My previous post on the chemistry of cast iron seasoning focused on fat polymerization – the transformation of an oil into a hard, slick glaze. After I posted that, someone sent me some links that talked about two other elements in cast iron seasoning: carbon and magnetite.
Carbon is the black stuff that’s left after something is burned. A certain amount of carbon gets bound up in the polymerized fat when food is cooked in the pan. This may darken the pan, but does it make it more nonstick? Some say it does, though I don’t see the mechanism.
More interesting to me is the third element: magnetite.
Black Rust is Protective
Magnetite is an oxidized iron, also called “black rust” or “black iron oxide”. It’s magnetic; lodestones are magnetite. Chemically magnetite is Fe3O4. Red rust (or “hematite”) is Fe2O3.
Well, that may not be what it is chemically. I also read this:
“…it is not Fe3O4, but rather FeO. Fe3O4 is a common term denoting the what you have is not pure “black rust” but rather a combination of Fe2O3 (red rust) and FeO. Fe2O3 + FeO = Fe3O4, technically inaccurate, but not all that important.”
Here’s a good overview of the different types of rust (from the perspective of car bodies): Water + Steel = Rust.
Unlike red rust, black rust is protective and prevents corrosion. Also, things bond better to magnetite than bare iron (for example, polymerized fat). Black rust is not sufficient by itself to protect cast iron from corrosion. It must be insulated from air and water with a layer of oil, and also it’s easily removed. But when black rust is bound up in polymerized fat, the result is probably a better seasoning.
How to Create Black Rust
So how do you get magnetite on your cast iron cookware? Black rust forms on iron that’s under water or otherwise in a low oxygen environment. The type of oxidation you get on iron depends on how much oxygen there is – lots and you get red rust, not too much and you get black rust (magnetite).
There are chemical products you can buy that convert red rust to black rust, but these are generally toxic – not something you want to put on cookware.
It may be possible to convert a thin layer of red rust to black rust by boiling the pan, then drying in the oven and immediately coating in oil (before the black rust turns back into red rust). This is the traditional way of “bluing” a gun. But you have to have just the right amount of red rust to start with, and there are many other factors that are hard to control.
Heating accelerates the creation of magnetite (and many other chemical reactions). A home oven can’t create the ideal temperatures, which would melt the pan, anyway, but heating at even 450°F may encourage some magnetite to form. Many people put bare cast iron in the oven at high temperature for an hour before adding oil for seasoning because it blackens the pan. I thought this was just aesthetic, but now I realize it may create a layer of magnetite.
It also may be that the reason cast iron seasoning darkens with use from brown to black is because repeated heating causes magnetite to form. Many think pans darken over time because carbon from burnt food gets bound up in the polymerized fat, but I think it’s more likely magnetite that gets bound up. Or maybe it’s both.
So to my previous advice on how to season cast iron, I’d add the advice to heat the pan first, before smearing oil, at 450°F for an hour. This will bind magnetite into the polymerized fat of your seasoning. The polymerized fat will bind better to the pan, and your pan will be better protected from corrosion.
I can’t think why magnetite or carbon bound up in the polymerized fat would make the seasoning more nonstick, as some say it does. But maybe I’m missing something. If there’s a reason I’m not seeing, I’d be interested to hear it!
I just did a small test on an old skillet. Here’s what I did:
1. Stripped the cooking surface to bare iron
2. Let the naked surface rust over for a day
3. Boiled a few cups of water in the pan itself for about 1/2 hour
4. Repeated step 3
5. Applied a thin coat of oil
The result was a nice coat of black rust up to the water line, about half way up the pan.
It seems to me that “bluing” cast iron this way may be easier than you think!
Another interesting observation was that as I was wiping the oil on the pan the pan seemed to grab the towel, it wasn’t nearly as slick as usual. I suspect that magnetite may make for a great surface to adhere a seasoning to.
Very interesting! Thanks so much for doing the experiment and posting your results!!
Now I’d like to try it but I don’t have any unseasoned skillets. I seasoned a gem pan recently and tried heating it unoiled in a hot oven for a hour first. That did darken it a little, but didn’t give it a layer of black rust like you describe. I just bought a second gem pan – maybe I’ll try it on that. It’s scary, though, to let it rust then boil it!
Let us know how the seasoning goes post-bluing.
If I were to do it on a pan I planned on really using I would definitely use a more refined procedure. I might try to better control the “flash rust” layer and completely submerge the entire pan in boiling water. I would then brush off all the black film I could and repeat the rust/boil procedure several times. Basically just follow a gun rust bluing procedure minus the rust inducing chemicals.
I would do MUCH more experimenting with some junk iron before trying this with a valuable piece.
To tell you the truth it seems like more work than it’s worth, it makes for an interesting experiment though!
I don’t mind taking time in the initial preparation of a high quality pan that I plan to use for a long time. You only strip and season once. How well you do this determines how well the iron is protected, and how good the nonstick surface is. To me, it’s worth some time and effort.
Right now I don’t have any cast iron cookware that I wouldn’t mind ruining. Maybe I can find something cheap at Goodwill for experimenting. I’m very curious to see the black rust effect you describe.
There are new and improved methods for doing pretty much everything.
Clean a cast iron pan…throw it in the fire..(Paula Dean Quote). Well we should all know that will do nothing more then ruin a good maybe priceless skillet.I will use your blog to say that Paula Dean is such a fake….tho a rich fake and her cookwear is “CRAP”..made in China by who knows what material. There exploding in peoples ovens as we speak.Theres many ways to clean cast iron cookwear and many seasoning methods. Improvement is always posable.
I have tried many ways as a collector and user of cast iron and my preferance has always came back to Criso.
All this scientific stuff means nothing to the average person. Smoke points Poly bla bla bla.
If you use Crisco to season with thats all you will ever need. I have an old skillet that was lard seasoned years ago and a egg will slip right out of it.
I know a man who does Dutch Over cooking and his preferance is Crisco seasoning . Multible times
Hi Harold. This post was for people like me who are interested in science. If you’re not interested in science and like using Crisco, carry on!
You mentioned being a “collector” of cast iron. I’m not really a collector. I’m a cook who likes to use quality cast iron, and that generally means old Griswold because it’s the best. So I’m an accidental collector.
I don’t buy cast iron with the intention of reselling, as many collectors do. I buy pieces to use, and I use them heavily. So I don’t care how long it takes me to season a piece or how much the oil costs, I just want the seasoning to be the hardest and slickest I can make it. Science helps me to do that.
I have no idea who Paula Dean is. I do know that heating cast iron cookware at temperatures high enough to clean it risks warping or otherwise damaging it. So I agree with you there.
I stumbled across your science of seasoning post after several hours of reading a huge range of seasoning techniques. Like you I stumbled across the word polymerization, which was what brought me here. Thank you! I’d been wishing that there was a site that explained the science for me. It explains why what I did in my first round of seasoning worked to create an amber-colored, plastic-like coating that is far more non-stick than any seasoned cast iron I’ve ever tried, and why my second round of seasoning formed blotches. Now to get some flax oil…
Hi Mark,
Having spent an absurd amount of time researching this stuff, I am delighted to read your comment and know that my efforts were helpful to someone else. Thanks for your note.
– Sheryl
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
Hi Sheryl – thanks for the research. A couple of searches make it easy to see why it took an absurd amount of time. I wanted to follow up with my experience using flax seed oil (unrefined). First, I didn’t re-read the chart before starting, and so mis-remembered the smoke point at around 400 F rather than the actual 225 F. After many coats and hours at 500 F, the glossy black finish came out of the oven flaking off. The outside of the pans, which I’d only oiled the first couple of coats, had bare iron areas on them. My theory is that too far above the smoke point results in damaging the polymerized oil, rather than hardening it. I’m experimenting; here’s what I’ve learned so far: 300 F for an hour, then a 2 hour cool down, was not quite enough to create a hard surface. I’m in the process of trying 350, and will let you know.
Mark,
I’d love to know more about your experiments with excess heat over the smoke point. Less is more, I guess, and it’d be interesting, and economical, to heat the oven to the *right* temperature.
Sheryl – your posts are inspirational to me. I have two seasoned pans, and I used lard to great effect. Another subject worthy of discussion (to me at least), is the composition of the pan itself. How are good ones made? What of? What’s so great about Griswold?
S.
The pans I’m talking about seasoning are made of cast iron – pure and simple. No other material involved. Griswold casting techniques were much higher quality than what companies use today. The inside surface of a Griswold skillet is glassy smooth. New cast iron skillets have a rough surface, and are coated with waxy chemicals.
I don’t think your seasoning flaked off because 500 F is too hot. That’s not too hot. I suspect you put the oil on too thick. I saw flaking early on in my experiments when I was putting the oil on too thick. You have to wipe it off so it looks dry.
I have a cast iron barbeque grill that I left in the barbeque during the winter. Some parts of it are badly rusted in the middle but the edges have the old coating from years of use and seasoning and they look good.
But I decided to start from scratch to have one smooth even surface so I put it in the oven (dry, straight out of the bar-b) on the self clean cycle which I believe is 500 F.
I can attest to the fact that at this temp all coatings will turn to ash and/or flake off. The grill is down to its original bare metal with the section that was rusted still rusted and the section that was coated now down to just the black factory coating that it came with.
I think Mark is correct in that 500 is too hot and damages the coating which is why I have rust in the middle of the grill and not the edges because the flames from the bar-b always come up from the center and burn off the coating and I can never adequately maintain the surface there but the sides always get just the right temp to make a nicely seasoned and hard coating.
During the self-clean cycle, the oven temperature is 900-950 degrees F, way higher than 500. Temperatures that high can actually warp the iron – soften it – so many people advise against doing this. That said, I’ve heard more reports of success with self-clean cycles than failure. My own oven doesn’t have a self-clean cycle, so I haven’t tried it.
I don’t have time to keep correcting errors so I’m thinking of closing comments on this topic. I’m busy with other things. I’ll leave comments open for a while longer but if people keep posting misinformation I’ll close it.
I’m using electrolysis to clean old, crusty pans and I believe the process creates black rust. The pans usually come out with a coating of a soft black film. I have been washing that off, but perhaps I should try doing only a cursory rinse to clean off the electrolyte water and then season right on that coating. Thoughts?
Paul
Paul, I think you should wash that off. You sure don’t want to eat it, and it could prevent the seasoning from adhering.
hi,
something you may want to consider about this is something I picked up from a website about iron Japanese tea pots. I’ll leave the science to you, but apparently the tannins in tea have something of the effect that you are looking for.
Maybe it would work well for one of the modern pebbly cast iron pans where it takes a heck of a lot of effort to remove all of the rust in the pits once it forms. I know that’s what I am testing it on.
BTW, I have use your flax seed oil method on an old pan from my grand mother, and it works great! People that balk at “linseed oil” could consider walnut oil which is *slightly* cheaper, and second to flax seed in all of the chemical properties that are desirable as far as drying, iodine level, etc.
I am thrilled to find your site and finally find someone that knows what they are talking about. I just purchased an old “Erie” pre-griswald skillet and need to refinish. One question, after you do the “black rust” oven bake do you allow the pan to cool before you oil and put it in the oven? Thanks for all your work.
>…after you do the “black rust” oven bake do you allow the pan to cool before you oil and put it in the oven?
Sometimes I do, sometimes I don’t. I don’t think it matters. If you don’t wait for it to cool, be VERY careful handling it. It’s HOT.
Hi Sheryl,
I went down the same path as you as far as looking for scientific fact behind all of these sometimes contradictory seasoning techniques and came to a lot of the same conclusions. I found your blog by searching for the results of PAM as a seasoning agent. I would never use PAM intentionally, but I forgot it was in a nonstick pan over 10 years ago at a friend’s house and no chemicals or physical effort could remove it. It was evilly strong and stuck to the nonstick PTFE coating like I wish seasoning would stick to my cast iron. Your discoveries are interesting and I’m planning on trying flax oil. I wanted to second what scooter said about using tea to remove rust, but further that knowledge a little. My first brush with using tea on rust was in a search for a chemical that would convert rust directly to magnetite, so that I could arrest rust on my car with something homemade. For that purpose, phosphoric acid and tannic acid were used in professional rust converter formulas, and tea contained “tannins” so I chased that lead and found that some old Chinese cooks start the seasoning of a new cast iron wok by boiling a pot of tea in it, all the way to the brim…. so I used my large stainless steel dutch oven as a giant tea pot for soaking my tools and car parts in tea. The hotter the tea, the faster the conversion to magnetite. I’ve been trying the same on my cast iron, and the way you know it’s working is that the tea gets dark, until its tannins are all bound with iron from the surface of the pot (tea is acidic and eats at the iron) and then you have a black pot filled with weak black ink. A concentrate of this ink was actually a popular ink at the time of the revolutionary war. For smaller cast iron items like mt waffle iron I boil them in a larger pot. One note of caution is that if you boil cast iron in a stainless steel pot you get some nice electron action that speeds along the rusting of your iron, but the point where the iron touches the stainless steel needs to be underwater or you get some SERIOUS rust. The handle of my waffle iron was sticking out of the water and touching the rim of the stainless pot, which almost lead to a rust pit. A last word of caution is that your nice acid bath will keep eating the cast iron beyond the point when it’s all magnetite coated, which is why you shouldn’t leave it in there much longer than it takes for the water to turn black or the item to turn black. I usually boil things for about 10-15 minutes the most, if the rust is superficial. If you want a thin coating of red rust before you tea bath things (as I wanted on my tools) I let them soak in H2O3 (peroxide), but be careful how far you let your item rust.
Please forgive the run-on sentences and lack of line-breaks, but I’m posting in a rush and really wanted to share everything I’ve been able to piece together that I haven’t read before in a “cast iron cookware” context”. Thanks for the scientific focus on such an unfortunately anecdotal topic.
Good Morning Sharon
I have read your Blog and it sounds great!
I just purchased a Weber Genesis Grill with Cast Iron Grates with NO INSTRUCTIONS on how to season the grates. So I’m planning to try your Seasoning process with the Flaxseed Oil… Since I didn’t see comments in your blog about Grill Grates I just thought I would ask if is OK to use the same flaxseed oil process as Cast Iron Pans???
Thanks
Karen,
New cast iron doesn’t generally need to be seasoned. It is “pre-seasoned” with a waxy coating. You wouldn’t want to season on top of that – it’s pointless. Some people strip new cast iron, but you don’t need to do that. Only antique cast iron – which is just iron with nothing on it – needs seasoning.
– Sheryl
hi Joe,
I would not use the tea method on any nice smooth piece of iron cookware. I would rather get all the rust out and let the black build up over time.
It did work great on some modern Lodge pieces that would have been a pain to scour out.
Your comment on phosphoric acid reminds me that some folks recommend Coca-Cola for treating rust. I didn’t know that phosphoric acid converted the rust. If folks don’t like the idea of Coke, you can find phosphoric acid in the nausea section at your drug store.
Hi,
I bought a cast-iron skillet at a garage sale that is very old. It is not rusty but when I used it to saute some onions, there was this black film on the onions. I stopped & cleaned the pan but when I wiped it out with a paper towel, the paper towel is black where I wiped it. I need to know what to do so I am able to use the skillet. Can you help?
Thanks!
When cast iron has no seasoning on it at all, some black will wipe off on paper towels. That’s actually iron. You need to season it before you use it. I think you should strip it first, just to be sure it’s clean. You can use oven cleaner, as I describe in my post.
Hey Sheryl (& all the other folks),
Thanks for the great advice. I’ve been following this thread since the previous post on the chemistry of seasoning cast iron. Being a student of science myself, I found it fascinating and quite useful since I recently purchased a Lodge double dutch oven (preseasoned, ::shudders::). Strange thing was, while the dutch oven did come seasoned, the lid (which doubles as the skillet) didn’t have any seasoning at all! Considering I was planning to use the lid, I had to season anyway, so I figured why not go ahead and do the whole thing.
Before seasoning, I baked at 500 degrees for an hour. Had there been any factory wax on the thing, I’m confident it’s been melted off. Turning off the oven, I allowed it to cool off while I ran some errands. What errands you may ask? Flaxseed oil! I ran down to the local VitaminShoppe and upon asking where said oil was, I was lead to the a fridge (good sign) with all kinds of oils. The smallest bottle, which happened also to be one made of glass sufficed. Cold-pressed organic.
When I got home, the oven was cooled off but the cast iron was still a tad too hot to handle barehanded. I busted open the flaxseed and began applying with a small paper towel. It left little towel bits everywhere unfortunately (not a machined surface) but it didn’t matter because I was wiping it “dry” with an old cotton shirt anyway. The oil went on the preseasoned dutch oven easily but the lid was thirsty and sopped up a lot of the oil (yay!)
It’s now in the oven for its first seasoning (450 for 1.5 hours, 1.5 rest, repeat). I’m probably going to get 2 more in tonight and 3 more tomorrow and another 3 for good luck.
As for your frustrations about “misinformation”. You’re the one in charge here. Everybody who’s commenting otherwise probably just isn’t as well-versed as you are. Closing the comment section is no way to deal with it. In fact, you don’t have to deal with it at all. Either ignore it or correct them. I don’t think anybody else coming to your blog is going to think “Oh well, based on other comments, sheryl must be wrong, I’m just gonna go ahead and listen to what xxxxx said instead.” 1 – you’re probably right and they’re probably wrong, so don’t waste your energy or emotions. 2 – if somebody’s going to take the advice based on one comment of this entire thread, that person doesn’t deserve properly seasoned castiron. That being said, I think you should keep the comments open.
Thanks again, Sheryl, for all the hard work (the science that is). I’ll update as soon as I get all 9 (or 11, haven’t decided yet) coats on.
Well, I decided 9 was enough. It was absolutely enough for the preseasoned dutch oven but the skillet lid (that originally came unseasoned) could maybe use one or two more. But it’s been enough heat and smoke for me so it’ll just have to season itself as I cook with it.
Here are my observations. The flax oil became easier to apply over the course of the seasonings. This made sense to me since the surface was getting slicker with each layer. It was also easier to apply onto a warm surface than a cold surface. I noticed the surface of the cast iron went from a dark charcoal grey (as on the unseasoned lid) to a dark hazy brown/black/purple mottled color. It isn’t uniformly black everywhere. At certain angles it’s more noticeable than others. It feels much, much slicker than when I got it originally. Awesome.
As others have noted, your house (or my studio) starts to smell like a deep-fryer, in a bad way. Although I didn’t see any smoke, I had to disable the smoke alarms because they did go off. We had the hood fan going, a window fan sucking air out, and a fan blowing towards the kitchen. Despite all that, we still smelled oil from the adjacent room.
Tomorrow morning we make our first meal with the double dutch oven. Fried eggs (appropriate, as it is on the Lodge logo) and home fries.
Hopefully this helps some people out. And another hand for Sheryl, the sultan of seasoning.
Hi Sheryl, and thatnks for not disabling comments yet. I know how my father always seasoned his cast iron, and how my grandmother and great-grandmother did theirs, and with my great-grandmother, she learned her technique in the late 1800s. But there’s so much disagreement on how to treat cast iron, even amongst my relatives, that I wanted to see if there was any science that’s been done. Remarkably, not so much. I am thrilled to have found your site.
I have one old skillet from my husband’s grandmother that has kind of an interesting surface. It’s smooth as glass on the inside. On the outside, it’s rough, almost scaly. I thought that was just how the skillet was, but a couple of big ole flakes came off the bottom yesterday to reveal smooth metal underneath. I slapped a quick coat of oil on the spot to arrest the oxidation, but I was wondering what the scaly stuff might be, and whether I should remove it.
A side note, sort of – I play brass instruments. There is much discussion among musicians about the quality of metal with various makers of instruments, and different years of manufacture within the same make. The consensus is that instruments of recent manufacture from overseas – chiefly Asia, but with globalization it shows up everywhere – are of poorer sound quality, owing to inferior metals. I am not at all surprised to find this discussion happening with cast iron.
Also, I had no idea my favorite cast iron skillet – a Griswold – had any particular appeal to collectors. Like you, I have my cast iron to cook in it, not to show it off, and I hope it lasts many lifetimes over.
>On the outside, it’s rough, almost scaly. I thought that was just how the skillet was, but a couple of big ole flakes came off the bottom yesterday to reveal smooth metal underneath. I slapped a quick coat of oil on the spot to arrest the oxidation, but I was wondering what the scaly stuff might be, and whether I should remove it.
That’s just crud – baked on carbon. If it’s beautifully seasoned on the inside, you probably don’t want to strip it. As long as the crud is on the outside and not getting in your food, you’re okay.
Hmmmmmm interesting site.. interesting theories.
Basically all the “seasoning” of the pan is doing, is turning the oils into a coat of varnish.
It’s a form of speed oxidising beyond that of air drying over time.
And varnish is a type of “plastic”.
The worlds first “teflon” style coating in a cooking vessel.
That is about all there is too it.
Re: Oven Cleaner to Strip Cast Iron Cookware
After reading your very informative blog, I am going to strip my Wagner cast iron skillets. I have a self cleaning oven, but I’m reluctant to use the self-cleaner; so I am going to use oven cleaner. What brand do you recommend? And do I follow the manufacturers instructions?
Thanks
I have been enjoying the seeking of scientific knowledge on seasoning, as well as the general overview of information of cast iron.
I have an old Griswold No.6 and recently bought new Lodge 12″,10.25″, and a few other of the smaller pans, a 5qt. dutch oven, and Pro Giddle.
I just found this blog (glad I did) but seasoned on top of the Lodge coating with a few vegetable oils coatings at 500 degrees. Second coatings, I went with the Crisco on top and these pans are all as nonstick as can be for the “rough surface”. While one day I hope to have all the beautiful old Griswold stuff, the 200 dollars I bought in Lodge cookware is very worth it and it frys/cooks just as nice and nonstick as the Griswold.
For fun I am going to smooth down my Lodge stuff before time to season new (with the flaxseed oil), which is going to be in a long while.
Do not discount the performance of the rougher surface Lodge stuff. It takes a seasonong very well and eggs slide off like oiled glass.
One question. After you heat the pan at 450 for one hour, and then put the oil on, do you put it right back in the oven for the time needed to season it?
Sheryl, thanks for all the information! I’ve been cooking with cast iron for the last 8 or 10 years, and my seasoning has been pretty random… hopefully having some actual information on how these things work will make future attempts a little less trial and error!
Hi Sheryl, would like to echo everyone else and thank you for sharing what you’ve learned. I’ve spent so much time and money following different bits of advice I gathered from around the web it was such a relief to find a site with logical, scientific information rather than more anecdotal opinions.
I stripped my skillets and have started to season them again but am running into some problems. I’m sure I am inadvertently doing something wrong and was hoping you could help me troubleshoot. I blackened the pans in my oven for an hour before applying the first coat of flax oil, they didn’t really darken all that much. More to the point, I am about to put on a third coat of oil and my pans are barely even dark brown. I am keeping them in the oven at 500 for about an hour and a half, then letting them cool for as much time if not longer. I know I only have two layers on so far, but reading through your other pages I saw that the photos you included of the gem pan were after only three coats of oil, and my pans are barely bronze after two. Can you think of anything I might be doing wrong?
Thanks again.
Hello, Sheryl.
Great info on this site. I recently stripped two pieces of old cast iron. One came from my grandmother, the other came from an estate sale down the street. My grandmother’s pan was re-seasoned with flax oil and has been used successfully now many times. But it’s barely getting darker. Still looks bare even though i keep a light coat of oil on it when not in use.
The estate sale pan was re-seasoned with coconut oil and has already achieved the dark glossy look after only a few uses.
Both are fantastic and i’m curious to see which one holds on to the seasoning better.
Now, on to my real question… or maybe more of a concern.
I took the advice of many people at various sites/blogs and stripped the old gunk off with Easy-Off oven cleaner. Since then, I have seen comments and talked to people in the real world about this and a lot of them think this is a bad idea. I’ve searched everywhere for info on whether or not the oven cleaner leaves a chemical residue or gets baked off with the hi-temp seasoning process. I can’t find a DEFINITE yes or no answer. I see a lot of people saying it’s the best way to clean of layers of gunk, but not a lot of folks can say whether or not the chemicals will stay with the pan, or worse, leach into the food cooked in the pan. I found a poison control site with some info on the actual oven cleaner and it stated that even though the chemicals could cause all kinds of problems when using it, it didn’t mention anything on what happens after the cleaning process. Although it did mention that none of the chemicals were carcinogens or carcinogenic.
I have a baby daughter and the thought of cooking her eggs in the skillet has me a little paranoid. Could you please shed some light on this or maybe one of your other readers has some “real” scientific info on the subject.
Thanks much!
-Ryan
I believe the tea boiling might be creating black iron tannate and not magnetite. Not sure if thats better or worse for high temp cooking. Alot of rust converters are based on tannic acid or phosphoric acid.
Electrolysis after rusting it sounds interesting. I’ve heard the red rust converts to magnetite and then metallic iron. If you stop it at the right point you’d have magnetite. If you didnt i imagine the end result would be metallic iron but very porous from the rust formation, so it might hold seasoning better.
Im looking to get some cast iron pans for searing. Should I buy a griswold on ebay or a new lodge?
Sheryl prefers Griswold, but they can be pricey. Someone posted on Amazon that you can sand off the bumpy surface of a Lodge skillet with a medium to coarse grit and then smooth it down with a fine grit. Then season the pan following the guidelines here.
mike, I had Lodge cast iron cookware and seasoned it according to Sheryl’s method, as published in Cook’s Illustrated. Even though seasoning with drying oils (I eventually switched to tung oil, it smells nice and dries into a harder absolutely waterproof film) works like a charm and the surfaces became absolutely non-stick and beautiful, I hade to give them away and replace them with a quality cast iron manufactired before 1950s, from e-bay.
It turned out that Lodge Pro-logic pans were heating up extremely unevenly and even after 20 min of heating on medium the temperature in the center of the bottom was different from the edges of the bottom, the difference about 100F in 3 or 4 places. Unacceptable
And Lodge Logic is much smoother and heats up much more evenly across teh bottom of the pan. Given no other choice I would use this particular line. However, my Lodge Logic skillet has a bump in the bottom that catches up the spatula each time I am trying to lift something off the pan – a pancake, a fried egg, etc. I don’t want to sand it off, because I am afraid of destroying the integrity of the iron. I heard people saying their cast iron cracks if they sandblast it too agressively, but I wouldn’t know where to stop. So I am giving that away too.
Sheryl, many thanks for the wonderfully useful, clearly explained science (& your saintly patience w/restatements!) on seasoning of cast iron cookware. I’m jazzed and have passed your fine advice on to other cast cookers!
Sharon
Hey, I’ve got a LodgeLogic Skillet and I’m going to sand it down because i’m unhappy with the course finish on it. While I’m at it I’m going to completely strip and reseason my pan – and I was hoping someone would give me the “go ahead” on what I’ve written down as steps to take, in order…
1. Thoroughly scrub and clean pan.
2. Use Lye bath (near somewhere warm), replacing water every 12 hours, to clean off gunk (till the gunk is gone)
3. Sand bottom of pan with around 100 grit aluminum oxide sandpaper – increasing the grit until satisfied with smoothness (after 100 i’ll likely use 150 or so and then stop)
4. Wipe off pan, then place it in a vinegar bath for less than 24 hours
5. Remove from vinegar, Neutralize vinegar residue with baking soda
6. Place pan in oven at 450 F for an hour
7. Immediately (and carefully) oil with flaxseed oil (or tea oil? anyone tried that?)
8. Wipe off excess and place upside down in oven for an hour at 450 F
9. Repeat steps 7-8 at least 6 times, until satisfied with seasoning.
Sound about right?
Hey Dexter,
Your steps look good except that I was reading in another response from Sheryl that you don’t want to use vinegar except as a last resort if the pan is very rusty.
Here’s what Sheryl said about vinegar in another post
“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.”
From my experience using the self-cleaning oven causing a pan to warp may depend on the shape. I messed up the seasoning on a cast iron wok, was touching up an area, first coat went fine, so I decided to add another coat, and I had way too much shortening on the wok, which caused an oven fire and destroyed much of the remaining seasoning (drip pan was not big enough, and I did not have aluminum foil to catch the excess). I used the oven in self cleaning mode (for itself and the wok) and that removed all the messed up gummy seasoning without any problem. However, The wok sits flat when upside down, as I had it in the oven. I think skillets don’t rest flat when upside down may have problems. It might not hurt a dutch oven that sits flat however when upside down. The space suit method seems safer on the cookware however, so I think I’ll do that next time I strip some cast iron cookware. Since I’m starting from scratch on the wok I’m sanding (by hand) the inside of the wok to make it smooth. Started at 60, now I’m at 80 grade. It will be still be a while until I finish it, as I’m not working on it all the time, and it was very bumpy to start out with.
I worked in the steel industry for a number of years and can offer a couple of comments.
1) The problem with red rust is that it flakes. This exposes fresh bare iron surface to the atmosphere to allow further rust.
2) Black rust is high temperature rust. It is a more cohesive, protective rust. If you have access to a steel mill, drive by and look at the yard. You will see a lot of dark gray blooms, billets, bars. After they sit long enough they will start turning red. That is why it is important to oil and season the stripped, heated cast iron quickly to keep oxygen away from the iron.
3) Stainless steel works because the oxide (rust) formed from that metal composition is tightly coherent. Stainless steel rusts, the difference is that the first coat of rust prevents any more exposure to oxygen. It also helps that the rust is relatively transparent so it looks good.
Some other comments:
1) I got my first cast iron skillet at an auction. Paid a dollar for a box of kitchen stuff and inside was an old Wagner cast iron skillet. Smooth as a baby’s bottom. If you can find an old piece of cast iron that has been treated right, it is much better than the modern stuff.
2) Wagner was based in Sidney Ohio and was a competitor of Griswold. At some point I understand that Griswold bought Wagner or Wagner bough Griswold. I find that Griswold cast iron is bought by collectors/antique dealers for resale at high prices. I find that Wagner cast iron is usually cheaper. Are you making an investment or are you planning on using it to cook with?
3) I have used the Self cleaning oven method on numerous cast iron implements and have not had one warp yet. I did have a fire start on a grill pan that had a lot of grease on it. Very scary. Black smoke pouring out of the oven vent. Can’t open the door until the temp goes down. I survived, but if your going to do it, wash the cast iron in heavy soapy water to degrease before running the oven cycle. If you are stripping then it doesn’t matter if the soap hurts the existing seasoning.
4) My experience is that a properly seasoned cast iron skillet will never be as non stick as a brand new latest generation non stick cookware. But it will be close. And when the non stick pan starts dying, there is nothing you can do other than replace it. With cast iron, you can re-season. Environmentally a much better approach.
Question:
I read somewhere that people used to fill there cast iron skillets with rock salt and put them in the oven for an extended time once a year to clean them. Does anybody know about this? Does it clean off food waste? Does it refresh the seasoning? What is the science behind it?
I’m not convinced about polymerization. When I started with cast iron, I assumed that this was what was happening; but now I suspect that the oil permeates the porous surface of the iron and black oxide.
I’m not as experienced with steel as Paul, but I do have a background in engineering and some experience in surface science. Paul is right that the black oxide is harder, and holds up better. It’s an incredibly common, inexpensive protective coat for steel. When I looked at the wikipedia article about it just now, much to my surprise, I learned that it’s a) porous and b) has to be protected with a coat of oil.
As for the nonstick properties, I have a hunch. First off, having a hard, inert surface is a huge help. Stainless steel (which has a tough layer of chromium oxide as Paul said) and anodized aluminum (which has a thickened layer of aluminum oxide, one of the hardest materials in the world) are far easier to clean than regular steel or aluminum. Second, because the surface is porous, oil gets trapped and stored there. There’s a reservoir of oil beneath the surface. This would be similar to sintered bronze bearings, which are very porous and are saturated with oil.
All of this is conjecture, though. I’m a little surprised that nobody seems to know what’s actually going on. None of us, myself included, is linking to any studies examining the actual chemistry of the surface; I’ve looked for them, and have never found one. It’s possible that nobody has actually ever studied it. It’s also possible that some cookware manufacturers have studied it, but are keeping the results as trade secrets. Or that they just haven’t bothered to publish them. If nobody has done the experiment, I sort of want to…
@ Channing:
You’re right that oil permeates a porous surface, and an oil-saturated surface would help maintain a layer of oil. But I can’t think of a reason why a layer of oil is preferable to a layer of solid polymer. Especially if assuming you put some oil in the polymer pan.
Bearings are smooth and run in smooth tracks, so maintaining a thin layer of lubricant by using sintered bearings is helpful. If you throw some grit (or salt) into the track, sintered bearings don’t work so well and can get damaged. I don’t think that system would be ideal for cooking pans.
Black oxide layers are also used to prepare iron surfaces for painting. It’s the same idea as scuffing or sanding before painting.
I agree that the hardness is also a good thing though. Reducing scratches and such will keep things smooth and not sticky.
You’re right that oil permeates a porous surface, and an oil-saturated surface would help maintain a layer of oil. But I can’t think of a reason why a layer of oil is preferable to a layer of solid polymer
Hi, here’s a thought.
Assuming:
1. black rust makes the surface more porous
2. that makes the surface hold a greater amount of oil, and
3. that yeilds more thermal expansion of oil
I could picture embedded hot oil (as from seasoning with lard or coconut oil) expanding to the surface and lubricating it, on one hand, and the black rust, while increasing the overall surface area, decreasing the surface area subject to adhesion. Further, with the porous surface underneath, the oil would be able to hide in it and be less subject to being pushed out (and sealed out) of the way by whatever food/spatula action was happening in the pan.
I don’t picture much thermal expansion with a solid polymer (as from seasoning flax oil) and I see, with increased smoothness, an increased surface subject to adhesion and a greater opportunity for sealing cooking oil away from underneath the food.
(The above is pitting, for me, seasoning a clean pan with something like flax oil versus seasoning a black pan with something like lard or coconut oil. I think I am leaning toward the black rust and lard method.)