Cast iron is naturally non-stick when seasoned properly. New cast iron is anything but non-stick, and it can easily rust. Seasoning — rubbing oil or fat into the cast iron and subsequently heating it — fixes both problems. Unsaturated fats work best (unsaturated means that some of the carbons in the fatty acid chains contain reactive double bonds). Nineteenth century American cooks typically used lard because it was readily available and unsaturated enough to polymerize well, but almost any oil will work. When an unsaturated fat is heated to high temperatures, especially in the presence of a good catalyst like iron, it is broken down and oxidized, after which it polymerizes –joins into larger mega molecules the same way plastics do – and mixes with bits of carbon and other impurities. This tough, impermeable surface adheres to the pores and crevices in the cast iron as it is forming. The surface is non-stick because it is hydrophobic – it hates water. Water soluble proteins make foods stick to their pan; a hydrophobic surface prevents sticking. The bits of carbon in the seasoning may also act as an additional release agent
There is no quick way to fully season a cast iron pan; the surface of cast iron becomes slicker and blacker the more it is used. Though most cast iron today is sold “pre-seasoned,” this cursory seasoning protects against rust, but not against sticking. A good non-stick surface forms over time, with use. The oil polymer on a well-used piece of cast iron is built of many thin layers deposited over time. Thick layers can flake off in large pieces. Thin layers will remain adhered to the pan and will slough off microscopically. A true seasoned surface will only form properly at temperatures well in excess of the 350-375 degree F temperature that some manufacturers recommend for seasoning cast iron. Low temperatures do not completely polymerize and break down oil and will leave a brown, somewhat sticky pan instead of a black, non-stick one. 400-500 degrees F is the effective range for seasoning