Other materials used include stone, often marble or agate, wood (highly absorbent), bamboo, iron, steel, brass, and basalt. Mortar and pestle sets made from the wood of old grape vines have proved reliable for grinding salt and pepper at the dinner table. Uncooked rice is sometimes ground in mortars to clean them. This process must be repeated until the rice comes out completely white. Some stones, such as molcajete, need to be seasoned first before use. Metal mortars are kept lightly oiled.