Chemistry of Cast Iron Seasoning: A Science-Based How-To
The post after this one on “black rust” describes why you should heat the pan before applying oil for seasoning. This helps the seasoning to adhere and makes the pan pleasantly black.
http://sherylcanter.com/wordpress/2010/02/black-rust-and-cast-iron-seasoning/
In a previous post, I illustrated how I cleaned and reseasoned an antique cast iron popover pan. This was my first attempt, and my seasoning technique was somewhat haphazard because I couldn’t find consistent, science-based advice. I used a combination of organic avocado oil and strained drippings from organic bacon. This worked pretty well on the popover pan, which doesn’t have a polished surface. But the smooth inner surface of a skillet showed an unevenness of color and texture, and the seasoning wasn’t hard enough. It was too easily marred by cooking utensils or scraping against oven racks.
I wanted to understand the chemistry behind seasoning so I’d know how to fix this, but there is nothing that addresses this issue directly. A Web page on cast iron posted by someone similarly obsessed with the science gave me two crucial clues, the phrases “polymerized fat” and “drying oil”. From there I was able to find the relevant scientific literature and put the pieces together.
The pictures below are both of the same antique cast iron skillet. The “before” close-up on the left is from a picture of the skillet in my previous blog post on making German Pancakes. I stripped the pan with oven cleaner and reseasoned it based on my new understanding. The “after” close-up on the right shows the result.
Griswold skillet closeups: old seasoning on left, new seasoning on right
Start With the Right Oil (It’s Not What You Think)
I’ve read dozens of Web pages on how to season cast iron, and there is no consensus in the advice. Some say vegetable oils leave a sticky surface and to only use lard. Some say animal fat gives a surface that is too soft and to only use vegetable oils. Some say corn oil is the only fat to use, or Crisco, or olive oil. Some recommend bacon drippings since lard is no longer readily available. Some say you must use a saturated fat – that is, a fat that is solid at room temperature, whether it’s animal or vegetable (palm oil, coconut oil, Crisco, lard). Some say never use butter. Some say butter is fine. Some swear by Pam (spray-on canola oil with additives). Some say the additives in Pam leave a residue at high temperatures and pure canola oil is best. Some say it doesn’t matter what oil you use.
They are all wrong. It does matter what oil you use, and the oil that gives the best results is not in this list. So what is it? Here are some hints: What oil do artists mix with pigment for a high quality oil paint that dries hard and glassy on the canvas? What oil is commonly used by woodturners to give their sculptures a protective, soft-sheen finish? It’s the same oil. Now what is the food-grade equivalent of this oil?
The oil used by artists and woodturners is linseed oil. The food-grade equivalent is called flaxseed oil. This oil is ideal for seasoning cast iron for the same reason it’s an ideal base for oil paint and wood finishes. It’s a “drying oil”, which means it can transform into a hard, tough film. This doesn’t happen through “drying” in the sense of losing moisture through evaporation. The term is actually a misnomer. The transformation is through a chemical process called “polymerization”.
The seasoning on cast iron is formed by fat polymerization, fat polymerization is maximized with a drying oil, and flaxseed oil is the only drying oil that’s edible. From that I deduced that flaxseed oil would be the ideal oil for seasoning cast iron.
As a reality check of this theory, I googled “season cast iron with flaxseed oil” to see what came up. The very first hit is a page written by a guy who seasons his cast iron cookware with linseed oil from the hardware store because it gives the hardest surface of anything he’s tried. (I’m not sure how safe that is; I don’t recommend it.) Below that were several sites selling traditional cast iron cookware from China, which they advertise as being “preseasoned with high quality flax oil”. I don’t know whether they really use food-grade flaxseed oil (which is expensive) or linseed oil from a hardware store. What’s significant is the claim. Seasoning with high quality flaxseed oil is something to brag about.
With this encouragement, I stripped one of my skillets and reseasoned it with flaxseed oil. As you can see in the picture above, the result was a dramatic improvement. The finish is smooth, hard, and evenly colored.
Seasoning Is Not Cooking: Different Principles Apply
The first time I seasoned a pan I chose avocado oil because it’s monounsaturated and doesn’t easily go rancid. It also has the highest smoke point of any edible oil, 520°F, so I could heat it in a 450°F oven without passing the smoke point. I knew that when cooking, you should never heat an oil past its smoke point because that causes the release of “free radicals”, which are carcinogenic. I was careful not to choose a polyunsaturated oil – and especially not an oil high in omega-3 fatty acids – because these are especially vulnerable to breakdown with heat and the release of free radicals.
Ironically, it’s for exactly these reasons that the best oil for seasoning cast iron is an oil high in omega-3 fatty acids – in particular, alpha-linolenic acid (ALA). Free radicals are actually what enable the polymerization. Drying oils, which produce the hardest polymers, are characterized by high levels of polyunsaturated fatty acids, especially the omega-3 fatty acid ALA.
The lard that was traditionally used for seasoning 100 years ago was much higher in ALA than fat from pigs today, because back then pigs ate their natural diet. Today they are raised on industrial feedlots and forced to eat grain, making their fat low in omega-3s.
Since lard is traditional but no longer readily available, many people substitute bacon drippings, but this is a bad idea. If it’s conventional bacon, you’re baking in carcinogenic nitrates. But even organic bacon is not good for an initial seasoning because it’s filled with salt.
The reason that Pam seems to work well in seasoning is that its main ingredient is canola oil, which is relatively high in ALA (10%), making it a “semi-drying oil”. Flaxseed oil, a drying oil, is 57% ALA. But it’s not a good idea to use a spray oil, no matter what oil it’s made with, because of its additives. You’re doing chemistry here. If you want good results, use pure ingredients.
Fat polymerization can be triggered or accelerated in a variety of ways. As best I can tell from my reading, the cast iron seasoning process is an example of “radical polymerization”. The process is initiated when something causes the release of free radicals in the oil. The free radicals then “crosslink” to form the tough, hard film you see in a well-seasoned pan.
So what is the “something” that initiates the release of free radicals in fat? Iron, for one thing. High heat, light, and oxygen, for some others. To prevent cooking oils from going rancid – i.e., breaking down and releasing free radicals – you need to store them in dark, tightly sealed containers in a cool location. To initiate or accelerate the release of free radicals, put the oil in contact with bare iron and heat it above its smoke point, which will cause even non-drying oils to release free radicals.
I haven’t defined “free radical” or “crosslink” because that gets into details of chemistry that you don’t need to understand to season a cast iron pan. All you need to know is that the molecular structure of the oil changes and becomes something else, something tough and solid. The process is initiated with the release of free radicals, which then become crosslinked, creating a hard surface.
Free radicals are carcinogenic inside your body, and also a cause of aging. So don’t ever heat oil you’re going to eat above its smoke point. If the oil starts to smoke, toss it out and start again. When you’re seasoning a pan, you’re not cooking food. By the time the seasoned pan comes out of the oven, there are no more free radicals.
The Recipe for Perfect Cast Iron Seasoning
The basic idea is this: Smear a food-grade drying oil onto a cast iron pan, and then bake it above the oil’s smoke point. This will initiate the release of free radicals and polymerization. The more drying the oil, the harder the polymer. So start with the right oil.
Go to your local health food store or organic grocery and buy a bottle of flaxseed oil. It’s sold as an omega-3 supplement and it’s in the refrigeration section because it goes rancid so easily. Check the expiration date to make sure it’s not already rancid. Buy an organic flaxseed oil. You don’t want to burn toxic chemicals into your cookware to leach out forever more. It’s a fairly expensive oil. I paid $17 for a 17 ounce bottle of cold-pressed, unrefined, organic flaxseed oil. As it says on the bottle, shake it before you use it.
Strip your pan down to the iron using the techniques I describe in my popover post. Heat the pan in a 200°F oven to be sure it’s bone dry and to open the pores of the iron a little. Then put it on a paper towel, pour a little flaxseed oil on it (don’t forget to shake the bottle), and rub the oil all over the pan with your hands, making sure to get into every nook and cranny. Your hands and the pan will be nice and oily.
Now rub it all off. Yup – all. All. Rub it off with paper towels or a cotton cloth until it looks like there is nothing left on the surface. There actually is oil left on the surface, it’s just very thin. The pan should look dry, not glistening with oil. Put the pan upside down in a cold oven. Most instructions say to put aluminum foil under it to catch any drips, but if your oil coating is as thin as it should be, there won’t be any drips.
Turn the oven to a baking temperature of 500°F (or as high as your oven goes – mine only goes to 450°F) and let the pan preheat with the oven. When it reaches temperature, set the timer for an hour. After an hour, turn off the oven but do not open the oven door. Let it cool off with the pan inside for two hours, at which point it’s cool enough to handle.
The pan will come out of the oven a little darker, but matte in texture – not the semi-gloss you’re aiming for. It needs more coats. In fact, it needs at least six coats. So again rub on the oil, wipe it off, put it in the cold oven, let it preheat, bake for an hour, and let it cool in the oven for two hours. The picture above was taken after six coats of seasoning. At that point it starts to develop a bit of a sheen and the pan is ready for use.
If you try this, you will be tempted to use a thicker coat of oil to speed up the process. Don’t do it. It just gets you an uneven surface – or worse, baked on drips. Been there, done that. You can’t speed up the process. If you try, you’ll mess up the pan and have to start over.
The reason for the very hot oven is to be sure the temperature is above the oil’s smoke point, and to maximally accelerate the release of free radicals. Unrefined flaxseed oil actually has the lowest smoke point of any oil (see this table). But the higher the temperature the more it will smoke, and that’s good for seasoning (though bad for eating – do not let oils smoke during cooking).
I mentioned earlier there’s a myth floating around that vegetable oils leave a sticky residue. If the pan comes out of the oven sticky, the cause is one of three things:
- You put the oil on too thick.
- Your oven temperature was too low.
- Your baking time was too short.
It’s possible to use a suboptimal oil for seasoning, like Crisco or bacon drippings, and still end up with a usable pan. Many (most) people do this. But the seasoning will be relatively soft, not as nonstick, and will tend to wear off. If you want the hardest, slickest seasoning possible, use the right oil: flaxseed oil.
Kyle:
Very interesting post Sheryl!
I have a question though: I was under the impression that once you heat an oil to much past the heat point and it’s done polymerizing it begins to carbonize, loosing the slick surface were after. If that’s the case wouldn’t the flax seed oil have to low of a smoke point, and baking it at 400 degrees bake it past the polymer stage?
January 29, 2010, 6:01 pmSheryl Canter:
Hi Kyle. Temperatures have to be way above 400-500 degrees to burn off polymerized fat. You can observe that just with the spatter mess that builds up in an oven. That’s polymerized fat. Once the chemical conversion to polymer has taken place, it’s not oil anymore and has a different burning point. A self-cleaning oven will burn the seasoning off cast iron (and the polymerized fat in the oven itself), but that’s 900-950 degrees.
January 29, 2010, 6:17 pmJosh:
A more direct measurement of an oil’s ability to polymerize is its iodine value. In a nutshell, this measures how much iodine an oil can absorb, which in turn is an indication of how many bonding sites are available for polymerization.
Now, you’re certainly heading down the right path with linseed oil, with an iodine value of around 185. Fish oils like sardine are up there too. (I’m sure you’ve heard that oily fish like salmon and sardines are rich in omega-3s.) The best oil available for cheap at any grocery store is soybean oil, with an iodine value around 130.
A couple links:
http://www.journeytoforever.org/biodiesel_yield.html#highiodine
http://vegburner.co.uk/oils.htm
January 30, 2010, 9:24 pmSheryl Canter:
Hi Josh. I’d read about iodine values, but it was just one more thing to explain in an article that was already long, so I left it out (though you explained it very succinctly!).
Thanks for the links. I wish canola oil was in the table with the iodine values. I’m pretty sure that’s higher than soy oil. I found a link with another table at one point, but I can’t find it again to save my life. Both canola and soy oils are semi-drying. But yes, soy is cheaper, mostly because of the massive government subsidies for corn and soy (but don’t get me started on that…).
January 30, 2010, 9:44 pmJosh:
Actually, canola is in all those tables – it’s also known under the less marketing-friendly name rapeseed oil.
January 30, 2010, 10:34 pmSheryl Canter:
Ah – there it is. I was skimming for the letter “c” (which I think is for “Canada”, isn’t it?).
Interesting – soy oil has a higher iodine value than canola. I knew sunflower oil was high because I’ve seen it mentioned in research papers on polymerization. But sunflower oil is very high in omega-6s and low in omega-3s, as I recall. So you’re right – omega-3 content is not the most direct measure!
Someone sent me a link yesterday that talked about the importance of carbon being bound up in the polymerized fat for slickness, but I don’t understand why that would make a difference. Polymerized fat is already very hard and slick. Someone just stated it in a forum post without explaining why, so I don’t know if it’s true.
January 30, 2010, 11:07 pmChris:
Sheryl,
February 13, 2010, 1:29 pmThanks for this. I am trying it out right now and I followed all your directions. My first coat came out gray with black spots. It looks dry, almost as if I didn’t season it except there are those spots. I did bake it at 550 for an hour. Also, I used a very thin coat of canola oil to keep it from rusting after I cleaned it initially. My flaxseed oil is filtered, cold-pressed from whole foods. I tried another coat this morning and it is cooling in the oven now. Do I need to start over? Thanks!
Sheryl Canter:
Chris,
It almost sounds like the pan wasn’t completely dry before you oiled it. I always dry the pan for a few minutes in a 200 degree oven before oiling, to make sure it starts out bone dry. Do you think it might have been damp when you put on the oil?
- Sheryl
February 13, 2010, 6:04 pmChris:
Sheryl,
February 14, 2010, 12:06 amActually, yes it may have been damp. I did dry it with a towel, but I didn’t put it in the oven before putting the canola oil on after my initial cleaning. I did put it in the oven at 200 before doing the flaxseed oil regiment.
Sheryl Canter:
That’s definitely the problem, then. Water will prevent the oil from adhering. You also could get rust beneath the seasoning if the pan wasn’t completely dry. That could ruin it, so I’d strip it again, make sure it’s bone dry (at least 10 minutes in a 200 degree oven), and then oil it for seasoning.
Or better yet, heat it stripped and unoiled in a hot (450 degree) oven for an hour to darken the pan with what I believe is “black rust”. I’ve started doing this. Just did it, in fact – the pan is still in the oven. Here’s a link with more info about black rust:
http://sherylcanter.com/wordpress/2010/02/black-rust-and-cast-iron-seasoning/
February 14, 2010, 12:33 amChris:
Thanks, stripped it down using the self-cleaning oven method this time, the lye is just too gross. It worked much better and faster! I just finished wiping all the ash off and put it in vinegar and water then rinsed with soda water and then regular water. Now it is in the oven baking at 450 for an hour. We’ll see if it works!
February 14, 2010, 7:23 pmSheryl Canter:
I don’t have a self-cleaning oven. But even if I did, I’d be a little nervous about using it because the high heat could warp the pan. I use oven cleaner with lye. I hated doing it at first because the lye made me so nervous, but after having done it a bunch of times now, it’s no big deal. It takes just a minute to spray it, I seal it in a plastic bag for a couple days, then rinse it off and give it a quick wash. Usually it doesn’t require much more than that.
I just seasoned a new gem pan yesterday – or rather, new to me. The pan was made by Griswold between 1890-1910, and it’s in great condition. I did the one-hour at 450 thing (no oil), then three coats of flaxseed oil. The only time I put the oil on hot rather than letting the pan cool in the oven first was after the initial no-oil baking. I have an idea this opens the pores of the iron, but I can’t say for sure that’s true. In any case, the pan came out great. Here are pictures, taken right after I finished seasoning it (no additional oil afterwards – this is the natural sheen from the seasoning). Today I made oatmeal scones in the pan and they slid right out.
February 14, 2010, 7:50 pmChris:
nice! I did the bake at 450 for an hour today on a bare pan and it did darken. I wiped some flaxseed oil on it after it cooled enough to handle and the paper towels keep on looking like they have stuff on them. They get grey on them from the inside of the pan and they get brown stuff from the outside. Is this normal?
February 14, 2010, 10:21 pmSheryl Canter:
> I wiped some flaxseed oil on it after it cooled enough to handle and the paper towels keep on looking like they have stuff on them. They get grey on them from the inside of the pan and they get brown stuff from the outside. Is this normal?
Yes, that’s normal for naked iron. What you’re actually seeing is iron, rubbing off onto the paper towel. If it’s slightly brownish, there might be some flash rust mixed in. Once it has a layer of seasoning, this no longer happens.
February 15, 2010, 12:07 amkatie:
just curious . . . can you use lard, if you have it? I rendered some pork fat and was going to use that. It’s what my great Grandma swore by?
February 24, 2010, 11:20 pmSheryl Canter:
Sure, lard is fine. If it’s from naturally fed pigs like your grandmother had access to, even better. Naturally fed pigs have fat that’s higher in omega-3s and produces a harder seasoning.
Really any fat will work fine as long as it doesn’t have a lot of additives (like bacon fat does). Some fats produce a harder seasoning than others, that’s all.
Don’t forget to heat the pan first, as described in the next post. This is important.
http://sherylcanter.com/wordpress/2010/02/black-rust-and-cast-iron-seasoning/
February 25, 2010, 8:34 amJohn:
Great read! I’m having problems finding flaxseed oil. I prefer to buy on the internet because of my rural location. This is an Amazon search and I would like to know if any or all of these are the right thing to buy: http://www.amazon.com/gp/search/ref=sr_nr_n_0?rh=n%3A16310101%2Ck%3Aflaxseed+oil%2Cp_76%3A1249146011%2Cn%3A%2116310211%2Cn%3A123382011&bbn=16310211&keywords=flaxseed+oil&ie=UTF8&qid=1267464416&rnid=16310211
Thanks!
March 1, 2010, 12:32 pmSheryl Canter:
Hi John,
Any food grade flax oil is fine – the one you link to is fine. Just don’t use linseed oil from a hardware store!
- Sheryl
March 2, 2010, 9:28 pmJill:
Hello Sheryl,
I have loved reading your entries about your experiments with cast iron cookware. I have enjoyed them so much that I have bid on a few Griswold items on ebay and hope to have a few pieces soon! I am excited to begin the cleaning/seasoning process.
Once your cast iron cookware has been seasoned, how do you clean it after cooking a meal? I read that a lot of people never use detergent while others say detergent is okay. Do you re-oil after cleaning?
Thanks for all the great information.
March 2, 2010, 9:30 pmSheryl Canter:
Somebody posted some great feedback on this seasoning method in another forum. I’ll quote the message, and also link to it:
http://www.permies.com/bb/index.php?topic=3385.msg28406#msg28406
March 2, 2010, 9:31 pmSheryl Canter:
> Once your cast iron cookware has been seasoned, how do you clean it after cooking a meal? I read that a lot of people never use detergent while others say detergent is okay. Do you re-oil after cleaning?
Hi, Jill. Glad you liked the articles!
I handwash my cast iron cookware with dish soap and a nylon sponge (nothing abrasive). I towel it dry, stick it in the oven, set the temperature to 200 degrees, and leave it in there until it’s bone dry (just a few minutes). Then I take it out and lightly oil it with avocado oil, which is highly monounsaturated and won’t go rancid. Olive oil would work, too.
If you have burned on food that’s stuck really badly, boil water in the pan and it will loosen. I have a cast iron grill that got really badly caked with burned whatever and I got it loose without any scrubbing by boiling washing soda mixed with water. Washing soda is like baking soda, but stronger. It’s next to the clothes soap in the store.
I don’t know where people get the idea that washing with dish soap will remove seasoning. It’s very difficult to remove seasoning – you need oven cleaner with lye. Dish soap doesn’t harm the seasoning at all.
March 2, 2010, 9:46 pmMyra:
Sheryl,
I just stumbled on your site and am so fascinated with this post. I would love to try it. I just bought a Griswold from that looks very clean and smooth, although I see some minor marks (probably from cooking with a fork?). I want the completely smooth surface you have with your pan. Again the skillet is very clean. Do you think I need to strip with oven cleaner before re-seasoning using your method?
Thanks,
March 3, 2010, 10:03 pmMyra
Sheryl Canter:
Hi Myra. Seasoning won’t remove imperfections in the iron itself.
March 8, 2010, 11:37 amanatoliy:
Hi Sheryl,
Glad that I found this post before seasoning my cast iron. I followed your instructions starting from the “black rust cast iron seasoning” and my first coat with unrefined flax oil came out perfect. However after my second coat, there were patches, for lack of a better description, of dark yellow-extreamly sticky-goo. After scratching at it, much of the rim started to peel off.
I had preheated the pan, and it was completely dry before rubbing the oil onto it. I left the pan in the oven at 450 for an hour and let it cool for 2 hrs in the oven as per your instructions. I tried washing off the residue, and noticed that the pan smoked aggressively when placed on a burner to dry. Any ideas on where I went wrong?
March 8, 2010, 7:24 pmReid:
Hi Sheryl,
March 8, 2010, 7:32 pmI don’t mind the possibility of losing a $16 lodge skillet to a self cleaning oven over the mess of soaking with oven cleaner for days. Was wondering if, in your opinion, the water/vinegar and the black rust process would be necessary after the high heat?
Sheryl Canter:
> However after my second coat, there were patches, for lack of a better description, of dark yellow-extreamly sticky-goo. After scratching at it, much of the rim started to peel off.
You put the oil on too thick. That never works – you can’t speed up the process this way. It just forces you to start over. You have to use the thinest coats of oil possible. Wipe it on, then take a dry paper towel and wipe it off until the pan looks dry. There is still oil on it – it’s just a very thin coat. That’s how thin it needs to be for seasoning.
> Was wondering if, in your opinion, the water/vinegar and the black rust process would be necessary after the high heat?
Vinegar is only to remove heavy rust – too much rust to scrub off with steel wool cleaning pads. I don’t know if self-cleaning ovens remove rust. I never tried one. But if it’s a Lodge pan that’s got the original gunk on it, it’s probably not rusty so why would you need vinegar? Vinegar – even diluted 50/50 with water – can damage cast iron. You only use it as a last resort to remove significant rust. No rust, no vinegar! I only had to use the vinegar solution on one pan. The rest had little or no rust.
I recommend always heating the pan first before applying the oil. It helps the seasoning to adhere.
March 8, 2010, 9:12 pmJohn:
Very thorough analysis Sheryl, much appreciated.
I also noticed, like anatoliy mentioned, that my pan begins to smoke when placed on a hot burner. I typically like to get my pan nice and hot before searing a steak, and an not sure if I’m overheating it or if the pan is not meant to be used at high temperatures.
After keeping the pan on high heat, (maybe a 15-20seconds) I noticed that the colour on the bottom and the inside bottom turned a lightish grey colour, almost like the seasoning evaporated in patches.
I would very much appreciate any thoughts.
March 9, 2010, 1:38 pm