Raku firing is quite different to a normal glaze firing.  Temperature goes from 0 degrees to around 1000 degrees very quickly, which causes significant thermal shock, so a robust clay body is required.  At close to 1000 degrees, pieces are removed from the kiln and are placed into a metal bin with combustibles such as paper or sawdust.  The combustibles ignite from the red hot pieces, and a lid is put on the container, thus reducing the oxygen in the bin.  The reduction in oxygen causes chemical reactions in the glazes – those with copper content go red, purple, green, blue, gold.  It is a bit unpredictable.  The glazes are formulated to crackle and carbon from the smoke is deposited in the crackle and where the pots are not glazed, carbon also deposits, so these areas are quite black.  Serendipity.