Scratchboard is a very unique medium created on a specially made board coated with a thin layer of fine clay. It's then painted or airbrushed with ink and the picture is made by scratching away the ink with a sharp blade or tool. For the color pieces, several layers of a diluted ink wash are added during the scratching process to build up color and contrast.
See how it's done -
21 hours condensed to 3 minutes: