Before you can season the nice, old polished skillet you found, the one that feels like porcelain under your fingers (“Griswold” brand, Goodwill), you must strip the pan’s flaking seasoning. First you’ll need some Easy Off oven cleaner, the kind with lye. The oven cleaner will dissolve all the grease and grime on your cast iron. It will eat right down to the bare metal. You do not want the kind of oven cleaner that says “No fumes”—that kind doesn’t have any lye in it. It’s even better if you can find pure lye, but that’s been hard to come by ever since the supermarkets figured out that lye has a small but crucial role in the manufacture of methamphetamine. Find some rubber gloves. Find an old long-sleeved shirt. Find an open window. Put your skillet inside your plastic bag, pull the gloves over your sleeves, and carefully spray the skillet with the Easy Off, letting the white foam coat it all over. Watch it bubble and rise. Do not get the spray on your skin. Seal the pan in a plastic bag, place the plastic bag in a glass baking dish, and set the dish on the windowsill in the morning sun. The sun is important—the higher the temperature, as we know from high school chemistry, the faster the reaction. After a few hours, re-glove, open the bag, and drag a fingertip through the remains of the oven cleaner, which will now be a brown-black goo. Take the pan over to the sink and gently rinse it under running water. Be careful not to splash; lye is caustic, and it will leave red welts where it spatters. (If skin contact occurs, hold the affected area under cold water for 15 minutes.) Throw away the bag. When all the oven cleaner is rinsed away, remove your gloves and wash the skillet and the glass dish thoroughly in hot water. The bare metal is the color of graphite.
Your skillet will now be cleaner, but the blackest encrustations on the pan, the spots that are basically charcoal, will still be there. That is because lye reacts with oil. It has no impact on the kind of food crud that’s so petrified no oil remains to react with; lye also has no effect on rust. If your skillet is clean but for a few bits of rust or blackened crud, take 000 steel wool and start scrubbing. Don’t forget the bottom and the handle. Anything too stubborn for steel wool can be handled with fine grit wet-dry sandpaper or a steel-bristled brush or, if there are nooks and crannies, like on a waffle iron, bamboo skewers. You could even cut up an old credit card and use it as a scraper.
After scrubbing, rinse the clean skillet in hot water and dry it immediately on the stove over a burner set to Low. Don’t ever put a room-temperature cast iron pan over high heat when it’s empty. It’s a misconception that cast iron is indestructible. Cast iron is very, very hard. But in the way of so many hard things, it is also surprisingly brittle. You should always treat cast iron tenderly, especially the old, polished pans, which are thinner and lighter than modern cast iron. (If you were to put your Griswold No. 8 next to a modern Lodge Logic skillet, you would see the Lodge is nearly twice as thick.) Never drop your cast iron pan, never store it in a place where it might fall, and never—tempting as it is, given its weight—use it as a hammer or a club; the handle could snap right off. Cast iron can handle high temperatures—its hardness means it holds its shape at heat settings that warp aluminum, stainless, or carbon steel—but you have to turn the burner up gradually.
So put the skillet over low heat. Do not let the pan air dry, because bare cast iron is extremely susceptible to rust, and any moisture allowed to dry on the metal will leave a rust spot, which you will have to scrub and rinse. Air-drying risks initiating a rinse-dry-scrub-rinse-dry-scrub infinite loop. When the skillet becomes too hot to touch with your bare hands, turn the burner off and wait.