Demystifying Cast Iron: Heat and the Polymerization of Oil

I have to admit, there are a million articles on the internet about seasoning cast iron, and I really don't feel like going in depth on yet another "how to season a cast iron pan" piece. I'm writing this because there remains a crazy mysticism around cast iron and the word, "seasoning", that I'd love to see fade into distant memory.

To that end, here are a few little notes to contribute to the discourse.

1. It's Plastic

What is the coating on a cast iron pan? In a word, it's plastic—specifically, a natural bio-polymer that's the result of burning oil. This is actually a lot easier to observe on stainless steel cookware. If you've ever burned canola or olive oil while cooking with stainless steel cookware, you'll certainly have seen some brown splotches appear, and you'll also have seen how difficult these splotches are to remove.

That's because you've actually turned your oil into plastic and bonded it to your pan. There's actually a bunch of interesting chemistry that happens when you heat oil (try How to Read a French Fry for a bunch of fun facts), but the important one for this discussion is just that whenever you apply heat to oil, some of it recombines into big polymers, and when you burn oil, a lot of it recombines into big polymers.

2. "Seasoning" Has Nothing To Do With Flavor

While I don't have an official history of the word "seasoning" on hand, I would bet a thousand bucks that it came from the fact that just using the cookware tended to build up this coating, and therefore a good coating was often tied directly to the good, "well-seasoned" food that invariably came out of the pan.

When you consider how difficult it is not to burn your oil in 2019, you can understand that it was basically impossible back at the turn of the 20th century, when skillets and big pots were slung over open fires or crude gas cooktops. Thus, every single meal resulted in another layer of polymer added to the pan—a thick "seasoning" to mark the number of mouths fed (and hours spent bent over a fire).

3. Store-Bought Cast Iron and the Manufacturer's Seasoning

When you buy a skillet from the store, it usually comes pre-seasoned. This is fine, but frustratingly, the manufacturers have stopped doing one of the most important steps before seasoning: sanding. Cast iron comes out of the mold all bumpy and rough. While this isn't the end of the world, it's annoying as hell, and a sanded skillet is well worth the extra effort.

So usually when I buy cast iron, the first thing I do is sand the surface down, which completely removes the manufacturer's coating. I do this using a cheap whet-stone for sharpening knives (though I'll probably just buy a drill attachment the next time I have to do it, since it usually takes like 45 minutes of steady sanding to get it done well).

4. Spot Repairs

Another interesting side effect of understanding the concepts behind seasoning is that you can actually "spot repair" your cookware. I often get rust spots on the bottom of my skillet because I wash it and then set it on the glass stovetop without fully drying it. Every once in a while, before I cook a meal, I simply polish the rust off with a brillo pad, wash the bottom really well, dry it, then spread a bit of oil across the bottom. When I cook with it, the oil does its thing and the bottom of the pan is restored.

Remember: All you're doing is creating a bonded plastic coating, so all you need to do is put oil on it and figure out how to burn it. (That can be somewhat difficult if you've got a weird-shaped cookware like a waffel maker or something....)

Sometimes if I feel the coating in general is getting a little thin, I'll just brush some oil across the pan and throw it on the stovetop on high for a few minutes. Yes, it smokes the room up, but it adds a nice new layer to my cookware, and the pain is temporary.

Finally, The Full Process: Seasoning Your Cookware

So: all you need to do to season a pan is heat oil in it. Whether you heat your oil to 350 degrees or 500 degrees simply affects the degree of polymerization (though it should be noted that too little will not yeild the result you want, and too much will actually burn the polymer and cause it to flake off).

Here's how I do it:

  1. Sand that shit down. Make the surface as smooth as you could possibly want it, since doing it again will be very labor-intensive. Try scraping it with your favorite spatula when you're done to make sure it feels good.
  2. Wash it. Always good to wash it first so there's as little between the bare metal and the polymer coating as possible. This includes polishing away any existing rust with steel wool or a brillo pad.
  3. Dry it. Dry it well, so you can be sure the oil bonds to the surface evenly. (Honestly, you don't even have to be all that careful about this. It'll work anyway.)
  4. Oil it. Cover the whole thing—top, bottom and sides—with a thin layer of oil.
  5. Cook it. I usually do around 400 degrees farenheit for about an hour. As I said above, if you go low, it won't polymerize enough, and if you go high, you could burn the polymer (though in my experience that's rare. As far as I'm concerned, there's not much of a practical upper bound for home ovens).
  6. Rub it. It really helps to add a little more oil while it's cooking and push the existing stuff around. I usually take an old rag, add some fresh veggie oil, then just rub the pan all over a couple times during the cooking process.

When you're done with all that, it should be good to go.

Resources

Here are a few more resources for all this. The first one is particularly good, and also short.

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