Raku is a bizarre technique: fire the glazed pot to 1000 deg. C. Immediately remove from kiln and place in a garbage can full of sawdust and newspaper. Close the lid and wait 20 minutes while the sawdust burns, smolders, and smokes a great deal.  Then plunge the pot into cold water. If the pot survives all this, you may have something quite unusual.

The key to raku is the smoldering: the sawdust uses up the oxygen in the can, reducing the oxygen content around the pot (thus referred to as "reduction").  The colorants (metal oxides) in the glaze respond to this reduction by giving up some oxygen and changing their composition. For copper, think of how a penny (unoxidized, or reduced) turns pale green (oxidized) over time—in raku the process is reversed.  As the flames and smoke lick the sides of the pot, a great variation in local oxygen content occurs, resulting in a wide variety of colors from the same glaze.  Copper is the most often used colorant for raku, probably because it can give many colors depending on the level of reduction.  Any unglazed surface will become a velvet black due to the carbon in the flames.  The final plunge in water helps to "freeze" the colors.  The technique is very unpredictable in effect—again, its best to leave your expectations at the door.  Here are some pieces, all made in Ann Arbor, Michigan, at the Potters Guild.

Candlesticks (approx 6" high). JPEG doesn't do a great job with the subtle
variations in color, but such color changes are another reason raku is prized.

Pot (approx. 6" dia.).  The bottom of this pot shows how the sawdust smoldered around it.

Pot (approx 7" dia.). This is a good example of how a single glaze
can change to many colors depending on the local reduction.  In fact,
the glaze is called "Egyptian blue" and does go towards blue at times.

Hex vase (approx. 12" tall). The form took about 6 hours to make

due to the slab construction.  This glaze has a lot of copper, as you might guess.