“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 pm