Why Cast Iron Pans Need to Be Seasoned

A well-seasoned cast iron pan develops a smooth, dark, naturally non-stick surface, but that surface has nothing to do with any coating applied at the factory -- it's created entirely by the cook, through a process of repeatedly heating a thin layer of oil directly on the metal. Raw cast iron straight out of the mold is porous and rough at a microscopic level, and food sticks easily to all those tiny gaps and ridges. Seasoning fixes this by triggering a chemical change in the oil itself. When a thin layer of oil is heated on the pan's surface to a high enough temperature, the fat molecules undergo a reaction called polymerization, in which individual oil molecules link together into long chains, forming a hard, plastic-like layer that bonds tightly to the metal surface. This polymerized layer fills in the microscopic pores and ridges of the raw iron, creating the smooth surface that food no longer sticks to as easily. A single seasoning session builds only a thin layer, which is why cooks repeat the process, heating and cooling additional thin coats of oil multiple times, since each new layer bonds on top of the last and gradually builds a thicker, more durable, and more effective non-stick surface. This same polymerized layer also protects the iron underneath from direct contact with air and moisture, which is why a well-seasoned pan resists rust far better than an unseasoned one.