“Black Rust” and Cast Iron Seasoning
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!
Kyle:
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.
February 10, 2010, 4:32 amSheryl Canter:
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.
February 10, 2010, 9:41 amKyle:
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!
February 10, 2010, 9:37 pmSheryl Canter:
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.
February 11, 2010, 2:01 amHarold J Dunfee II:
There are new and improved methods for doing pretty much everything.
February 21, 2010, 1:47 pmClean 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
Sheryl Canter:
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.
February 22, 2010, 2:51 pmMark:
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…
February 27, 2010, 3:56 pmSheryl Canter:
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
February 27, 2010, 4:34 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, 9:32 pmMark:
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.
March 9, 2010, 2:44 pmSteven Hoy:
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.
May 5, 2010, 4:02 pmSheryl Canter:
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.
May 5, 2010, 5:11 pmGlenn:
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.
May 6, 2010, 11:53 pmSheryl Canter:
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.
May 7, 2010, 7:48 amPaul:
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
May 11, 2010, 11:10 amSheryl Canter:
Paul, I think you should wash that off. You sure don’t want to eat it, and it could prevent the seasoning from adhering.
May 11, 2010, 9:11 pmScooter:
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.
May 12, 2010, 8:25 pmGloria:
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.
May 17, 2010, 11:51 amSheryl Canter:
>…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.
May 17, 2010, 1:27 pmJoseph R. Kennedy:
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.
May 21, 2010, 1:56 pmKaren Parker:
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???
May 22, 2010, 8:35 amThanks
Sheryl Canter:
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
May 22, 2010, 10:20 amScooter:
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.
May 30, 2010, 2:26 pmLori:
Hi,
June 3, 2010, 8:25 amI 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!
Sheryl Canter:
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.
June 3, 2010, 8:31 amJack:
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.
July 20, 2010, 7:55 pmJack:
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.
July 23, 2010, 3:10 amShannon:
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.
July 26, 2010, 4:40 pmShannon:
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.
July 26, 2010, 4:51 pmSheryl Canter:
>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.
July 26, 2010, 5:33 pmJahm Mitt:
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.
July 29, 2010, 5:42 am