I like using seasoned cast-iron skillets. Cast iron, before it is seasoned, is a metallic gray color and can rust quickly. Cooking food in an unseasoned cast-iron skillet will likely lead to a massive stuck-on clean-up job. But a glossy black seasoned cast-iron skillet is practically non-stick and resists rusting by comparison. So what’s going on? What is this mysterious “seasoning” thing, anyway, and how do you transform a cast-iron skillet into a seasoned one?

Hey, I know! Let’s look at the internet to find out!

Okay… (Googling madly) Hmmm… heat the skillet, coated with a thin layer of fat or oil, everyone says that. But then the disagreements start. The trick is to use animal fat, that’s how your great-great-Grandma did it. Or wait, not animal fat, but vegetable oil — any vegetable oil. But no, in particular, flaxseed oil, because it polymerizes. Yeah, that’s the ticket, polymerization. And don’t heat it to the smoking point, heat it to 200° F, or maybe 350°, or no, 400–500°.

Okay, got it? Yeah, me neither.

Being a chemist myself, this situation seemed rather unsatisfying. There is no consensus about exactly how to season a cast-iron skillet or what precisely it is that seasoning does. Maybe there are scientific papers out there on the subject but they haven’t appeared in my internet searches yet. So what can my chemistry training bring to this discussion?

First, let’s start with some observations, either from my own experience, or things that everyone seems to agree on. As mentioned above, cast iron starts out gray. Seasoning, which involves heating some oil or fat in the skillet, produces a black, glossy, hard, non-stick surface. This surface can be removed, it is said, by harsh lye soap or cooking acidic foods like tomatoes in the skillet. Leaving water in the seasoned skillet for an extended period leads to rust, which then must be scraped off with sandpaper or salt, and then the skillet must be re-seasoned. Repeated seasoning is reputed to build up a layer of seasoning that works better.

What does all this mean? To a chemist, one key word in the paragraph above is “surface”. Many strange things happen at the surface of a metal, especially when organic compounds are involved. All sorts of chemical reactions, like hydrogenation, de-hydrogenation, carbon-carbon bond splitting, and carbon-carbon bond formation can occur. And yes, polymerization.

Which brings me to the subject of flaxseed oil and polymerization. As much as I would like to congratulate the people who came up with this hypothesis on their impulse to use a scientific explanation to address the issue of seasoning, I would even more strongly like to point out the inconsistencies in their position. First (and don’t take my word for it, look up “seasoning cast iron flaxseed oil” on the internet), the idea is basically that using an unsaturated oil like flaxseed is best to season cast iron because unsaturated oil polymerizes on the surface and that is what makes the surface non-stick.

Unfortunately for this hypothesis it runs into several problems. First, if it were that simple, then using a thick layer of flaxseed oil would be best, because after heating it produces a detectable plastic-like layer of polymerized oil. Which, however, is sticky, soft, and not a suitable cooking surface. Second, practically any fat or oil, saturated or unsaturated, is reputed to result in a good layer of seasoning, provided it is thinly applied, so unsaturated oils producing polymerization may not be the essential thing. Third, seasoning is a black, hard, glossy layer, and polymerized flaxseed oil is none of these.

Which brings us back to the main question about seasoning: What is it? If it’s not a polymer, what could it be?

Well, I’m not 100% certain that polymerization isn’t part of the answer. But the description of a hard, black, glossy layer does suggest something: carbon. More precisely, carbon and iron, or “iron carbide” which isn’t a single compound but rather a solid solution of carbon in iron: an interstitial compound of varying composition. Cementite, epsilon carbide, and Hågg carbide are names of interstitial iron carbides of various Fe/C ratios. Cementite for example, according to our good friend Mr. Wikipedia, is a hard, brittle ceramic material. Ceramics tend to be good non-stick cooking surfaces but usually don’t conduct heat very well. In this case, it would be a thin ceramic layer on top of metal, fine for cooking.

Okay, what does that mean for the process of seasoning? It could mean that when oils or fats are heated on cast iron they decompose to elemental carbon which dissolves at the iron surface to make a thin ceramic non-stick coating of “iron carbide”. This would imply that the composition of the starting oil or fat isn’t important, but a high enough temperature to decompose the oil or fat is crucial. Since the iron might act as a catalyst for the decomposition, that temperature might be below the measured smoking point of the oil or fat; without measurements I have no specific ideas about that. Subsequent seasonings and continued use might allow this carbon layer to penetrate a bit farther into the metal, producing a thicker non-stick surface.

And that’s it. Let me be the first person to emphasize that the foregoing is a hypothesis, a guess that fits the facts as I see them, and could turn out to be wrong in light of more facts or measurements. As such, I encourage anyone to point out any faulty reasoning, or facts that would tend to show where it is incorrect. That’s how science works. Is there anyone with LEED (Low Energy Electron Diffraction) measurements of seasoned cast iron?

Now, people may swear by their particular method of seasoning (whatever it is) and claim that their method gives much better results than other methods. To which I would say, provide evidence. Make objective measurements of some kind, to show that flaxseed oil, or bacon grease, or coconut oil, or doing it during a full moon, or whatever, gives the best results. Because without objective measurements, it’s just a lot of opinion about what’s best, and it’s not science but alchemy. However, objective measurements are difficult to come by in this game. So maybe we’re stuck with alchemy for now.