the bottom part of the furnace is called the "hearth". when it has filled with liquid pig iron and slag, the slag is removed. this is called "skimming". slag is lighter than iron and does not mix with iron. it floats on top of the iron. a hole is made in the hearth at the level of the slag with a special drill. the liquid slag moves out through the hole into a container called "slag pot". the iron is then drained from the hearth. this is called "tapping". a hole is made at the bottom and the liquid pig iron comes out. it is either used directly for steelmaking, put into a special railway wagon called "torpedo car" or made into molds. when all the pig iron has been removed, fire-resistant clay is used to close the two holes. the clay becomes solid very quickly because of the high heat.