Raku pottery or Raku ware originated in Japan in the 16th century. The master potter, Chõjirõ, after firing, rapidly cooled his pots. Because of his masterful work, Chõjirõ was given a gold seal bearing the mark Raku. The family adopted the name and continued the firing process that bares their name.
Reading about "Raku-ware" in a book by renown English potter, Bernard Leach, an American potter, Paul Soldner started experimenting with the process of removing pots from the kiln red-hot. He found different glazes and chemicals caused different effects when placed in combustible materials. Paul Soldner is attributed the name the "Founder of Western (or American) Raku."
Today, Raku is defined as ceramics removed from a kiln at a high heat and combined with combustibles to create a cause & effect with the glazed ceramic piece.
Or how the heck did he get all those colors on his pots. The process shown below is specifically for copper matt glaze. There are many variations of the Raku process that will yield different effects, colors and textures to different glazes and clays. This is how I do my pots and not a "bible" on how to do all Raku.
The pieces are pre-glazed and fired in an electric kiln to mature the glaze on to the pieces. In the "post-fire reduction chamber" I use wood chips and shredded newspaper as my combustibles.
PLEASE NOTE: If you watch specifically at the 1:20 mark on the video "The Burp" you can see the colors develop on the pieces.
Enjoy! (FYI: the music is Simon & Garfunkel's Anji from their Sound of Silence Album)