In the western style of raku firing, a reduction atmosphere is created by closing the pieces, removed from the kiln, while still glowing hot, in a can with combustible materials.
A reduction atmosphere induces a reaction between oxygen and the clay minerals, which affects the colour. It also affects the metal elements of the glaze. Closing the can reduces the oxygen content after the combustible materials such as sawdust (or paper, leaves) catch fire and forces the reaction to pull oxygen from the glazes.
Glazes gets metallic and iridescent effects just with deprivation of oxygen.
A particular marked craquelé on the glaze can be obtain by sprinkling flour on the pot while still glowing hot.
Pieces with no glaze have nowhere to get the oxygen from, so they take it from clay minerals. This atmosphere will turn clay black, making a matte colour.
The Western Raku cannot be considered a Japanese traditional practice.
Raku ware (楽焼 raku-yaki) is a type of Japanese pottery traditionally used in Japanese tea ceremonies.
The process is the following: the pieces are removed from the kiln while still glowing hot and then left to cool at open air.