Positively Polymerized

Here’s en excellent question from reader Pete:

I like your idea about clafoutis in cast iron, but I get nervous about using my cast iron pan because I don’t know if it’s properly seasoned. Can you tell me a little about seasoning and how to do it right?

Pete, I would love to. Seasoning is the process by which porous die-cast metal pans are transformed into smooth, virtually non-stick cooking implements. In the old days people didn’t think much about seasoning since it just happened with use. Today with the renewed interest in cast iron cooking, home cooks are taking a more clinical attitude toward seasoning. Most cast iron pan manufacturers provide instructions on how to season a new pan on the tag. The procedure calls for a fat of some kind (usually solid) to be applied to the pan in a thin layer, then the pan is baked in a low (300 degree) oven for about an hour. Done correctly, the procedure yields a slick surface which, if the pan isn’t scrubbed out with soap, works very nicely.

But how exactly does seasoning work? It all has to do with the breakdown of fat molecules. Some of you may recall from previous posts on fat that fat molecules are “E”-shaped: three long fatty acid molecules attached to a “backbone” of glycerol. Heat them though, and they begin to break down. The individual fatty acids break loose from the backbone, at which point they’re free to galavant about and bond with whatever type of molecule catches their fancy. If they happen to be near iron, they’ll bond to that, with their polar end down and fatty end up. The end result is that all the tiny pores in the metal get plugged up and the surface becomes permanently slick.

Many cast iron aficionados kick the whole seasoning procedure up a proverbial notch by employing a liquid fat (oil) and high heat. What does this do? Well, as opposed to merely affixing fatty acids to the metal surface, the high-heat method breaks the fatty acid’s hydrocarbon chains down into pieces, pieces which, in the presence of metal and oxygen, rearranging themselves into even longer chains known as polymers. These polymers inter-weave with one another to create an incredibly hard and dense plastic-like film. If you’ve ever spent hours trying scour blackened drips of burnt fat off the exterior of a sauté pan, you’re familiar with the stuff. You find it on the outside of pans and on cooktop surfaces because that’s where the big heat is.

To create a polymer film on the inside of a pan you need oil (because less saturated fats make harder polymers) and an oven temperature of 500 degrees or more. It’s a stinky, smoky process and one that in my opinion isn’t strictly necessary. However if you’re curious you can find some very good instructions here. Does deep frying in a cast iron pan achieve the same result? Actually no. You’d be in imminent danger of a grease fire if you heated a pan full of oil to 500 degrees (all the smoke will be a clue that something is awry). To my mind the best way to season a cast iron pan is to employ the low-heat method, then use it as much as you can. In time you’ll get the shiny, slippery well-seasoned patina you’re after.