Cast iron cookware provides a cooking surface with great heat retention properties and typically is fairly cheap. And seasoned properly, this surface can be fairly non-stick as well – not as good as Teflon, but with a bit of care, able to release delicate fish filets and eggs. The key phrase here is “seasoned properly.” Seasoning refers to the layer of polymerized fat/oil which develops on the cast iron while cooking – it is this layer which provides the non-stick properties of cast iron. And while pretty much all modern cast iron pans come preseasoned out of the box, the factory seasoning is nothing compared to a fully developed seasoning layer. However, the much of the instruction for maintaining cast iron floating around the Internet derives from, it seems, ancient lore or someone’s grandmother. Some of the advice seems contradictory and in general, doesn’t seem well defined. I’ll write a little of what I understand about the process and what I’ve decided to do as a result of my research. I have never stripped my cast iron down and reseasoned, just built on top of the factory seasoning on my pieces.
On the science side, Sherry Canter has written a pair of great science-backed articles on both black rust and a procedure for seasoning a pan from scratch. Cook’s Illustrated even published the seasoning method, citing Canter, in their January 2011 issue. The gist of it seems that the seasoning layer is composed of a polymer in addition to some magnetite/”black rust”. The polymer is created when a fat or oil is placed on bare iron and heated at a high heat. This causes the oil to undergo a chemical transformation whereby free-radicals are formed and those further cross-link to polymerize. This material is very slick and hard and will not break down unless you heat it well above even the highest cooking temperature. If you do, say, put it in an oven during it’s self cleaning cycle, or over a charcoal fire, where the temps get into the 800-1000 degree Fahrenheit range, for an extended period of time, the polymer will turn to ash. This is typically how you would strip a pan if you wanted to reseason it from scratch. Alternately, you can also use oven cleaner or lye, if you don’t mind harsh chemicals.
So, as I was saying, this is a pretty tough layer. Embedded in much of the advice surrounding cast iron is the dictum, “NEVER USE SOAP ON CAST IRON!” This is bunk. This polymer is similar to the splatter on the walls of your oven and roasting pans – it’s even made by similar processes. If you familiar with this stuff, you’ll know that soap and water doesn’t take this stuff off at all. This advice may have been more applicable when soap was made from lye, which can strip this stuff off, but modern dish soaps are a much milder detergent.
Usage and Maintenance
When using my cast iron, I always start by wiping the pan with oil or a fat (if you’re frying bacon, just grab a slice and wipe). Don’t just pour in oil! You have to wipe with a paper towel or something! Getting some grease in the pores of the pan just helps improve the non-stick properties. This is a good thing to do with other non-stick pans too, regardless of the coating. Heat the pan slowly, at most on medium-high, so that the pan has time to heat thoroughly. Remember, cast-iron is both a relatively poor heat conductor and has an enormous heat capacity! If you heat it too fast, you’ll just have one gigantic hotspot in the middle of the pan. If you want to get the pan really blazing hot, say to sear a steak, preheat the pan in a 400 degree F oven, and then place the pan on the stove on high heat. The bonus here is that once you’ve seared your steak, you can pop it into the oven to finish. Don’t cook acidic foods (tomatoes, lemons, wine) in the pan! This will strip off the seasoning! Boiling significant amounts of water is a bad idea as well, for the same reason.
To clean up, you want to first use hot water and a nylon dish brush
Now this is important – you want to dry the pan as soon as possible. Don’t leave the pan sitting for hours to dry. You want to avoid rust, because if your pan starts to rust, the seasoning layer will come off and you’ll have to strip everything off and start from scratch. Immediately (well, in a reasonable amount of time, an hour, say) wipe it dry with a towel, then put the pan on the stove on moderate heat to dry it out. Once it’s dry, wipe the inside with a thin layer of vegetable or canola oil. I mean thin – you should have pretty much removed all of it. Crank up the heat to medium-high and continue to heat the pan until it starts to smoke. Turn off the stove and let it cool. Wipe out the pan with a dry paper towel. You’re good to go.
A factory seasoning typically is a dark brown color. You’ll note that over time and meals cooked, your pan will turn a deep, dark, black. This is a sign of well developed seasoning layer.