The other day, one of my cast iron skillets started leaving little black flecks in the food. It got me to chasing some things, and you all get to bear the brunt of my rambling.
Now, there are a couple of things this could be. One is food that didn’t get fully removed in the cleaning, and the other is seasoning that isn’t staying bound to the pan. I’m going to play with seasoning first.
One of the things that is rather peculiar is that “seasoning” is two things. Seasoning is the barrier formed that prevents rust. It is also the very slick surface They’re both the same thing (I think).
I THINK, based on a lot of research, that what’s going on is a low-temperature carbonitriding of the pan, working in conjunction with the catalytic properties Iron has in the hydronegation/hydrogenolysis process of dealing with hydrocarbons. Yeah, that seems quite a mouthful. Let me take it in smaller bites.
Carbonitriding is a surface treatment method where carbon and nitrogen are applied to iron in a heated environment. The carbon and nitrogen interpenetrate the iron matrix, and the result is a slightly hardened surface that has decreased slip. It’s a lower temperature effect than carburization, which incidentally doesn’t do so well at reducing slip.
Now, “proper” carbonitriding requires temperatures hotter than the average oven is going to attain, and requires that the carbon and hydrogen remain in contact for sufficient time that the migration to the matrix can be achieved. It’s done at temperatures between 570 and 700 degrees C. (Above 590 is considered better because the iron turns to martensite when quenched. This improves the hardness and slip.) The problem with all this is three-fold from the normal kitchen cook’s perspective: 1) most home ovens are not reaching any 570C; 2) most home hydrogen/carbon materials (oils) will burn off completely at these temperatures; 3) most people aren’t going to have the proper equipment to safely and effectively quench cast iron. Even so, even at lower temperatures the carbon (and hydrogen) can migrate into the matrix provided they’re free to do so (not bound to tighter bonds).
Now the other thing to keep in mind is the role iron can (and does) play in hydronegation/hydrogenolysis of hydrocarbons. Bluntly, it’s very powerful, and while not the best it’s effectiveness in conjuntion with relative cost makes it very useful. Oh – killing the fancy words, it helps form and break down long-chain hydrocarbons and hydrocarbon polymers. Exactly which it does depends on what specific elements/hydrocarbons are present and the temperature and the time, but still – it’s a multipurpose tool. I’m going to add a critical here – I have not yet found what polymerization happens at “home oven” temperatures in the presence of “food oils” of various types. I do know, however, that canola oil heated above the smoke point polymerizes anyway.
I’m now ready to delve into the realm of speculating about what happens when you “season” the cast iron skillet, though you can probably guess. I think we get two chemical processes operating simultaneously – a low-grade carbonitriding with some of the carbon bonds linking to a polymerized long-chain hydrocarbon.
The higher the temperature at which you season your pan AND the longer you can keep it at temperature while the carbon (and nitrogen) remains in contact, the better the carbonitriding. At the same time, you want your hydrocarbon chains to polymerize, not ignite. If you get your pan too hot, the polymers ignite – and in the process they burn any carbon that has not fully carbonitrided (which at the very-low temperature process we’re using is most of it.) Oh – and I also suspect that if your oil is too thick during the process you get two “bad” problems. First, your carbon layer is too thick. Second, you get a “pool” of polymer. Actually I’m sure of the latter. Returning to the numerous guides you often see discussions of what to do if you get a “sticky pool” in your pan – a low-grade polymer.
Now demonstrating all that properly requires some tools I do not have. Using it as a theory does, however, give me a decent seasoning. I can get an outstanding first season by coating my pan with an oil and cooking it for an hour at a temperature that is consistently above the smoke point but below safely below the flash point of the oil I use. Intuitively, the higher temperature oil I use, the higher temperature the resulting polymer can bear during cooking. I know that isn’t necessarily true but barring experimentation it is my working hypothesis.
My first seasoning of cast iron is meant to emphasize the carbonitriding. So I use an oil or fat with a smoke point of around 350-375 (vegetable shortening at ~370, coconut oil at ~350) in an oven of ~400 to ~425 for about an hour. I end up with a rich black surface that is tightly bonded. It’s also somewhat slick, but not ideal. So I run a second seasoning meant to emphasize the polymerization. This time I either use a hotter oil (like refined canola or refined corn) or a slightly cooler oven. Recallling my working hypothesis I prefer the hotter oil. I thinly coat the pan and cook it for an hour at 10-20 degrees above the smoke point for that oil. (Refined canola is ~400, refined corn is ~450). This surface tends to be “slick” once heated – unless I made it too thick. Then it works more like glue – the joys of polymers.
So now I’ve got all the background, what about the flakes? As I said, there are two possibilities.
Possibility one is that I didn’t clean it well enough and this is left-over food. Now people will tell you to only clean your cast iron with salt and a towel – never soap and water. This is a mixed truth. The problem with water is that it CAN dilute the polymer, and if left long enough can soften the outer layer(s) of the partial carbonitride of the pan – well, the outer carbon-carbon bonds anyway. since it’s not a perfect bond this will expose iron, and iron likes oxygen. Sorry, let me simplify that – if you soak it too long it will RUST. Soap, on the other hand… Two things here.
Part of this is history. Old-fashioned soap – particularly dish soap and clothes soap – tended to be heavy in lye. Lye WILL strip your pan of the carbon layer. (Remember when I said there are a couple of other ways to remove the seasoning?) Now today’s dish soaps are better buffered, and so long as you don’t leave it sitting overnight it’s FINE to wash the pan in soap and water. Except there’s a second issue.
The way soap works is the molecule has two “grabbing” ends. One likes to grab water. The other likes to grab hydrocarbons – oil. The milder the polymerization, the more susceptible it is to soap. I note this because some folk season their pans at a much lower temperature than I recommend. (This emphasizes the polymerization over the carbonitriding. I am… not certain which is “best” as both produce great surfaces on which to cook.) Anyway, what this means is that soap will pull some of the seasoning. The milder the polymer OR the stronger the soap OR the longer the soap is left in contact, the more “seasoning” it will pull off the pan.
All this is why classic wisdom is that cleaning the pan consists of some salt used as a scouring agent in a hot pan which is then swept out. The problem with doing that is that it is quite possible to miss small portions. Another method is to bring some water (no soap) to a boil and then srub with a stiff-bristle brush or a chore-boy (woven steel or copper srub pad). Finally, there are plenty of restaurants which successfully hand-wash their cast iron in soap and water. Each of the latter two are more likely to get all the food particles. I… tend to use the salt method. Since I don’t put food into the cold pan – instead I preheat – I know any bugs still present are going to be dead. Thing is if I am not thorough on the cleaning i get a build-up. And yes, I have seen it. It tends to be on the sides – any residue on the bottom is obvious, with the exception of right where the bottom meets the sides. Guess where many black flakes come from.
Second source – the carbon layer is too thick. This tends to be a problem of the “mild seasoning” folk. Over time they end up with a somewhat thicker layer of polymer. The layer next to the pan tends to gradually become carbon, but it doesn’t do quite as good a job of bonding to the iron matrix. If this happens, instead of a bond you’ve got a series of close layers which can be “popped” loose due to the shrinking and expansion of the metal during heating.
Third source – the seasoning wasn’t done in a timely fashion. Cast iron will start to oxidize – rust – almost instantly. Now rust is a natural protective layer – not a great one, but it does work that way – which isn’t well-bonded to the original iron. But because it’s carrying iron in a matrix, you can get a carbonitriding effect over it. What happens, of course, is that you end up with spots where the layer is bound to itself and to the rust, and only areas around the spot are bound to the iron matrix of the pan. As I noted the rust can separate easily from the matrix, and if the spots are large enough they’ll break the carbonitride layers’s links with adjacent properly bonded.
So given these three, what do you do? The first thing I do – and did – is to thoroughly clean the pan. Fill the pan 2/3 full of water and bring to a boil, use a little dish soap, and scrub thoroughly with a stiff bristle brush. Move it all to the sink and wash the outside while I’m at it. Hand dry and heat on the stovetop to drive away the last of the moisture. Then to be sure, add a VERY thin layer of oil to the interior. ummm…. it is very easy to use too much oil. I use a liquid oil like canola, put a paper towel against the mouth, turn it upside down, hold for one second, turn it right side up, and use that over the whole interior surface of my 12 inch skillet.
That didn’t work. Which meant step two – remove it all, and re-season, because whether it’s a poor bond mechanically or it’s got rust, it needs re-done.
I said lye works – an oven cleaner is an outstanding example. It is not my preference. My preference is extreme heat. Specifically, I put the pan in the self-cleaning oven and run a cycle.
The self-cleaning cycle runs at about ~900F for roughly 3 hours. Now, that temperature is actually really good for carbonitriding – or it would be, if carbon/hydrogen could be kept in contact for most of the time. Unfortunately for the process, that temperature is well above the ignition point of both carbon and the polymers. If I REALLY wanted to “burn in the carbon”, I’d need to do figure out a way to keep a high-carbon and hydrogen fluid (gas or liquid) against the metal for the whole time. It isn’t going to happen in a home oven. Instead the polymer layer and most of the original pseudo-carbonitride layer burn off. Umm, about that second, I mean the carbon layer that hasn’t migrated into the matrix or which is so carbon-heavy that the iron can’t act as a heat-sink safety valve. In addition, this temperature is hot enough to force the iron oxide to separate as well.
In other words, I end up with an unseasoned cast iron pan with functionally no rust. (As an aside, if the pan was heavily rusted and I’m trying to clean it, this works but I may have to brush it after I’m done. The rust will separate but, well, the stuff clings like any fine dust.) I need to rub the pan with oil or shortening immediately whether I’m going to season right now or not to prevent rust. Seasoning should be done within a couple of days.
Now in a perfect world i never need to strip the seasoning and start over. I’ve got one pan for which that’s true. Thing is I USE my pans, but use them for a variety of things that can damage the season. I have a spaghetti sauce, for example, that’s tomato based and spends part of the time on the stove top and part of the time in the oven (under the broiler). The acid picks at the seasoning just a bit. And the sugars from the onions do tend to caramelize and then get very, very hard to remove. Which means some of my pans get re-seasoned every year when i just can’t get the flaking to stop, or when the interior isn’t black and slick.
Two final points. First, an extension for readers who aren’t used to cast iron. Heat the pan, then add the food. If you heat them together the food will stick. If you heat the pan first it forms a non-stick barrier. Now this is also true of most other pans for a variety of reasons, but it’s especially true of cast iron.
Second, a reminder that I am not a chemist or physicist and do not have the tools or skill to verify my hypothesis. I could be wrong. In fact, I think it fair to note that there are some very, very good cooks who use cast iron who would say that the final step for every pan being put away is to put that fine coat of oil on it (see cleaning, above), and therefore the TRUE seasoning is an almost microscopically thin layer of oil that’s “filling the pores” – or maybe bonding to the carbon layer. Regardless, the slick is from oil, not polymers or carbon. They might be right. Since they’re not chemists or physicists either, I can’t say. I’m just basing on the fact that the carbonitriding and polymerization processes exist in production environments. Oh, and on the fact that it works for me.