Cast iron ordinarily contains silicon up to 4% or slightly more, although "silicon pig." the form in which silicon is handled for foundry and similar uses, is made in the blast furnace containing up to 18% of silicon; and ferro-silicons, made by the electric furnace, may contain up to 95% of silicon.

Next to carbon, silicon is most important in determining the suitability of cast iron for foundry use, because its presence up to 2% assists the softness and fluidity of the metal. It softens the metal, making it tough and less brittle, by decreasing the per cent of combined carbon, which acts as a hardening element. Also, in increasing the fluidity of molten cast iron, silicon contributes to the prevention of blow holes in making foundry castings by allowing time for the gases formed or entrained in the molten metal to escape.

Beyond 2%, silicon renders iron weak and hard. The solution of carbon in iron is rendered more difficult by the presence of silicon, as iron dissolves silicon in preference to carbon.