When fat or cooking oil is heated for a long enough time in cast iron, its fatty acids oxidize and reorganize together (or “polymerize”) into a new plastic-like layer of molecules. This layer becomes trapped within the pitted surface of the pan and bonds to the metal itself, creating the slick coating known as seasoning. Repeated exposure to hot oil continues to build on this coating, making it more slippery and durable. That’s why even though most skillets these days come with a factory seasoning, the surface will become even more nonstick with repeated use.
A well-seasoned skillet will have a dark, semiglossy finish and won’t be sticky or greasy to the touch. It won’t have any rust or any dull or dry patches. An easy way to test a skillet’s seasoning is to fry an egg (heat 1 tablespoon vegetable oil in skillet over medium heat for 3 minutes, then add egg). If your pan is well-seasoned, you should not experience any major sticking.
The more unsaturated the oil, the more readily it will oxidize and polymerize. We have found that flaxseed oil, which oxidizes and polymerizes faster than other vegetable oils, forms a particularly durable seasoning. But cheaper oils such as sunflower and soybean are also highly unsaturated and work fine.