Cast irons represent the largest tonnage of all cast metals. They can be divided into a couple of distinct categories distinguished by how their high carbon content is contained in the iron matrix. Gray cast iron is chemically balanced such that a large portion of its carbon is distributed throughout the casting as free or graphitic carbon in flake form. . Gray cast iron always presents a gray sooty surface when fractured. Nodular iron is a variation of gray iron that has been specially treated to have the graphite in the form of nodules in the casting. White cast iron has a composition such that after solidification its carbon is present in chemically combined form as iron carbide. White iron presents a white crystalline surface when fractured. Malleable iron is also a variation of white iron which is heat treated to precipitate the carbon from the carbides to chunk graphite form in the casting.