[theboozler]

I've recently obtained some cast iron cookware and have began studying up on how to season them. While I've found tons of valuable information on seasoning them I've found very little information on the chemistry of it all.

From what I gather, the seasoning is simply slathering on a thin coat of oil/grease on the iron and heating it to a point where it fully polymerizes. I've got an elementary understanding of what that means... Bottom line is the fat molecules rearrange to form a thin, hard, elastic film which protects the iron.

What I would like to know is a little more of how and why the process works, with cooking fats in-particular.

At what point do oils begin to polymerize? Before/At/After their smoke points?

At what point does polymerization break down?

I found a discussion about seasoning where a retired chemist slightly touched the subject and now I'm pulling my hair out to get some more details on the polymerization as it pertains to cooking oils. If any one cares here is the discussion: http://ths.gardenweb.com/forums/load/cookware/msg0918171717144.html  The chemist's posts are from Dan (danab_z9_la)

I'm coming to you guys because I can't seem to get a hold of 'ol Dan from that forum... It's a rather old thread.

[JGK]

http://sherylcanter.com/wordpress/2010/01/a-science-based-technique-for-seasoning-cast-iron/

[Kallaste]

Hi guys. I usually don't resurrect old threads, but I believe this one was done an injustice. The original poster asks for help understanding the chemistry of oil polymerization, and he gets pointed to Sheryl Canter without a word? Sheryl Canter is not a chemist. In fact, she is exactly the reason people out there might be searching for such information, as her blog post started a big to-do about pot seasoning and yet is fairly lacking--and even inaccurate--in terms of real science.

I was hoping there might be some here who would care to discuss the actual chemistry behind this phenomenon. I would love to understand the specific qualities of oils that contribute to the effect (such as linoleic vs. linolenic acid, perhaps?), what the ideal temperatures are for polymerization, what different oil characteristics lead to a harder or softer polymer, etc. What role do carbon atoms play? Iodine? Saturation vs. unsaturation? Other factors? And the end polymer matrix is what substance, exactly?

These are just a few examples of the things the above blog post glosses over or leaves out entirely. At one point Ms. Canter even suggests you need temperatures of 900 to 950 deg F to burn off polymerized fats, which is just blatantly false. Therefore, any and all information from knowledgeable parties here would be very much appreciated.

Thanks!

[CAyson]

Alright, I was actually searching for the answer to a lot of these questions too, but I couldn't find a good explanation and ended up attempting to research most of this stuff myself. I'm relying on a few semesters of OChem that I took several years ago, but here's my best understanding:

The basic polymerization reaction involves the cross-linking of carbon-carbon double bonds within fatty acids using oxygen and possibly other things, including iron, as catalysts. http://en.wikipedia.org/wiki/Drying_oil

Like most reactions, heat speeds up this process, but for polyunsaturated fats (containing multiple double bonds) this reaction is actually able to happen at room temperature, albeit at a slow rate. Based on this, it seems unnecessary to reach an oil's smoke point (at which point the oil begins to degrade) and it's possibly optimal to hold the pan a bit below the smoke point for an extended period of time. It's worth noting however, that the thread in the OP's post states that it's necessary to go above the smoke point to get the polymer to bond with the iron, but I wasn't able to find any scientific explanation for this theory.
http://www.firesciencereviews.com/content/1/1/3
http://naldc.nal.usda.gov/naldc/download.xhtml?id=25340&content=PDF

The more unsaturated the fat is, the more C=C bonds it contains, and the quicker the polymerization occurs and theoretically, the stronger the resulting polymer is. Therefore, my guess is that picking oils with the highest percentage of polyunsaturated fats would be the best for seasoning a cast-iron pan. http://en.wikipedia.org/wiki/Cooking_oil

Based on all this theorycrafting, I'd like to try seasoning my cast iron skillet by coating it with safflower oil and holding it at 450 F until a solid film forms.

Edit: found another article that states that although fatty acids can polymerize at room temperature, the degree of cross-linking isn't enough to form significant hardness until 250-300 F. This is above the smoke point of flaxseed/linseed oil which explains why the other thread stated you need to bring the oil above it's smoke point.
http://naldc.nal.usda.gov/naldc/download.xhtml?id=24870&content=PDF

[Enthalpy]

Beware the most unsaturated oils are deleterious precisely because they react with air at room temperature. This is known for linseed oil and contributed to ruin the health of 19th century painters who used it in their colours and varnishes. Is such a coating in cookware any desireable?

[CAyson]

I think you'll have to qualify what you mean by "unsaturated oils are deleterious". In what way do you believe they negatively impact a person's health? Most of the evidence seems to be to the contrary.

[curiouscat]

I think you'll have to qualify what you mean by "unsaturated oils are deleterious". In what way do you believe they negatively impact a person's health? Most of the evidence seems to be to the contrary.

Rancidity?

[Enthalpy]

I think you'll have to qualify what you mean by "unsaturated oils are deleterious". [...]

You didn't cite me fully. I wrote:
"the most unsaturated oils are deleterious"
and gave the well-known example of linseed oil. It's healthy if new and cold, but degrades quickly by contact with air at room temperature or by heating, after what it is deleterious.

Based on that well-known example, I wonder if a pemanent layer of degraded oil is desireable in a pan.

[CAyson]

I don't think that really applies here. The oil is a thin coat that gets chemically changed so that it becomes a polymer coating that's cemented onto the pan. It's not like we're deep frying our food in a vat of rancid oil.

And the reason I asked for qualification is because you made an example of 19th century painters, and I wasn't quite sure what you were trying to imply. Paint fumes? Skin contact? And then what happened to them? If we want to assess the toxicity of a substance we need to understand it's mechanism of action.

But still, if such a conclusion were true, then it would bad for anyone to do any cooking with unsaturated oils, which I doubt is the case.