Continuous casting is a solidification method used to mass-produce long, semifinished shapes of constant cross section from basic metals. Using this method, over 800 million tons (730 million metric tons) of steel, 20 million tons (18 million metric tons) of aluminum, 1 million tons (900,000 metric tons) of copper, and other metal products (such as nickel) are processed annually worldwide. Cross sections can be rectangular (for later rolling into a plate or sheet), square for long products, circular for wire and seamless pipes, “dog-bone” shapes for I or H beams, thin strips, or rods. Continuous casting is distinguished from other solidification processes by its steady-state appearance. That is, the molten metal freezes against the mold walls and is withdrawn from the bottom of the mold at a rate which keeps the solid/liquid interface at a constant position with time, relative to an outside observer. The process works best when all of its aspects operate in this manner.