Chemistry of Seasoning Cast Iron Cooking
When oils or fats are heated in a pan, multiple degradation reactions occur, including: autoxidation, thermal oxidation, polymerization, cyclization and fission.
The development of a seasoned cast iron pan is actually a two part process: polymerization and carbonization. The first part involves developing a thin layer of polymerized oil on the cast iron.This is done by applying a very thin coat of unsaturated oil (e.g., canola, flaxseed or grapeseed oil) to the cast iron surface and heating it in an oven until it dries. Unsaturated fats work better since they have less hydrogen's and therefore have less non-carbon components. Once the polymerization process is complete the layer of oil cannot be easily removed. To complete the seasoning, which involves laying down of a carbon matrix on to the cast iron surface, heat must be applied slightly above above the smoke point of the oil. If you do not heat above the smoke point only the polymerized oil coat will be present instead of having an added rich black carbon matrix.
How to Season a Cast Iron Pan
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.( Note: Lard from the 19th century was more saturated than today because the feed was more natural). 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 nonstick because it is hydrophobic – it hates water. A well seasoned cast iron pan will have a slick and glassy coating that is best achieved by baking on multiple "very" thin coats of oil.
