Seasoning a cast-iron skillet means heating a very thin layer of oil above its smoke point until it turns into a hard, black, water-resistant layer that helps prevent rust and sticking. The process is done with a refined oil whose smoke point is above 200°C / 390°F, in an oven at 200–230°C / 390–445°F, for 1 hour in each of 3 to 6 rounds. The key to daily care is not avoiding soap; it is never leaving the skillet wet.
The nonstick properties of cast iron come not from a factory coating but from the seasoning layer that builds up on its surface. When oil applied to the skillet is heated above its smoke point, it does not simply burn away; it reacts with oxygen, and its molecules bond together to form a hard film attached to the iron. This process is called polymerization. The result is no longer oil but a thin, water-resistant coating: moisture cannot reach the iron, which helps prevent rust, and the smooth surface helps eggs and fish stick less. Oil that is not heated enough does not polymerize at all.
An oil suitable for seasoning is a refined liquid oil whose smoke point is below the oven temperature; the smoke points of refined sunflower, canola, and grapeseed oils are above 200°C / 390°F. Extra-virgin olive oil is not preferred for seasoning because of its lower smoke point and sediment. The amount of oil, rather than its type, is the common source of mistakes: apply less than one 4 ml measure of oil to the inside, outside, and handle of the skillet, then wipe it as if you were removing all of it. If the skillet looks oily, too much oil remains and can leave sticky spots.
The process works as follows: first wash the skillet with hot water and, if necessary, dish detergent to remove the factory protective oil from a new skillet. After applying oil, set the oven to 200–230°C / 390–445°F, slightly above the oil's smoke point. Turn the skillet upside down and place it on the middle rack, with a tray underneath to catch drips. Leave it in the oven for one hour, then let it cool inside the turned-off oven. Turn on the range hood because the process produces smoke. Repeat this round three to six times; the layer formed in a single round is thin and wears during the first uses.
The rule that soap must never touch cast iron comes from a time when soap really did remove oil: old lye-based soaps saponified and removed free oil. Modern dish detergent is not soap but a synthetic surfactant, and it does not affect the polymerized layer because that layer is no longer oil but a hard coating. A moderate amount of detergent and a soft sponge do not ruin the seasoning. Other things can damage the layer: soaking the skillet in water, putting it in the dishwasher, scrubbing it abrasively with steel wool, or simmering acidic food in it for hours.
If rust appears, the seasoning layer has come off at that spot. Scrub the orange-colored layer with coarse salt and a scouring pad until gray metal is visible. For thick, rough rust, soak the skillet in a one-to-one mixture of vinegar and water for no more than half an hour; if left longer, the acid will also eat away at the iron. Then rinse it, dry it immediately, heat it on the stove to drive off moisture, and season it again from the beginning. Daily care follows the same three steps: wash while warm, dry, heat on the stove, and put away with a thin film of oil. Enamel-coated products should never be seasoned.