A Seasoned Cast Iron Pan Is Plastic, Chemically Speaking
The black slick on a well-used skillet isn't soaked-in oil. It's a polymer, formed the same way alkyd paint cures.
Strip a black, well-used cast iron skillet down to its surface and you are looking at a thin film of plastic. Specifically, a cross-linked polymer of fatty acids, formed by the same kind of chemistry that hardens an oil painting or a coat of alkyd house paint.
The process is radical polymerization. When you heat oil on bare iron past its smoke point — roughly 400 to 500 degrees Fahrenheit, depending on the fat — the triglycerides break apart. The glycerol burns off as smoke. The fatty acids that remain throw off free radicals, which is what makes them so eager to bond with each other and with the iron underneath. They link into long chains, and chains into a matrix. The result is a tough, hydrophobic film that has more in common with cured varnish than with cooking oil.
This is why polyunsaturated oils season faster than saturated ones. Saturated fats are stable; their carbon chains are full up on hydrogen, with nothing reactive to offer. Unsaturated chains carry double bonds — weak points in the molecule that crack under heat and start the radical cascade. Flaxseed oil is roughly 57% alpha-linolenic acid, which is why it polymerizes into the hardest film and why it is also what painters use as a drying oil. Lodge's factory seasoning is a soybean blend chosen on similar logic.
What you scrub off when you ruin a pan with soap and a steel pad isn't grease. It's the polymer layer, plus the carbon byproducts that turn it black with use. Rebuilding it is a chemistry problem disguised as a cooking one.
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