bizenware is a type of japanese kiln-fired pottery made in okayama prefecture. the pottery is unglazed, and the various effects are the result of entirely natural processes during the firing in the red pine wood kilns. straw placed around the pot burns off into red stripes, ash congeals and leaves brownish markings, or charcoal drops and marks the pots.
bizen pottery site.
e-yakimono bizen site.
these images via japanese pottery.