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Raku
The Story as I Know it
The process was initially developed by potters in Japan.
Tea bowls were made and bisque fired prior to a party. The potter then
built a small temporary kiln on-location at the party. The bisqued bowls,
paint brushes and glazes were provided for the guests to decorate as they
arrived. While the party progressed the potter quickly glaze fired and
cooled the tea bowls. The clay and glazes were specially selected
for this purpose. The clay had to be able to stand the shock of quick heating
and cooling and the glazes had to melt at a fairly low temperature.
The fired tea bowls were then used by the guests and they took their bowls home
with them. The basic process remains nearly the same over time but
technology has made it a bit easier.
Even though raku had it's beginning with tea bowls, it doesn't
mix well with food and drink. The pots are very porous and the glazes are
soft and sometimes contain lead. Raku should never be used to serve food.
Because of this you will commonly see raku art pieces that hanging a wall or in
a garden. Exposed clay on raku pieces is typically gray or black and the
glazes are frequently crackled or have a metallic luster.
The crackling of the glaze happens when the pot is exposed to
cold air as it is removed from the kiln. Some raku glazes are specifically
formulated with this effect in mind. The cracks are darkened by the
reduction process described in the next paragraph.
The blackened look and the copper luster happen when the hot
pieces are placed in an air tight container with burnable material. The
oxygen is quickly used up inside the container but there is still plenty of heat
from the piece and fuel from the burnable material in the container. This
creates a reduction (oxygen deficient) atmosphere in the container. For
combustion to continue oxygen molecules are pulled directly from the copper
oxide in the glaze and it is reduced back to a mixture of copper and glass on
the surface of the piece. Any porous parts of the piece like unglazed clay
absorb the free carbon in the atmosphere and turn gray or black.
My wife, Sheila glazing for raku firing. Notice the four
fish laying on the table to her right.
The following pictures show a raku firing. Notice the SNOW. It is
Fairbanks, Alaska after all. The temperature was about +10F Fahrenheit.
Doing raku in the cold allows the pieces to cool more quickly and increases
crackle but is still quite workable. The coldest I've fired so far is -5F.
This is the raku kiln. It is sitting on a pad of firebrick placed
directly on the snow. Notice the melted spot to the right from our last
firing. This is how I get traction in my driveway! Tongs laying to
the left of the kiln are used to remove the hot pieces from the kiln. The
kiln is simply ceramic fiber blanket fastened inside a cage made of 1/2"
mesh hardware cloth. The fiber is used to line furnaces and can be
purchased at a builders supply store here in Fairbanks. The propane
burner is kind of a special deal I bought from a ceramics supplier, but a friend
of mine uses a weed burner to fire his kiln. It works very well.
Notice the hole in the top of the kiln that is partially covered by a piece of
fiber blanket to control the draught.
The next few shots are looking into this hole as the firing progresses.
We fire the kiln to 1800F.
 
The pieces cool quickly when the kiln is lifted off, but you can
still see the shelf glowing in the picture below. The green color of
the fish is from copper in the glaze.
Next the fish goes into a metal container with some combustible material.
We use paper, sawdust and even leaves in the fall. The container is closed
as tightly as possible to encourage a reduction atmosphere inside.
The Final Product
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