What is Raku?
Raku is a firing process to achieve unusual effects with ceramic objects. Starting with a previously fired unglazed clay object, one applies one or more glazes, particularly copper glazes, of various types to the cooled object. Next, one places the object in a kiln, firing until the glaze matures at about 1925°F. The object is removed from the kiln with tongs while red to yellow hot and placed in a reduction container. A reduction container is a garbage can or other fireproof enclosure full of loosely packed combustibles such as news print, magazines, sawdust, dried dung, etc. with a tight lid. 
The idea is to cause the combustibles to ignite consuming the oxygen in the can and then clamping the lid down tight to starve the internal atmosphere for oxygen. The combination of the glaze, smoke, fire and shortage of oxygen (reduction atmosphere) to which the object is exposed contributes to the extraordinary colors typical of Raku. The Raku process is an imprecise one because the object is exposed to many loosely controlled environmental factors during its creation. Since the elements are constantly changing to some extent each object is always one of a kind. Raku actually dates back some 500 years where Raku tea bowls were treasured by Japanese tea masters 
who felt that the Raku bowl enhanced the flavor of the tea.
Although Raku is centuries old, its history in America is little more than 50 years - the Art Institute of Chicago being the first to display Raku in 1941. However, it was not until the l960's and the rising interest in Zen-Buddhism in America that Raku became popular here. The word Raku, when translated, means simply enjoyment, contentment, pleasure and happiness.
Raku today!
In the few decades since Raku began to gain interest in the US, artists have applied the techniques to works of art more than practical items such as tea sets.
Examples of these are: