Raku ware (楽焼 raku-yaki)
is a type of Japanese pottery
that is traditionally used in the Japanese tea ceremony
most often in the form of tea bowls. It is traditionally characterized
by hand molded, rather than turned, clay, which results in each piece
being "one-of-a-kind"; fairly porous vessels, which result from low
firing temperatures; lead glazes
; and the removal of pieces from the kiln
while still glowing hot. In the traditional Japanese process, the fired
raku piece is removed from the hot kiln and is allowed to cool in the
open air or in a container filled with combustible material. Raku
techniques have been modified by contemporary potters worldwide.
Raku means "enjoyment", "comfort" or "ease" and is derived from Jurakudai, the name of a palace, in Kyoto, that was built by Toyotomi Hideyoshi (1537-1598), who was the leading warrior statesman of the time.
In the 16th century, Sen Rikyū, the Japanese tea master, was involved with the construction of the Jurakudai and had a tile-maker, named Chōjirō, produce hand-moulded tea bowls for use in the wabi-styled
tea ceremony that was Rikyū's ideal. The resulting tea bowls made by
Chōjirō were initially referred to as "ima-yaki" ("contemporary ware")
and were also distinguished as Juraku-yaki, from the red clay (Juraku)
that they employed. Hideyoshi presented Jokei, Chōjirō's son, with a
seal that bore the Chinese character for raku.
Raku then became the name of the family that produced the wares. Both
the name and the ceramic style have been passed down through the family
(sometimes by adoption) to the present 15th generation (Kichizaemon).
The name and the style of ware has become influential in both Japanese
culture and literature.
In Japan, there are "branch kilns" (wakigama), in the
raku-ware tradition, that have been founded by Raku-family members or
potters who apprenticed at the head family's studio. One of the most
well-known of these is Ōhi-yaki (Ōhi ware).
After the publication of a manual in the 18th century, raku ware was
also made in numerous workshops by amateur potters and tea practitioners
in Kyoto,and by professional and amateur potters around Japan.
Raku ware marked an important point in the historical development of
Japanese ceramics, as it was the first ware to use a seal mark and the
first to focus on close collaboration between potter and patron. Other
famous Japanese clay artists of this period include Dōnyū (grandson of
Chōjirō, also known as Nonkō; 1574-1656), Hon'ami Kōetsu (1556-1637) and Ogata Kenzan (1663-1743).
Raku tea bowls play an important part in Rituals, a 1980 novel by the Dutch author Cees Nooteboom.
In raku firing, the aluminium container acts as a reduction tube, which
is a container that allows the carbon dioxide to pass through a small
hole. A reduction atmosphere is created by closing the container . A reduction atmosphere induces a reaction between oxygen and clay, which affects the color . It also has extraordinary effects on the metals inside the glaze. Reduction is a decrease in oxidation number .
Closing the can starves the air of oxygen after the combustible
materials such as sawdust catch fire and forces the reaction to pull
oxygen from the glazes and the clay 
. For example, luster duster gets its interesting color from
deprivation of oxygen. The reduction agent is a substance from which
electrons are being taken by another substance
. The reaction uses oxygen from the atmosphere within the reduction
tube, and, to continue, it receives the rest of the oxygen from the
This leaves ions and iridescent luster behind. This creates a pleasing
metallic effect. Pieces with no glaze have nowhere to get the oxygen
from, so they take it from clay. This atmosphere will turn clay black,
making an unattractive, matte color, without sheen.
Reduction Firing 2
Reduction firing is when the kiln atmosphere, which is full of
combustible material, is heated up. "Reduction is incomplete combustion
of fuel, caused by a shortage of oxygen, which produces carbon monoxide"
(Arbuckle, 4) Eventually, all of the available oxygen is used. This
then draws oxygen from the glaze and the clay to allow the reaction to
continue. Oxygen serves as the limiting reactant in this scenario
because the reaction that creates fire needs a constant supply of it to
continue; when the glaze and the clay come out hardened, this means that
the oxygen was subtracted from the glaze and the clay to accommodate
the lack of oxygen in the atmosphere. Consequently, the raku piece
appears black or white, which depends upon the amount of oxygen that was
lost from each area of the piece. The empty spaces that occur from the
reduction of oxygen are filled in by carbon molecules in the atmosphere
of the container, which makes the piece blacker in spots where more
oxygen was retracted.
Western raku techniques
Raku became popularized in America in the late 1950s with the help of Paul Soldner.
Americans kept the general process of firing, that is, heating the
pottery very quickly at high temperatures and then cooling it very fast.
Besides this, America has formed its own, unique style of raku.
It is raku's unpredictable results and intense color that attract
modern potters. These patterns and color result from the harsh cooling
process and the amount of oxygen that is allowed to reach the pottery.
Depending on what effect the artist wants, the pottery is either
instantly cooled in water, cooled slowly in the open air, or placed in a
barrel filled with combustible material, such as newspaper, covered,
and allowed to smoke
. Water immediately cools the pottery, stopping the chemical reactions
of the glaze and fixing the colors. The combustible material results in
smoke, which stains the unglazed portions of the pottery black. The
amount of oxygen that is allowed during the firing and cooling process
affects the resulting color of the glaze and the amount of crackle.
Unlike traditional Japanese raku, which is mainly hand built bowls of
modest design, western raku tends to be vibrant in color, and comes in
many shapes and sizes. Western raku can be anything from an elegant
vase, to an eccentric abstract sculpture. Although some do hand build,
most western potters use throwing wheels while creating their raku
piece. Western culture has even created a new sub branch of raku called horse hair raku.
These pieces are often white with squiggly black lines and smoke-like
smudges. These effects are created by placing horse hair, feathers, or
even sugar on the pottery as it is removed from the kiln and still
Raku is a unique form of pottery making; what makes it so unique is the
range of designs that can be created by simply altering certain
variables. These variables--which include wax resist, glazes,
temperature, and timing--ultimately
determine the outcome when firing a piece of clay. Wax resist, which is
painted over the bare untainted clay, results in the suspension of wax
in water 
before the raku glaze goes on. This is done so that the glaze does not
cover the area where the wax resist was applied, thus creating a design.
When in the kiln, the wax melts off and the carbon, that results from
oxygen reduction, replaces the wax.
This is the result of the combustion reaction. Raku glazes contain
alumina, which has a very high melting point. Therefore, carbon will not
replace the glaze as it does the melted wax. Raku glazes also contain
metallic compounds such as copper, iron, cobalt, which produce different
colors. After the glaze has reached its a certain temperature, the
metal in the glaze assumes its color.
For example, cobalt produces dark-blue, and copper produces green but
produces a red when the oxygen in the glaze is completely gone.
Any unglazed areas turn black due to the carbon given off from the
reduction of oxygen. Next, the clay is moved from the kiln to a
container(usually a trashcan), which contains combustible organic
materials such as leaves, sawdust, or paper. Once the lid of the
container is closed, the reduction oxidation (redox) process begins.
The temperature change from the kiln to the container is where the
magic of raku occurs. The change in temperature and in the redox
sometimes cause cracking or crazing. Crazing is a consistent cracking in
the glaze of a piece, as is seen on the white crackle glaze. This
either enhance or detract from the design. The timing of removal and
placement in water directly affects the shades of each color.
Introducing the water to the raku at the right time is incredibly
important; if the raku does not cool enough before placement in water,
the Raku can crack, break, or even explode!
(Raku ware - Wikipedia)