At What Temperature Does Oil Polymerize?

The internet is full of information on the best way to season cast iron.

The problem is there is no consensus.

Since seasoning is all about the oil bonding to the iron (polymerization), it might be helpful to know a few things such as, “At what temperature does oil polymerize?”

If you have a basic understanding of polymerization, then seasoning your cast iron won’t be quite as confusing.

So, let’s get started.

At What Temperature Does Oil Polymerize?

Cooking oil polymerizes when it hits its smoke point. In other words, all oils are different. For example, the smoke point of grape seed oil is 420°F. When the oil reaches that temperature, the fat molecules oxidize and bond to the iron in a series of chemical reactions, converting them into a layer of polymers. The process is called polymerization. Additionally, any cooking oil will polymerize with heat and enough time.

The Relevance of Smoke Points and Oven Temperatures

TYPE OF OILSmoke PointFlavor
Avacado (Virgin)520°F/271°CNO
Light/Refined Olive465°F/240°CYES
Rice Bran450°F/232°CYES
Soy Bean450°F/232°CYES
Extra-Virgin Olive375°F/190°CNO
Vegetable Shortening360°F/182°CYES

There is a lot of confusion surrounding smoke points and oven temperatures regarding seasoning cast iron.

So much so, it makes your head spin.

Here are three things to keep in mind:

1. The oil hasn’t polymerized if it comes out sticky or tacky no matter what the smoke point of the oil is or at what temperature you seasoned your pan.

2. The polymerized oil will burn off if your heat is too high and/or you leave the pan heating for too long (stovetop or oven).

One morning after putting my pan on the burner at medium-low heat to get it dry, I forgot about it. Needless to say, the seasoning started burning off and flaking. Luckily, I was home and quickly discovered the pan once I recognized the burning smell.

3. It isn’t difficult to season your pan if you know what you’re doing. It’s helpful to have a basic understanding of polymerization and/or follow a trusted source.

Once you find what works, stick with it. But don’t claim it’s the only way. Or even the best way. Because even the experts disagree.

Technically, smoke point is more about carbonization which is what happens after polymerization. Ideally, you want your oil to carbonize, not just polymerize.

Generally, when you talk about seasoning your cast iron, you are talking about both polymerization and carbonization.

Follow a Trusted Source

I follow Lodge. Lodge has been in business since 1896 and has been making quality cast iron for over 100 years.

On their website, they tell you to choose an oil with a higher smoke point and bake it for an hour at 450-500°F.

They also say the best way to season your pan is to cook with it.

I was genuinely confused about how Avacado oil would reach its smoke point, which is 520°F, by following their process.

Nor did I really understand how it worked on the stovetop.

So, I reached out to Lodge, and this is what they said:

Whichever oil you choose, it’s important to make sure you heat up your pan to that oil’s smoke point. When the oil hits the smoke point, a chemical reaction called polymerization occurs, bonding the oil to the pan to create a layer of natural seasoning (from their website). This process is only for during the cooking process. To do the seasoning process in the oven for an hour, don’t go past 500°F. The higher the temperature, and baking for an hour at a time, will burn off the seasoning no matter what oil you use.

So smoke points and oven temperatures do matter, but it’s not an exact science.

Any oil will polymerize with heat. However, how long it takes depends on:

  • What oil you use and what its smoke point is
  • How hot your oil and pan get while cooking or being seasoned in the oven combined with
  • How long the pan is being used to cook or season in the oven

Since Lodge says oven season for an hour at 450°-500°F, that covers pretty much any higher smoke point oil.

RELATED > > > > > Seasoning Your Cast Iron Skillet – New or Old 

Understanding Cooking Oil Polymerization

Fat/OilTotal Fat
Soy Bean14428
Flax seed143110
Grape seed142210
Rice Bran14534

Lodge says you can season cast iron with any oil, but they specifically recommend melted shortening, vegetable, or canola oil. They also sell a canola seasoning spray.

They base their recommendations on availability, affordability, effectiveness, and having a high smoke point.

Lodge also recommends olive, vegetable, sunflower, and grapeseed oil for cooking.

However, they don’t recommend Flaxseed, Light/Refined Olive, or Sesame oil.

The more unsaturated the fats are, the more C-C bonds it has, and the quicker the oil will polymerize.

Therefore, choosing oils that are high in unsaturated fats are the best fats for seasoning cast iron.

What Are Polymers?  

I’m not a chemist so the following is me trying to explain what takes place during polymerization.

When oil bonds to the iron to form a protective layer against rust and to make the pan nonstick, it’s called seasoning.

And also the patina. Seasoning is what you do and the patina is what you get.

Seasoning happens through a chemical reaction called polymerization, in which the oil hardens to form polymers.

And the polymers bond to the iron.

Low temperatures produce sticky polymers, and higher temperatures produce dry polymers.

Once the oil reaches its smoke point, it forms a carbon layer as well. The carbon layer is black and why the hardened oil turns black.

A polymer is simply a bunch of monomers (single molecules) that combine to form a chain. And that chain becomes part of the seasoning when it dries and bonds to the iron.

The Chemistry of Seasoning

Final Thoughts

At what temperature does oil polymerize?

That depends on the oil.

All cooking oils polymerize with heat.

But to get a hard, dry layer of polymerized oil, you need higher heat for most.

You don’t need to understand all the chemistry that goes into polymerization to have a basic understanding.

Thank goodness, because I have spent hours trying to understand the science, and I’m still confused.

However, knowing that oil polymerizes (and carbonizes) once it reaches its smoke point can be helpful.

Still, following a trusted source is a good idea because they break it down and help you know what to do.

How much do you know about polymerization?

Are you one who says, “Just tell me how,” or does understanding the science behind seasoning and polymerization interest you?

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