Cast iron is an alloy of iron. The term is usually used for grey cast iron. There are two main types of cast iron: grey cast iron and white cast iron. The differences between the two are mainly based on the amount of silicon in the alloy. All cast iron has about 95% iron in it. Grey cast iron has 3% silicon and 2% carbon. White cast iron has less silicon.
The silicon in grey cast iron causes the carbon to change into graphite. This causes it to have a dark grey or almost black color. With less silicon, the carbon in white cast iron changes to cementite or iron carbide (Fe3C). This causes white fractures to form on the surface and give white cast iron the name.
Grey cast iron is very good at conducting heat. Because of this, it is often used to make cookware.