Mining was not practiced in Japan until the 7th or 8th centuries. Prior to this, all metals were imported into Japan from China and Korea. Deposits of iron ore were scarce in Japan, so, around the 8th century, iron-making technology developed with the use of ironsand ("satetsu") as the raw ingredient. Because of the loose nature of the sand it was difficult to smelt in a normal bloomery, or to use in a blast furnace to make pig iron, so the Japanese developed an open-topped bloomery called a tatara. The tatara was built with a low, tub-like shape, resembling a horizontal blast-furnace, into which ironsand could be poured and contained, and smelted in stages. Unlike with other methods, the charcoal was piled on top of the sand and smelted from above, keeping it from being blown about by the blasts from the bellows. Instead of brick or stone, the tatara was made of clay so that it could simply be broken apart to extract the metal bloom. This method allowed smelting of much higher volumes of ore than other types of bloomery smelting.