Seasoning refers to the layer of polymerized fat bonded to the surface of a cast-iron pan. When you heat fat in non-enameled cast iron over high heat, the fat molecules attach to the pan and transform into a hard surface. Starting with a clean cast-iron pan is a must: Any residue left on the surface will fossilize between layers of seasoning, creating an uneven surface and damaging the pan’s nonstick properties.