowl with drawers for cocktail forks and spoons, 925 silver, 24k gold, black onyx, walnut (inside drawers) 16 x 11 x 20", 1987

Though somewhat rare in the United States, and almost unheard-of in the commercial industry here, Kum-Boo is a familiar technique in Korea and has been wide use for the past decade.

Overview

The term Kum-Boo is derived from Kum, which means gold, and Boo, which can be translated as attached. Because of the extreme delicacy of the sheet, conventional soldering is not appropriate and would probably be impossible. In Korean Kum-Boo, 24k sheet gold is diffused onto silver alloys through a combination of pressure and relatively low temperatures (500° - 700°F, 260° - 370°C). The primary tools used in this technique are a stove, hotplate or torch to create the proper heat, and variously shaped polished steel burnishers to press the overlay against the parent object.

In fabricated pieces, all soldering must be completed before beginning the Kum-Boo process. When the piece is fully assembled (except, of course, for elements to be attached with rivets or similar cold connections), the work is cleaned thoroughly in fresh pickle. In order to establish a skin of fine silver, the work is heated again until it shows brown spots, then it is again dipped in the acidic pickle. The piece is rinsed in water and lightly scratchbrushed. This cycle is repeated from three to six times, or until the metal retains its white color even when heated. The work is then dried carefully, with care taken that the delicate skin is not touched.

Overlay pieces are cut from thin 24k gold sheet with scissors or a sharp X-Acto® blade. The gold is laid into place and the work is set onto a hotplate or other heat source and brought up to the correct temperature. The foil overlay is then burnished into the silver, where a diffusion of the molecules of the two metals creates a permanent bond between the two sheets.

When burnishing the gold overlay, it is important to hold the object steady, which may require the use of a second burnisher or tweezers. Keep a container of water handy for cooling the burnishers.

Equipment and Materials

  • Hotplate, camp stove, torch, electric or gas stove
  • Gum tragacanth powder
  • #00 or #1 artist's paintbrush
  • 24k gold foil .03 - .05 mm If Thompson's gold foil is used, double or quadruple layers are used as a single sheet.
  • Tracing paper
  • Sharp scissors or X-Acto® knife
  • Several shapes of curved steel burnishers
  • Fine-pointed tweezers
  • Cotton gloves (not gardening gloves - too bulky)
  • Hot pickle (Sparex®)
  • Fine-pointed sewing needle

Optional

  • Optivisor®
  • Tripod (for large pieces)
  • Hole punches or design punches to cut out repeated shapes
  • Steel or brass scratchbrush

Preparation of the 24k Gold Foil

Purchase 24k gold in the thinnest form possible. The desired thickness is .03 to .05 mm, which can be achieved in a rolling mill if necessary. An electric rolling mill is preferable, but a manual mill will work. Cut two pieces of copper or brass a little wider than the size of the gold sheet and considerably longer. Heat these to build up an oxide layer and allow them to air cool without pickling. The purpose is to create a scaly surface that will not adhere to the gold foil. Set the gold between these sheets and pass it through the mill in the conventional way.

Moonlight Promenade, Box container, 925 silver, 24k gold, titanium and hematite, 9¼ x 10½ x 3⅛", 1987

After every two or three passes, the gold will need to be annealed. This is most easily done on a hotplate set on High. Use a sheet of 20-gauge copper, brass or steel to diffuse the heat. Work in subdued lighting and heat until the gold begins to show red, then air cool for a few moments and quench in water.

Note that if Thompson's gold foil is used, limit the number of pieces in an application to five or six. Because the foil is so thin, the prolonged heat needed for more complex applications is liable to diffuse the gold into the silver alloy, where it will almost disappear.

Process

  1. Complete the work through all soldering and finishing steps. Patination, stone setting and cold connections can all be achieved after Kum-Boo. Any other procedure should be completed before the overlay process is begun.
  2. Draw the pattern onto tracing paper. Fold this to create a sleeve into which the gold foil can be placed. Cut the overlay piece(s) with a knife or sharp scissors. Leave the gold in the paper until ready for use.
  3. Using a permanent marker, draw the overlay outline onto the work, making the drawing a little larger than the overlay piece. This outline will ensure the proper location of the overlay.
  4. Place the gold pieces on the work with a dampened brush or burnisher. Simply dip the brush in a dish of water and shake off all excess before using the brush to pick up the gold pattern. If the patterns are large or complicated, use a very thin mixture of gum tragacanth instead of water. I prefer to mix my own solution starting from powder rather than use a premixed commercial liquid solution.
  5. Set the work on an unheated hotplate or burner and set the temperature on High. In order to provide a flat surface, you may want to set a piece of steel or metal screen on the burner and allow this to distribute the heat. As it dries, the water or gum tragacanth will secure the foil to the surface. When the object reaches 500° - 700°F (260° - 370°C), apply light pressure with a burnisher to tack the foil into place.
  6. When each section of the overlay is tacked down, lower the heat and burnish the entire piece briskly. It is important that the object be held steady, which might require an additional burnisher or a pair of tweezers. Note that you are working directly on the hot burner, which is why gloves must be worn.
  7. If the burnisher gets too hot, the gold foil will adhere to it. To prevent this, keep a cup of cold water close at hand and dip the burnisher into it periodically as you work.
  8. When the application is finished, examine the edges of the overlay closely to see that they are well attached. A loupe or Optivisor is useful for this process. If the diffusion is complete, set the work in hot pickle until the metal is completely white and all burnishing marks are removed. The piece can now be patined or polished, using a rouge cloth or a scratchburhs lubricated with soapy water.
Story Hour, Three containers, 925 silver, 24k gold and red jasper, 3½ x 3 x 4", 1987

Variations

  1. It is possible to overlap layers of foil using the Kum-Boo techniques described above. The gold is so thin it is semi-transparent, which will effect a color change as layers are built up. When working on a flat or nearly flat object, it's most efficient to apply pieces directly onto the object while it's hot. To do this, pick up the prepared piece of foil with a dampened burnisher and carefully set it into position. Keep a slight pressure on the piece until the water stops sizzling, then gently begin to burnish the foil down. When it appears to be tacked into place, increase the pressure and speed of the burnishing action. Remember to quench the burnisher in water when it heats up, and to change burnishers when a new shape is required.
  2. Sometimes complicated pattern or unusual contours of the object make the positioning of the overlay pieces difficult. In these circumstances, use a thin mixture of water and gum tragacanth to glue the piece into place. Use a fine paintbrush to coat the area that will receive the foil and to pick up the overlay piece. When all the pieces are in position, set the work aside to dry completely.
    When dry, set the object on the hotplate, turn the heat to High, and selectively burnish areas of each piece to tack down a couple of spots on each pattern piece. Turn the temperature to Medium and burnish each piece fully. Remember to cool the burnisher periodically. When burnishing is complete, allow the piece to cool, then examine each edge for bonding. If the diffusion was complete, set the object in hot pickle.
  3. For a large object the hotplate might not provide sufficient heat. Diffusion will not occur if the parent metal is not hot enough. In this case glue the overlay pieces with the gum tragacanth mixture as described in B (above). A torch can be used in conjunction with the hotplate. Obviously this process will be easier if you can enlist the aid of a friend.
Wedding Ceremonial Set, Wine set with tray, 925 silver, 14k, 24k gold and hematite, 7 x 6⅛ x 4", 1½ x 2 x 3", ¾ x 11⅞ x 6⅛", 1987

Repair of Kum-Boo

Poor adhesion is usually indicated by tiny bubbles in the gold. To repair, reheat the area with a soft torch flame, then, turning the flame aside briefly, reburnish with considerable pressure. If there are many bubbles present, puncture each one with a fine needle, reheat the area and burnish quickly and vigorously.

The preceding article is extracted from Metals Technic, a book of collected technical articles to be published soon by Brynmorgen Press.

Komelia Okim is a professor of art and head of the metals program at Montgomery College, Rockville, MD.

By Komelia Okim
Metalsmith Magazine – 1992 Spring
In association with SNAG‘s
Metalsmith magazine, founded in 1980, is an award winning publication and the only magazine in America devoted to the metal arts.

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Komelia Okim

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