Sheryl Canter's post on the science of cast-iron pan seasoning is a fascinating and practical tale of flaxseed and kitchen chemistry. It's a long process — you need to season the pan six or so times, each time taking a couple of hours — but the science is sound and the proof is in the hard, nonstick coating your pan will have when you're done.
The basic idea is this: Smear a food-grade drying oil onto a cast iron pan, and then bake it above the oil's smoke point. This will initiate the release of free radicals and polymerization. The more drying the oil, the harder the polymer. So start with the right oil.
Go to your local health food store or organic grocery and buy a bottle of flaxseed oil. It's sold as an omega-3 supplement and it's in the refrigeration section because it goes rancid so easily. Check the expiration date to make sure it's not already rancid. Buy an organic flaxseed oil. You don't want to burn toxic chemicals into your cookware to leach out forever more. It's a fairly expensive oil. I paid $17 for a 17 ounce bottle of cold-pressed, unrefined, organic flaxseed oil. As it says on the bottle, shake it before you use it.
Strip your pan down to the iron using the techniques I describe in my popover post. Heat the pan in a 200°F oven to be sure it's bone dry and to open the pores of the iron a little. Then put it on a paper towel, pour a little flaxseed oil on it (don't forget to shake the bottle), and rub the oil all over the pan with your hands, making sure to get into every nook and cranny. Your hands and the pan will be nice and oily.
Now rub it all off. Yup – all. All. Rub it off with paper towels or a cotton cloth until it looks like there is nothing left on the surface. There actually is oil left on the surface, it's just very thin. The pan should look dry, not glistening with oil. Put the pan upside down in a cold oven. Most instructions say to put aluminum foil under it to catch any drips, but if your oil coating is as thin as it should be, there won't be any drips.
Turn the oven to a baking temperature of 500°F (or as high as your oven goes – mine only goes to 450°F) and let the pan preheat with the oven. When it reaches temperature, set the timer for an hour. After an hour, turn off the oven but do not open the oven door. Let it cool off with the pan inside for two hours, at which point it's cool enough to handle.
Link/ [Sheryl Canter]