Avoiding carbon on cast-iron when cooking hotter than usual

Question

I don't fully understand the chemistry of "seasoning" cast-iron but I have the basic understanding that fats polymerize when they're past their smoke point, that this is the coating, and that various oils and fats polymerize at different temperatures.

One part of the chemistry I do not understand is when things burn into carbon vs. polymerizing and forming a good coating, and this is the nature of this question.

I've had cast-iron pans for about 5 years now and there's a problem I consistently have: If I cook for a long time (say a few weeks) at relatively low temperatures (say, frying eggs, veggies, bacon), then one day I cook something at an extremely high temperature (say, searing a steak), I end up with a lot of carbon deposits when I go to that high temperature, and it noticeably affects the taste of the food. I'm not seeing huge flakes of burnt stuff, I can just taste it on the food, and if I wipe the pan the rag ends up black.

Now, if I cook for a few days at extremely high temperatures eventually this stops happening. If I go back to lower temperatures for a long time, though, it kind of "resets" the pan, so when I switch to a high temperature once again I end up with black powder and an ashy, burnt taste for a while. It's the transition from low to high temperature cooking that does this.

I want to find ways to avoid this.

I'm pretty sure I understand why this happens, and it makes sense: At "normal" cooking temperatures there's a bit of natural selection. The fats that polymerize do, the ones that burn get worn off, the ones that do nothing get washed off when I clean. So one day if I go to a higher temperature, some of those polymers that were happy to be hanging out suddenly find themselves above some critical temperature where they burn off and leave carbon.

I'd like to solve this problem through better care of my pans. My goal is to have a cast iron pan where I can cook at "normal" temperatures to my heart's content, but not have a period of burntness when switching to higher temperatures. Any advice?

For what it's worth, fatty things I often cook with at "normal" temperatures include butter, light olive oil, canola oil, sesame oil, various pork and cheese fats, etc.

---

I do already have some ideas. Here is my normal cleaning routine:

• Cook.
• Scrape off debris with metal spatula, rinse out with soap (usually) and water, scrub only if it's really gooey.
• Put back on high heat, let water dry.
• Spray with canola oil, wipe.
• Let it just start to smoke, heat off, wipe, then put it away.

I do that like clockwork. Every time the wipe is carbon-less, just clean canola oil, so I felt good about it.

Now I just found and read this great article about cast-iron seasoning and flaxseed oil. It seems to me like my use of canola oil might be one of the problems. It also made me question a few things, in particular:

• I use spray canola oil. I think this is a mistake now.
• Canola oil doesn't have nearly as much omega-3 as flaxseed.
• Canola oil smoke point is high, 400F, and so during my cleaning process when I get it to just start to smoke, I kinda doubt its at 400F at that point, based on the behavior of water on the surface (although I've never tested), which says to me that it's not the canola oil smoking, it's either additives (possibly from the spray formula) or something else. I might be wrong on this though.

But I don't know enough to be able to develop a better technique. Is my understanding mostly correct? Is any of this my problem? What can I adjust?

Another idea I thought of is to perhaps just use two cast-iron pans. One for "normal" cooking and one for "high heat" cooking. That seems like a waste, though.

Yet another thought is maybe I should do a full "re-seasoning" in the oven with flaxseed oil every once in a while to burn off some of the lower-temperature stuff, perhaps this happens because I'm letting things build up over time (my pan does not have visible "caked" layers on it, though).

I hope this question is clear, sorry if it was a bit long-winded, I'm sort of researching and typing at the same time.

Answer 1

Interesting question here. I'll preface my answer by saying that I am also not a chemist.

Short Answer: During the post-cooking clean-heat-spray-heat-wipe cycle, you need to get the pan much hotter.

Longer answer: Canola Oil has a smoke point of around 400F. As you mentioned in the question, the oil needs to be just hotter than it's smoke point so it begins to polymerize. When cooking at lower temperatures, the canola oil likely never reaches it's smoke point, and therefore does not polymerize during the cooking process.

When you wash your cast iron after cooking, you are then cleaning most or all of that canola oil off of the pan as it has not been polymerized. When you spray canola on during the cleaning cycle, and don't let it get above 400F, this oil is also not polymerizing and the seasoning on the pan is not being improved. Have you found the oil left on the pan is often somewhat sticky? This is a common sign of the oil not actually polymerizing.

So, you can absolutely use the pan to cook with canola oil at less than 400F and you can still avoid the carbon issue. The solution is just to make sure you let the oil go past its smoke point when you heat the pan after cooking. That way you are essentially "always cooking with high temperatures" and you avoid the transition from cooking at low temperatures to high temperatures, and therefore you will avoid the bits of carbon on your food.