Now that your skillet is clean, rust-free, and bone-dry, it is time to season the thing. Seasoning must immediately follow stripping. If you were to put your skillet in a cupboard in its present state, overnight you might find a thin layer of rust blooming over the surface—the naked cast iron wants so badly to rust that it will react with even the moisture in the air. Cast iron is highly porous, and it needs oil for protection and performance. The porosity of old cast iron is an impressive thing to contemplate: imagine what your pan might absorb over time, what it will witness. Pick it up—you can easily hold it with one hand—and think about how old it might be. The Griswold factory in Erie, Pennsylvania, shut down in 1957.
What you are doing when you season a pan is covering it with a thin layer of oil, then causing a series of irreversible changes in the molecular structure of that oil. These changes are known as polymerization. Under certain conditions, any oil will polymerize—the molecules will start to break down, free radicals will be released as fumes, and polymer chains will start to form. Some oils polymerize under the right amount of heat, others will do it in sunlight, or even just the oxygen in the air (linseed). If you season correctly, the polymerizing oil bonds with the pores of the iron, and under further exposure to heat, the polymer chains link up to form a durable, hydrophobic, and slick cooking surface.
Everyone agrees that to season cast iron cookware you need oil, heat, and time. But the exact kind of oil, the optimal temperature, and the right length of time are all subject to debate. Your great-grandmother seasoned her cast iron with lard; your grandmother with Crisco; your mother with sunflower oil. Every one of them thought she was right. A highly controversial 2011 Cook’s Illustratedarticle favored, of all things, flaxseed oil from the health-food aisle. In the face of so many conflicting claims and such marginal differences in performance, the best oil is probably the one you have nearby when the pan is stripped and dry and it’s time to begin (canola). We can all agree that using good olive oil to season cookware is just a waste.
By now, the heat from the burner will have opened the metal’s pores, and your skillet should be hot to the touch. Take a paper towel and carefully rub about a half a teaspoon of your oil all over the pan. Bowl and spouts, back and front, handle and bottom. Then take a fresh paper towel and rub as much of that oil off as you can. Buff every side. This might require a third paper towel. You want your skillet to be covered in only the thinnest micron of oil. That’s when you put your skillet into your cool oven—upside down, to prevent pooling—and bake it for 30 minutes at a temperature that’s above the smoke point of whatever oil you used, say 400 or 450 degrees Fahrenheit. Allow it to cool with the door closed. Prepare to spend all day at this.
Remove the pan from the oven. Were you expecting a big reveal, a new skillet all instantly dark and shiny? Sorry. We’re a long way from frying our first pancakes. Seasoning cast iron is like painting your nails or varnishing a table: multiple thin coats are better than a couple thick ones. Before proceeding with the next layer of seasoning, inspect the bowl of your skillet. You may notice a faint spiderweb pattern that is shinier than the rest of the pan’s surface, or even streaks of dull, brownish grease that baked but didn’t polymerize. Those are the tell-tales of an oil coating that was either too thick, unevenly applied, or both. If you don’t remedy them now, the next layer of seasoning won’t adhere properly, and sooner or later it’ll flake off. You’ll need to scrub those oil artifacts away with steel wool and salt, then rinse and dry the pan. Be more careful next time. Expect it to take at least five layers of seasoning to get your pan glossy enough to use.