During the process of smelting, the liquid iron absorbs and combines with a considerable quantity of carbon, sulphur, silicon, phosphorus, and manganese from the ore and coke. Some of the carbon is chemically combined with the iron in the form of iron carbide, while the remainder exists as a form of free carbon called graphite. While the fusibility (ease of melting) of iron depends upon the percentage of carbon that it contains, too high a per cent of carbon weakens the iron. The slower a casting cools, the larger the amount of graphite formed and thus the softer the iron. Pig iron is graded according to the appearance of its fracture.