Make a stoneware piece and fire it in an electric kiln. Apply glaze to the piece leaving an uncovered space of 5 mm at the bottom. You can also leave some surfaces without glaze for decoration.
Fire the piece a second time in a special “raku” gas kiln. When the temperature of the piece reaches 1,000 degrees (after about 45 min.), take it out of the kiln with a pair of long tongs. The thermal shock provokes cracks.
Place the piece in a metal container with a small quantity of combustible materials (leaves, sawdust, paper, etc.). As the clay piece is very hot, it sets the combustible materials on fire. Cover the container with a lid, and wait about 10 minutes. The smoke penetrates the cracks and the non-glazed surfaces. Then remove the piece and dip it in water, which provokes a new thermal shock and a second network of smaller cracks (which will not be blackened by smoke).