Iron-carbon alloys containing approximately 2-5% carbon are classed as cast irons. Much of the carbon is present either in its combined form, iron carbide or cementite – white cast iron or white iron -or as free carbon, graphite flakes in a matrix consisting of varying proportions of the eutectoid pearlite or ferrite – grey cast iron or grey iron. When liquid cast iron solidifies, white cast iron will form if the cooling rate is sufficiently rapid, grey cast iron if the cooling rate is slower. The cooling rate is dependent on the presence of quite small quantities of certain impurities in the metal. For instance, silicon promotes the formation of graphite, whereas phosphorus (above 0.1%) promotes the formation of cementite. An intermediate cooling rate can result in mottled iron, which has a white iron matrix with roughly spheroidal patches of grey iron dispersed within it.