Why does using very unsaturated oil for seasoning cast iron make the coating brittle?

Question

It's a point mentioned in this video. I don't quite get it. The more polymerization= stronger coating... till a point? Why does too much polymerization make the actual coating ultimately weaker?

Answer 1

I can indeed confirm that flax oil is a very poor choice for seasoning, and it is indeed due to its being overly reaction-friendly. * Let's look at the seasoning process in depth, first taking the average case

Case A: average fat

When you season a pan with some common cooking fat, it first stays liquid. But at some point, you get enough energy in there for some bonds to open, and the now naked atoms are looking for new partners - potentially atoms from other oil molecules. When they form these bonds, these are the cross-links mentioned in your video. A polymer starts to form.

At first the polymer is sticky - it has a lot of naked atoms grasping around for a bond. This is the "brown sticky" stuff mentioned in the video at 2:35 (it may be present as spots as shown, or as a single tacky light brown polymer sheet, which novices tend to mistake for proper seasoning). But with some time, a lot of these atoms form new, more stable bonds, and you get a strong, well-linked polymer, which is no longer sticky. This is a really good time to stop the process and consider your pan seasoned enough for further use. **

If you continue heating the pan after this point, at some time even the stable polymer bonds fall apart, and the carbon atoms form even stronger bonds with each other. Instead of a seasoning, you now have charcoal on your pan.

Case B: reaction-lazy fat

Different fats (and their polymers) have different properties. If you pick some fat that is not so reaction-friendly (which is correlated with high saturation), the phases look the same as in the diagram of the average case, but are somewhat shifted.

This fat stays liquid for a longer time before it starts polymerizing, and spends less time in the polymer area. When using optimal conditions, it is more difficult to catch it at that perfectly seasoned point, as opposed to underseasoning it (sticky layer) or overseasoning it (burnt, brittle, flaky carbon). Also, something which is not visible in this diagram: it has less tolerance for deviations from the optimal conditions. If your layer is even slightly too thick, the oil will stay liquid forever without ever starting to polymerize, resulting in a very obvious failure. This is why many people prefer to not use it at all for seasoning, opting instead for more forgiving fats. It is also one reason for the "more unsaturated is better" mantra which leads to indiscriminate suggestions of flaxseed oil in many internet sources.

Case C: Flaxseed oil

Flaxseed oil is incredibly reaction friendly, to a point where its results feel qualitatively different from other oils (which is most important in industrial uses outside of food production). When you use it for seasoning, its curve is shifted the opposite way of the reaction lazy oils.

It stays liquid for a much shorter time, and is very happy to build a polymer. It is stickier the whole time than the other cases. And it is so sticky that it doesn't even go through a phase of being a non-sticky polymer. Instead, it burns into charcoal before it loses its stickiness. And this makes it such a poor choice for seasoning.

"Brittle" and "gunky" seasoning

As noted above, if you underseason your pan, it stays tacky. So, it is common wisdom that you just continue seasoning until it stops being tacky. With almost all fats, this leads to a good, strong, C-to-H bonded and crosslinked seasoning.

But if you apply this advice to a layer of flaxseed polymer, it just burns. It reaches the stage of hard, brittle, flaking-off C-to-C bonded charcoal before it loses its stickiness. This is what the video means by it producing a seasoning that is "too brittle". They could have as well said that it produces a seasoning that is "too gunky", if they would have stopped the process before the seasoning burns. With flaxseed, there just isn't a sweet spot to aim for, the zones of the two unwanted properties are not just close, they overlap.

And this is why you don't want to use flaxseed, saturation or not: it is very easy to create a polymer layer with it, but the layer is of no use as seasoning.

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* I will use unscientific descriptors like "reaction-friendly" throughout my post. This is because I am not sure that saturation correlates with seasoning quality all that well; my hands-on experience is that other measurements of general willingness to create bonds, such as the iodine number, may in fact be better predictors. Since I haven't made any systematic tests, or seen any literature that goes into depth, I will try to be agnostic of the actual measurement used, and stop talking about saturation and such.

** nitpick here: most people call this just "seasoned", which misleads novices to think that a pan is either seasoned or not. In fact, a good seasoning is built up with repeated polymerization during pan use; this first step just lays down a much-needed foundation.