[smuscarella]

Introduction:
Hello Everyone. I'm working to develop an airtight, scientific explanation of what happens during the seasoning process of cast iron cookware and I'm still missing a few key details.
 
Overview:
Seasoning = applying thin layers of fat to cookware and then heating (usually in the oven). The objective, over the course of multiple rounds, is to bake on layers that blacken the pan, protect the iron from rust, and provide a non-stick cooking surface.
 
The lore surrounding seasoning is both extraordinary in volume and diverse in opinion. Everyone and their grandmother (literally) has an opinion of how it should be done. And they all provide their anecdotal evidence --"it works for me!" But "works" is a subjective word and it seems there are techniques that are far better than others.
 
Objective:
To provide a comprehensive set of science-backed theories that act as solid guidelines for the individual pursuing his or her own seasoning methodologies.
 
 
Background Material:
1. There are two methods outlined that yield good results, but understanding why and how they work, is still not 100% clear, mostly because they seem to contradict each other.

2. The Sheryl Canter Method which states that flaxseed oil is THE oil to use because it's a drying oil. It has a low smoke point of 225F, but Sheryl recommends baking at 450F or higher reasoning that an oil needs to heat above its smoke point in order to release the free radicals which lead to polymerization. Sheryl also states that flaxseed oil will produce the hardest polymer. This method takes at least 6 rounds. http://sherylcanter.com/wordpress/2010/01/a-science-based-technique-for-seasoning-cast-iron/

3. The Jeffrey Rogers Method uses Crisco vegetable shortening that has a smoke point of 440F. Jeffrey seasons at 400F for two hours. Through home experiments he found that 500F for an hour blackened the pan but did not leave it shiny. And 400F for one hour led to a gummy residue left on the pan. This method takes 2-3 rounds. Jeffrey highly discourages using flaxseed oil due to issues with flaking.
https://www.facebook.com/theculinaryfanatic/posts/874338525978951:0

4. Additional articles that seem on target, but lacking in scientific citation:
http://www.cookingissues.com/2010/02/16/heavy-metal-the-science-of-cast-iron-cooking/
http://scienceofcooking.com/cast_iron_cooking.htm

Research:
Two reactions (should) occur: polymerization and carbonization.
Polymerization works best (only works?) with an oil high in polyunsaturated fat.
Both reactions require heat.
More heat --> faster reaction.
Too little heat = gummy oil residue result
Too little time = gummy oil residue result
Most home ovens only go to 450-550F
In order to prevent a spotty finish, extremely thin layers of oil are needed so oil doesn't pool.
A cast iron piece placed in the self-clean cycle of the oven (900F) will turn everything on its surface to ash and will leave just bare cast iron.

Questions:
1. What is actually happening when I place my oily cast iron in the oven? If it is just polymerization and carbonization, how do those things work? The oil polymerizes and then if I continue to heat it, it will begin to carbonize? Or do both things happen at the same time? How can my understanding of this process point me in the right direction for choosing an oil, oven temperature, and duration?   
2. How does my choice of oil affect the seasoning process? Polyunsaturated is pretty well covered, but what about smoke point? Other factors? What happens when I use a saturated fat? 
3. What temperatures should I use? Does it depend on the oil? Does it depend on the smoke point? If I heat at 500F or more for too long will I just burn everything off similar to what happens in the self-cleaning cycle?
4. What is the chemical composition of seasoning? Is it the same whether I use crisco, flaxseed, canola, grapeseed, etc...?
5. How do I get the most durable seasoning?
6. Why would the flaxseed oil seasoning flake off? How hot can these oil polymers go before they burn away?

If you've made it this far, thank you for reading! I hope I wasn't too longwinded to get the discussion going.

[Corribus]

We had a brief thread about this a long time ago:

http://www.chemicalforums.com/index.php?topic=72038.msg260615#msg260615

I'm not sure it's necessary to season in an oven. I do my seasoning on a stove, with canola oil, and it works pretty well.

[smuscarella]

Just to clarify: I'm not too worried about seasoning technique. There seems to be an infinite amount of those. I've tried most of them. Some are good, some aren't.

I'm looking for a better understanding of exactly what's going on, chemically. And then, with that understanding is there a way to point to some overarching guidelines about what oil to choose, what oven temperature to use, how long to heat for, and any other factors that come into play.

[Enthalpy]

Flaxseed oil notoriously becomes toxic under many conditions. Is it a good idea to heat it in cookware?

Of course it hardens easily, that's why impressionists used it for painting - but that's too why some got neurological diseases, and they didn't even swallow it. The very same transformations that harden it make it dangerous.

[smuscarella]

The very same transformations that harden it make it dangerous.

Does that apply to just flaxseed oil? Or to any polymer formed from food grade oil?

[Corribus]

Hardening is largely caused by cross-linking, which occurs when oxygen bridges two sites where olefins (double bonds) are near to each other. A lot of unsaturated oils will semi-harden upon prolonged heating in the presence of oxygen.

[smuscarella]

Hardening is largely caused by cross-linking, which occurs when oxygen bridges two sites where olefins (double bonds) are near to each other. A lot of unsaturated oils will semi-harden upon prolonged heating in the presence of oxygen.

Right! So that the polymerization. But it seems like more than that is occuring because the pan also darkens.

My hypothesis is that polymerization and carbonization work together in a lot of ways. An important note is that all my testing has been with fairly raw, silver, vintage cast iron using very thin coatings. Observations thus far:
1. If the oil doesn't polymerize, it will just get burned up leaving the pan the same as it was before you did anything. I had this experience with coconut oil and to a lesser extent, bacon fat.
2. If you heat the pan just below the oil smoke point for long enough (2hrs), you'll polymerize the oil, but get very little carbonization -- a shiny, silver pan.
3. If you heat the pan above the smoke point of the oil, you'll get mostly carbonization -- a matte black pan.
4. If you have a thin polymer (shiny) surface on the pan, but then heat the pan at 500F for a while, the pan won't be shiny when you remove it from the oven. It will be darker.

Can you explain the blackening portion of these observations? What's happening there? How does the carbon bond to the pan? I've read about magnetite but can't really make sense of it.  Does darkening require the polymerization to happen first to form the bond with the iron?

Thanks for the help. The language about polymerization is the exact kind of description I'm looking for.

[Enthalpy]

The very same transformations that harden it make it dangerous.

Does that apply to just flaxseed oil? Or to any polymer formed from food grade oil?

Flaxseed oil is usually not recommended for cooking, and even at room temperature, it gets noxious when aging. That's a big difference with most other oils, but I can't tell whether it's just a matter of transformation temperature or if other transformations occur.