To ancient Greeks black represented the underworld, separated from the world of the living by the river Acheron, whose water was black. Those who had committed the worst sins were sent to Tartarus, the deepest and darkest level. In the center was the palace of Hades, the king of the underworld, where he was seated upon a black ebony throne. Black was one of the most important colors used by ancient Greek artists. In the 6th century BC, they began making black-figure pottery and later red figure pottery, using a highly original technique. In black-figure pottery, the artist would paint figures with a glossy clay slip on a red clay pot. When the pot was fired, the figures painted with the slip would turn black, against a red background. Later they reversed the process, painting the spaces between the figures with slip. This created magnificent red figures against a glossy black background.