Scratchboard is a unique material to work on. A board of masonite is coated with a white kaolin clay, then coated with a thin layer of black ink. Removing the black ink reveals the white underneath. Tools used to remove the ink include art knives, fiberglass brushes, tattoo needles, rotary tool bits, steel wool, and sanding pads. Each tool creates different effects. The lighter the touch, the less ink is removed. Heavier scratching, more layers of scratching, and closer lines all remove more ink and therefore show more white.
The International Society of Scratchboard Artists, or ISSA, is devoted to the promotion of scratchboard as a fine art. Check their website for more about this medium.