Cast-iron skillets used to be sold as raw, uncoated, unseasoned iron, so the home cook would have to season them entirely from scratch to prevent rusting and sticking. But all that is in the past: New cast-iron skillets are now sold factory preseasoned. This means that the manufacturer sprays on a proprietary food-safe oil and bakes it onto the pan at a very high temperature. However, if the seasoning on a new pan becomes damaged or if you acquire an old-fashioned uncoated pan—inherited from a family member or found at a yard sale—you will still need to know how to season (or reseason) it yourself, in which case you might be wondering: What exactly is seasoning, why do you need it, and how does it work?
In this context, seasoning is what we call a coating of polymerized triglyceride molecules that is many molecules thick. When fat is heated at a certain temperature for a particular length of time, it polymerizes. For cooking oils, polymerization means the linking together of hundreds of molecules through the formation of new chemical bonds between the fatty acids in the oil. This bonding creates a plastic-like layer of large polymers (many hundreds of molecules linked together) that is physically trapped within the pitted surface of a cast-iron pan and partly bonded to the metal itself. The metal atoms catalyze, or speed up, the reaction.
In other words, by applying oil to the surface of your skillet and heating that oil, you can cause the fat molecules in the oil to break down and reorganize into a layer of new molecules that adhere to the pan, creating a fairly durable coating that acts much like an all-natural Teflon.
Heating the pan is crucial to seasoning. The degree of polymerization is directly related to the temperature to which the oil is heated. An oil at its smoke point is rapidly oxidizing, which is great for polymerization. However, heating the pan too hot can actually cause the coating to break down, or depolymerize. A 500-degree oven is the easiest way to bring the pan’s temperature just past the smoke point without allowing it to get too hot, so we use the oven for our recommended seasoning technique. However, the stovetop also works well; simply heat the pan to the oil’s smoke point for a few minutes. Heat helps activate the creation of polymers and encourages the seasoning process, building up the layer of protective molecules that will form a barrier between the reactive iron in the pan and water or food that will cause it to rust. Because the main component of cast iron is iron, which combines more easily with oxygen than other metals do, cast iron that is not seasoned tends to rust quickly and easily when exposed to moisture. In theory this whole process may seem like a lot to worry about, but it’s actually pretty simple in practice—we have straightforward instructions to get started with your skillet. And remember: Every time you cook in your cast-iron pan, exposing it to heat and oil, you’re improving the pan’s seasoning.