The internet is full of cast-iron seasoning advice — most of it is wrong. Here is what actually happens on the metal surface at 450°F, why grape-seed oil beats flax, and how a single seasoning cycle can last decades.
What seasoning actually is
Seasoning is not oil coating the pan. It is polymerized oil — carbon-carbon-bonded molecular chains chemically bonded to the iron surface. Once formed, it is essentially a very thin layer of plastic. That is what makes the pan non-stick.
The polymerization reaction happens above about 375°F. Below that, oil just sits on the pan and eventually goes rancid. Above about 550°F, it burns. The sweet spot for building seasoning is 400–450°F — hot enough to polymerize, cool enough not to burn.
The right oil
Grape-seed oil is what I use. High smoke point (~420°F), high polyunsaturated content (the polymerizing part). Flax oil polymerizes fastest but the result is brittle and flakes off. Vegetable oil, canola, and even lard all work but take more cycles.
What you should NOT use: olive oil (too many free fatty acids that go rancid), butter (dairy solids burn), and any spray oils (they include propellants and lecithin that leave residue).
The method
Start with a clean, dry pan. Warm it on the burner for a minute to open the pores. Add a teaspoon of oil. Wipe it aggressively with a paper towel — as much off as you can get. It should look like there is no oil at all. This is the biggest mistake people make: too much oil.
Place upside down (drip tray under) in a 450°F oven for one hour. Cool in the oven with the door closed. Repeat three times. That is your factory seasoning.
Ongoing care
Every time you cook with fat in the pan, you are adding to the seasoning. A pan gets darker and glassier over years of use. That is the seasoning maturing.
Do not use soap on a seasoned pan. Warm water and a stiff brush are enough. If something is stuck, coarse salt + a paper towel scrub works without stripping the seasoning. Dry immediately on a warm burner. Rub a bead of oil in while still warm.