This is an AJM article from John Shanahan that discussed Keum Boo in
Depth “John Shanahan”
Korean for gold added, keumboo is a technique for applying gold foil
to metal. It is a fairly simple method for dressing up a piece of
jewelry. It can take the place of inlay, can be added to a previously
finished pieceand it can be good for you.
Koreans believe that if you intake gold, it will improve your
health, explains Komelia Okim, a professor of jewelry and
metalworking at Montgomery College in Rockville, Maryland. Okim has
been working in keumboo since taking a trip to Korea in 1982.
Traditionally, keumboo was used on silver utensils in Koreaspoons,
bowls, or at the ends of chopsticksso that when youre taking in food,
you also get a bit of gold.
Although the health benefits of sucking on a keumboo earring have
yet to be proven, the virtually limitless license for design it
allows jewlery makers is undeniable. But those who use keumboo in
their work agree that while this is not a difficult technique to
master, it can be time-consuming and exacting: Rolling the foil alone
can take several hours, and the metal must be annealed at each pass;
when placing the foil, bubbles can appear if the base metal is not
clean; and if the foil is too thin, it can literally disappear. But
the results are very much worth the effort.
The process begins with the creation of the gold foil. There are
commercially available gold foils that can be used, but the jewelers
profiled here prefer to make it themselves, for the control it gives
them over their finished work. (Okim now buys her metal from Korea to
save time.) I buy a sheet of 24k gold from my refiner, says
Cumberland, Maine-based designer Jayne Redman. I start with a piece
2.5 inches wide by 12 inches long in 40 gauge thickness. I cut those
into three 2.5 inch squares and one square thats 2.5 inches by 2
That gold is then put through a rolling mill to thin and elongate
it. At 40 gauge, Redman says, the sheet is about as thin as the jaws
of her rolling mill will go, so she sandwiches the gold between two
sheets of 18 gauge copper about 3 inches by 2 inches. Regardless of
what gauge gold you start with, eventually it will get too thin to
roll and will have to be placed between sheets of a thicker metal.
This is where the first bit of keumboo caution comes into play.
Charles Lewton-Brain, a jeweler, instructor, and author from
Calgary, Alberta, Canada, makes his sandwich with only one piece of
24 gauge copper and the gold foil which he dirties up with oil. If
you were to take clean copper and put the gold on it and run it
through the press, theres no oxygen in there, he notes. Its quite
possible, at room temperature, to stick the gold to the copper.
Redman does much the same, but uses a liberal coating of WD40 on the
copper, jokingly referring to it as the mayonnaise in her sandwich.
After rolling the gold the first time, it must be annealed before
putting it through the mill again. This should be done after each
pass, but because the foil is so thin, it must be done quickly and
carefully. I have annealed the foil with a [cigarette] lighter,
Lewton-Brain says, noting that an alcohol lamp or a fast pass with a
torch will also work. It anneals almost instantly, so you have to be
Okim adds that careful annealing and rolling is required to ensure
the metals integrity at a microscopic level. If you dont do this
right, the edges will be broken. Even 24k gold can become very
brittle as it work hardens. As it passes through the rolling mill, it
can crack around the edges.
Ideally, the gold should be rolled until it is as thin as possible.
Redman works her sheet down to 0.02 mm. This takes between 12 and 15
passes through the mill, she says. Lewton-Brain takes it even
further, to the point where the sheet no longer registers on his
micrometerwhich would be about 0.001 mm. ( In my estimation, you cant
make it too thin, he says.)
When the foil is complete, it can be cut into various shapes to be
applied to a base piece. Typically and traditionally, that base piece
will be sterling. Redman also uses 18k for the color contrast with
the 24k foil. Whichever metal is used for the base, it must first be
heated to depletion gild the surface. In depletion gilding, metal is
heated with a torch until it begins to oxidize (look for the first
signs of brown or black), and is then plunged into pickle. Okim
recommends a cold pickle, but it can also be done hot. This removes
the various alloys in the metal and brings the pure gold or silver to
the surface. This is important because any material that will oxidize
when the piece is heated again to apply the foil will impede the
To prepare it to receive the foil, the base piece is then heated.
This is best done on a hotplate. Lewton-Brain suggests that a
flat-topped hotplate is preferable to one with a coiled heating
element, as a piece of foil can easily be dropped through the coil.
If you only have access to a coiled hotplate, a thin sheet of
stainless steel placed over it will create a larger and gap-free work
The question of how hot is hot enough for keumboo is a bit tricky,
since hotplates dont have exact temperature control. Silver has to
reach a temperature of 650 F before it will accept the foil; 18k has
to reach about 800 F. To get there, Redman turns her plate up as high
as it can go to heat it up quickly, then turns it back down to medium
heat after about five minutes. Lewton-Brain notes that a clean piece
of steel placed on the hotplate will turn blue at the temperature
thats best for keumboo using silver.
When the base is hot enough, completing the process is as simple as
laying down the foil and burnishing it onto the surface. This is
essentially pressure weldingbut only the slightest amount of pressure
is required. The burnisher should remain relatively cool to avoid
having the gold adhere to it rather than the jewelry piece.
The combination of pressure and heat allows the metals to alloy at
the contact point with no melting, Redman explains. She also cautions
not to leave pieces on the hotplate too long. Ive walked away and
forgotten about it and ended up heat hardening the pieces, which made
them hard to bend and form.
During burnishing, air bubbles can get trapped under the foil. Thats
why its a good idea to give everything a wipe with alcohol to
degrease it before you actually heat things up, says Lewton-Brain.
Small bubbles can be burnished with a fingernail. Larger bubbles
should be pricked with a thin needle; the metal is then re-heated and
re-burnished, pushing it toward the bubble hole to cover it over.
Designers who are familiar with enameling foil may be thinking that
it could act as ready-made keumboo foil. While it will work, keep in
mind that enameling foil is several times thinner than keumboo
foiland therefore more delicate and difficult to work with. One
problem is its color properties. Its so thin that when it is applied
to silver, it reads green to the eye, says Lewton-Brain.
The light passes through the foil, hits the silver, and comes back.
This can be negated through the use of additional layers, he notes,
but in most cases its easier to use standard keumboo foil.
Another problem with enameling foil is that if it is heated too
high, it will literally disappear into the surface of the base
piecebut this drawback can be a design benefit.
For design reasons, I might put a piece of enameling foil down, then
put another overlapping the first so that I end up with tones of
green depending on the thickness, Lewton-Brain says. Also, you can
deliberately heat enameling foil so that it merges with the surface.
Then, if you oxidize the piece, anywhere the foil was will stay
white. It acts as a resist.
When a piece is done and returns to room temperature, it can be
finished. This is typically a matter of getting rid of the burnishing
tool marks on the gold. Lewton-Brain touches up his pieces with a
hematite- or agate-tipped burnisher and a bit of soapy water. The
quality of the shine is much better than I get with steel, he says.
Redman, who prefers a satin finish, opts for the softer touch of
pumice. “I use a lot of texture in my work,” she explains. “A harder
burnisher [such as hematite] would leave shiny streaks. I use the
pumice to even out [the tool marks] and to get a flat finish. Then I
brass brush the piece with soapy water to get a shine.”
Keumboo pieces in silver are frequently oxidized to add color,
contrast, and interest. Redman immerses her finished pieces in liver
of sulfur to turn the silver areas blackbut some care must be taken
to prevent the gold from being affected as well. Redmans partner,
Sharon Sauerwald, has worked out a technique.
You have to remove any surface grease left over from handling, she
explains. Otherwise youll get flaking and the oxidization wont be
uniform across the surface. So I rinse the piece under hot water
first. The temperature of the liver of sulfur has to be just below a
simmer. There should be bubbles on the bottom, but no actual boiling.
The time varies depending on how much of the silver is showing. It
takes anywhere from 30 seconds to 2 minutes. You have to watch it. If
you leave it in too long, the liver of sulfur will oxidize the gold
as well. After pulling the piece from the liver of sulfur, it is
quenched in cold water to stop the oxidation process.
The end result is a piece that isnt entirely what it appears to be.
Its a silver earring at a silver price, but it looks more like a gold
earring because what you see is the black metal with the gold and the
gold wire, Redman says.
This is emblematic of the bottom-line appeal of keumboo. It can
quickly add a gold look to jewelry without adding a gold costto
either the jeweler or the customer. A gram of gold will make up to
about 2.5 square inches of keumboo foil, Lewton-Brain says. For the
cost, the coverage is pretty good.
I do my jewelry in both sterling and 18k lines, says Redman. The
basic metal cost for the one in sterling might be $2; in gold it
could be $35 for the same size piece. With the gold foil, the gold
cost might be $2 to $3. Whatever the cost, keumboo delivers a
priceless look to jewelry.