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Whenever I season my cast iron pan with canola/rapeseed oil it turns sticky after a few weeks. It also gets sticky on the bottle and glues the cap on sometimes. When I season it with coconut oil, it doesn't get sticky and looks beautiful. Why does rapeseed oil get sticky but not coconut oil?

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It is a chemical quality of the oil called "iodine number". There is nothing you can do about it, it is as inherent in the oil as its smoke point. Oils with a low iodine number create hard polymers, and oils with a high iodine number create soft, sticky polymers.

If you want a hard, nonstick surface on the pan, choose the right oil. Coconut oil, Palm oil or lard give you a good finish, while many polyunsaturated oils give you a soft finish. Linseed oil is the worst of all, it's very gunky.

Note that many sites will suggest seasoning with high iodine oils, especially linseed, because it is easier to get them to polymerise. I personally don't like this recommendation - as you discovered, the resulting polymer is low quality.

  • I often use my cast iron pans for searing and after seeing this, I come to find that avocado oil has one of the highest smoke points. Do you know how it performs for finishing a pan? – JimmyJames Nov 15 at 14:54
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    I don't follow "isn't much reason to look at the smoking point for seasoning fats". I care about the smoke point because I don't like the house filling with smoke (and odor) and having to open the windows in the winter. I also have sensitive smoke detectors that upset my children. – JimmyJames Nov 15 at 16:41
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    @JimmyJames The smoke point might have some effect on choice of cooking oil (although good searing is done at higher temperatures than the smoke point of any cooking fat), but it's irrelevant for the pan finish. It was oil once, it isn't now, and has completely different physical and chemical qualities from oil. – rumtscho Nov 15 at 16:51
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    "Iodine number" is one measure of unsaturation. High iodine number oils are highly unsaturated (usually polyunsaturated), which means they combine with oxygen from the air more easily. Standing at room temp for a long time, this makes them rancid (and sticky). At high temp, they oxidize enough to decompose (smoke), and get sticky. Highly saturated or monounsaturated fats are probably a better bet. – jeffB Nov 15 at 20:34
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    @JimmyJames I think you're missing the point. Rumtscho isn't saying you should use this oil for cooking with, but for seasoning the pan (meaning, creating the nonstick finish on a cast iron pan). It's entirely unrelated to cooking with the oil, and the temperature you cook things with later on isn't the issue here. See lodgemfg.com/discover/cleaning-and-care/cast-iron/… for more details. – Joe M Nov 15 at 20:37
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From what I've read, the answer is that the oil will be sticky when the oil is not cooked sufficiently to finish both polymerizing and the carbon deposition. This is a chemical reaction, and like any endothermic chemical reaction takes some combination of time and heat to occur.

From Science of Cooking:

Note 3: If highly unsaturated oils are used and too low a temperature they will not completely polymerize leaving a sticky layer. The problem with this sticky layer is that it is still prone to further oxidation and can therefore turn rancid. Low temperatures do not completely polymerize and break down oil and will leave a brown, somewhat sticky pan instead of a black, nonstick one. 400-500 degrees F is the effective range for seasoning.

Canola (rapeseed) oil has a smoke point of 400°F or so, while Coconut oil is much lower - 350°F. That smoke point is about where the process should be - it can be above it, but below and you won't get as good of results (and it will be more likely to be sticky). See this explanation of how the process works (same site as above):

The development of a seasoned cast iron pan is actually a two part process: polymerization and carbonization. The first part involves developing a thin layer of polymerized oil on the cast iron.This is done by applying a very thin coat of unsaturated oil (e.g., canola, flaxseed or grapeseed oil) to the cast iron surface and heating it in an oven until it dries. Unsaturated fats work better since they have less hydrogen's and therefore have less non-carbon components. Once the polymerization process is complete the layer of oil cannot be easily removed. To complete the seasoning, which involves laying down of a carbon matrix on to the cast iron surface, heat must be applied slightly above above the smoke point of the oil. If you do not heat above the smoke point only the polymerized oil coat will be present instead of having an added rich black carbon matrix.

You also may have too thick a coat of oil - as more oil will of course take longer to polymerize.

One recommended oil to use is Flaxseed oil, because it has such a low smoke point that it can even polymerize without heat (though you don't get the nice black coating if you don't hit that smoke point).

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