“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!

84 thoughts on ““Black Rust” and Cast Iron Seasoning

  1. To season my 10 in Wagner, I heat the oven to 500 & pan & lid also, and by scientifically studying the fattest Ribeyes, I insert them in the hot pan on a burner at hi for 30 seconds ea side, then in the oven at 500 for 6-8 minutes depending on thickness. My skillet is a sick as frogs hair. Mainily, I just cook in it using medium to high heat and things dont stick except for some fried potatoes which will come off some day.

  2. How long you been researching? I found this in 5 minutes.
    Posted by danab_z9_la (My Page) on Wed, Mar 7, 07 at 17:17

    I really enjoyed reading your article on the care and use of cast iron cookware. You have collected and shared some really good information. However, I’d like to point out that the claim to “never use soap” to clean cast iron is based on old folklore and has no scientific basis that I am aware of.

    In the “old days” it was very common for folk to make their own soap. Soap simply is the potassium or sodium salt of a fatty acid…….it is the end product of a reaction between Potassium or Sodium hydroxide with an animal or plant fat. Lard and Olive oil were used quite extensively for soap making.

    Early folk boiled wood ash extract to get Potassium Hydroxide for making a soft soap. By the early 1900’s lye (or Sodium Hydroxide) became readily available in cans and was the raw material of choice for making soap. One of the advantages folk realized in using lye was it produced a hard soap and they didn’t have to boil all of that wood ash extract.

    Folk used cast iron vessels in which to boil the wood ashes or lye, with animal fat to produce soap. One of the observations they made in the soap making process was that it removed the seasoning of the cast iron kettle. This is where the folklore to “NEVER USE SOAP” began. The error here is that it is not the soap that removes the patina or seasoning on the kettle; rather, in reality it is the Potassium or Sodium hydroxide alkalie that removes the pot seasoning.

    Patina development on cast iron is a two part process. The first part involves developing a thin layer of polymerized oil on the cast iron. This is accomplished by applying a thin coat of oil to the cast iron surface and heating it in an oven until it dries to the surface. When done properly this layer of polymerized oil CANNOT be removed by either soap or dishwashing liquid. The only way to removed this layer is by vigourous mechanical scrubbing (i.e. brillo pad), by caustics (lye, draino, or oven cleaner), or by burning it off at temperatures greater than 500 deg F (on BBQ pit or in Self Cleaning oven).

    The second part to true Patina development on cast iron involves the actual lay down of carbon on the cast iron surface. This happens at temperatures slightly above the smoke point of the seasoning oil. You MUST heat cast iron above the smoke point to get actual carbon black into the patina matrix. If you do not heat to the smoke point you will only have polymerized oil in the coating……..this is a protective coating but it is not as slick a surface as a mixture of both carbon and poly molecules.

    Keep in mind that grease splatter inside of an oven undergoes the same chemical reactions as what goes on in the cast iron seasoning process. If soaps or detergents really were able to remove seasoning from a pot, then cooks could actually clean the inside their ovens with Ivory soap or Dawn liquid soap/detergent. We all know that doesn’t work and is why oven cleaner and self cleaning oven cycles were invented!

    I’ll comment on the following in the near future:

    1) Oils used for seasoning.
    2) Old cast iron is superior to today’s stuff.
    3) Proper cleaning procedures

    Thanks for making your article available for comment.


  3. Dear Sheryl,
    Thanks so much for all of your useful and informative info. I bought a naked thin cast iron wok from China and used your method from the thread on seasoning your wok. Afterwards I cooked chives until they were black as they suggested and now have a wonderful slick surface with the carbonization having begun. If not for your site I would have not used flaxseed oil and would not have learned the science behind it. I am thinking that you have long ago left this post in the dust, but I did want to thank you.

  4. Rather than buy a bottle of flaxseed oil, I just used 6 capsules of Costco’s 1300 mg Flaxseed Oil that I take for cholesterol issues to do the first coat on my new large dutch oven. Cheap, handy, good for me, and very fresh since they are individually sealed!

  5. What a long but interesting post. I bought an old 9 inch cast iron skillet (Lodge) at a garage sale in 1970, seasoned it well by heating the pan to 450, removing it from the oven and quickly wiping it with Crisco using a paper towel. After coating it well inside and out, I then wiped off any excess Crisco and returned it to the oven for an hour or more. I used it for 20 years, re-seasoning it maybe once. My 30 year old son is now using it.

    Some Thoughts: Using a fat that is solid at room temperature seems to make more sense to me…Crisco, lard, coconut oil. After the pan gets a really nice black, slick surface you can use some Joy to clean it without any removal of the finish. The only use that seemed to be hard on the finish was cooking a tomato based sauce for long periods.

    Thanks for all the wonderful input from so many diverse people.

  6. I just bought a 15gallon or so cast iron kettle with the intentions of making some burgoo. The kettle is not rusty at all, really has a pretty slick black finish on the inside. If you rub the outside you get a black substance that seems more like carbon than oily. The guy I bought it from said to build a fire in it under it and around it to (kill it) as he put it, then rub it down with grease before cooking with it. Does anyone have a good recommendation for how to treat a kettle, its obviously not going to fit in my oven.

  7. We tried our best to follow your instructions and we aren’t sure where we went wrong but….

    We received 3 stripped down old cast iron skillets for Christmas. We followed the your instructions and they were looking good. We tried to cook with one after only 5 coats and it didn’t go so well. Now the seasoning is flaking off inside and out, even off the pans we didn’t cook with. Did we use the wrong type oil (organic cold pressed, pure and unfiltered Flax oil)? We tried very hard not to use too much oil and never got any drips.

  8. how do you keep an iron kettle from turning water black when you cook in it? i have an old kettle my grandmaw used and it was givin to me. i wanted to use it now. so i fixed some pinto beans and it turned the water balck. help please.thanks. ronnie m

  9. i got my grandmaws iron kettle and when i tried to use it. my water that i was fixin beans in turned balck, what caused this and how do i fix the problem. it is very old and seasoned. she used it to cook all the time. thanks ronnie

  10. So it wouldn’t surprise me that seasoning cast iron hasn’t been researched. I have not conducted a search of scientific literature or Lexus-Nexis on this subject.

    After skimming your black-rust-and-cast-iron-seasoning forum a few things strike me.

    1) Some people advocate seasoning woks with green onions. It would seem the onions could form sulfuric acid and perhaps produce some iron salts during this process?
    2) The “salts” of iron can speed up the drying of Flax oil. from http://en.wikipedia.org/wiki/Drying_oil if this is correct, which it seems.
    3) Taken together perhaps some salts or other catalyst could be added to the linseed oil to improve the polymerization process.
    4) It appears all of the “basic” chemistry of seasoning cast iron is known it just needs to be put together.
    5) iron + sulphuric acid = iron sulphate + hydrogen or another way to say it “might be” cast iron +onions=iron sulphate
    6) So what would happen if you heated you cast iron with some onions and oil.. much like you do with seasoning a wok, just to create some iron salts. Then perhaps wipe it dry of the oil and season in the oven at high temperature 500 F for an hour as Sheryl has suggested? Would enough salts be created to make a difference?

    I haven’t been in the lab for years, but it would be really interesting to look at the process using a mass spectrometer and even a electron microscope. I’d like to look at the surface of the cast iron after each successive layer of oil is applied and polymerized.

    Frankly studying this stuff could lead to some factory applied seasoning that outperforms anything known.

  11. The only reason I can think of for attributing non-stick qualities to the presence of carbon is the idea that graphite (one of the forms of carbon) is extremely slippery, so much so that it is used as a dry lubricant. However, char, which is the only form of carbon that would show up from the seasoning process, is not graphite, so I think this is a mis-attribution.

    In any case, even if char were non-stick (not so from my experience!; it sticks to pans and grills, that’s for sure), it should flake off with use and get all over your food. Graphite certainly wears away, unless you have specially prepared graphite solids.

    If there were enough char or graphite to contribute significantly to the non-stick qualities of seasoning, wiping the well seasoned surface (seasoned by your flax seed oil method) with a paper towel should show lots of black stuff coming off. If the surface were washed clean, and wiping comes off clean, there shouldn’t be any char left on the surface at all.

  12. thank you for this and your other articles. I found them very interesting, and since I’m newly obsessed with cast iron, I’ve been trying to figure out building up a good set of pieces. My mother still uses her grandmother’s, but I’ve resorted to not trying to co-opt any of those, as she still uses them. I won’t go on and tell my whole story, but I found a no-name dutch oven from salvation army. Bought it for 10 dollars, and decided to try and do a proper and thorough seasoning from the ground(iron) up. I found the relation to ‘bluing’ metals (guns, machine parts) very interesting and thought at least to give it a try before I went through the tedious process of the various coats of seasoning.

    I took the dutch oven (and a Logic skillet that had seasoning troubles, I suspect because of pre-seasoning) down to the metal using 2, 24hr settings of Easy-Off oven cleaner, and thorough rinsing and scrubbing (left some old bits of seasoning on outside of dutch oven, but I’m not that concerned about outside). The one thing I regret was not washing with soap/degreaser after rinsing the Easy-Off. Honestly, I feel like the lye had somewhat of a degreaser effect, and it came off as a slick film, so I could feel the DRY iron underneath, especially in tandem with scrubbing with a wire brush. The iron was very raw as far as I could tell, so I rinsed very well and dried thoroughly, and then put it into the oven for an hour on 400 or so. I have a very large stainless steel pot that I had filled with water and boiled. I initially tried to put the skillet into the water, but because of an intense sizzle when touching the water slightly, decided that the temp difference was too much. The boiling point of water is about 212deg F, so I let the iron cool in the oven to a 200deg setting, then it went smoothly into the water without any ‘reaction’. After 30min, I took the skillet out, put it back in the oven to dry, and put the dutch oven bottom in. After 30 min, took it out, put the top of the dutch oven in.
    I didn’t document it THAT well at all. I didn’t take pics before the Easy-Off, and the pics I took weren’t that consistent in terms of lighting, etc. BUT, I did notice something that I wanted to share. I think all the Iron came out darker (less red, darker grayish) after I put it in the boiling water for 30min. The dutch oven bottom, however (which was clearly heavily used before, has some pitting and scratches on cooking surface, but not any noticeable rust before I removed the old seasoning) came out of the water with a kind of speckled, or spotted yellowish/greenish residue that easily brushed off with a paper towel. I have no idea what this is, but I think it has something to do with this black rust procedure. Neither the new-ish Logic (which went in first) nor the top of the dutch oven (which went in last, by itself) had visible residue.

    I will try and embed pics…if that doesn’t work, I will include pic links.

    I am going to try and get these pieces well-seasoned hook or crook, but if I were to do it again, and had a really nice piece of cast iron, I wouldn’t hesitate to do it again. Assuming you, like I did, make sure the temperatures match, I don’t see how it could hurt the metal in any way at all. If I did it again, however, I’d probably try to do it (as in the bluing proceedure) more than once. I’d do it a number of times and really see how well the blackening effect worked. I was in a rush to get to an important obligation (SLIGHTLY more important than perfect cast iron; babysitting my neice), so I rushed… and just slathered some oil on it afterward so I could leave and not worry about rust developing. I’d be interested to know what this residue is, and if that’s a sign that the black rust was indeed forming… They still had red hue to them after the boiled water, but were definitely darker. I wonder if I did it over and over again if they would turn a nice dark color and form that protective layer that gun ‘bluers’ go for.





  13. I wish I could edit my comment after leaving it… n e ways, here is the first pic with bad link…


  14. Hi sheryl and all, just bought a new 20 inch cast iron roaster. going out tomorrow for some organic flax seed oil will keep you posted on the results

  15. Thank you so much for this post and the one about using flaxseed oil to season cast iron pans, I’m going to buy some flaxseed oil on my next shopping trip!

  16. Hi everyone. Loving this blog and loving this thread, and I’m using this information to recondition an old skillet.

    But… I tried the black rust pretreatment and got the wrong result, but I can’t figure out why.

    I stripped our old skillet down to bare metal using power tools because there were some grooves scored around the perimeter of the cooking surface that I wanted to smooth out. I washed it thoroughly in soap and water to be sure there was no residue left from the sanding, dried it thoroughly with a towel and then stuck it in a preheating 450 deg oven. An hour later it came out with reddish, not black, rust.

    I understand that moisture is an issue, but heating the pan IS the way to dry it out, so I figure it should have dried out quickly, then turned black.

    I wiped as much of the rust off as I could with paper towels, but the color never completely went away. Then, following an inexplicable logic, I rubbed some flax oil into it while it was still hot, hoping that might change the color. Not. Now it looks really rather beautiful and shiny, but definitely reddish.

    I’m wondering if I need to start over. But more importantly, I don’t know why it would turn red. Or more accurately, how can you dry it out without creating red rust, so you can get to the part where you make black rust. It’s like a Catch 22.

  17. When you are reading an article, there is a Facebook link. The link goes to http://www.networkedblogs.com/blog/sheryls_blog?&ref=fb, but does not let you log into Facebook.
    So, we (Earth), cannot share this particular article. I can share everything you have written at networkedblogs.com, that is true. Buy I may not agree with each and every article or want to read each and every article.

    In the non-maximized efforts world, I am sure that each article can have a link. But not in the super-efficient, aggregation of efforts world that we live in.

    I liked your cast iron skillet article and thought others would benefit from it.

  18. Where are getting that Networked Blogs link from? I don’t see anything with a www in front of it.

    The icons on the upper right are for subscribing to the blog or following me on Twitter (not that exciting to follow me on Twitter). But you want a Share button – that’s different. I have that on another blog but forgot to put one here. I’ll add it. Not right now because it’s 3am, but in the next few days. Thanks for suggesting it.

  19. I just purchased a set of cast iron skillets, and i washed them and dried in the oven at 200 degrees and then oiled them with canola oil(couldnt find it) but after the first seasoning they had a nice sheen to them but not as dark. so i wiped them down with a second coat if canola using a micro fiber cloth and seasoned for a hour more. am i doing this correct?

  20. Hi Sheryl, et al:

    Because I’m a meticulous, retired printer, I re-wrote and synopsized your article on seasoning cast iron to make it easier for me to follow while standing at the stove. My photographic memory no longer works — when I retired twenty years ago, I ran out of film! Let me know if it accurately represents your suggested process:

    Recipe for Perfect Cast Iron Seasoning:

    • The basic idea is this: Smear a food-grade drying oil onto a cast iron pan that’s been stripped down to the iron 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. Use organic flaxseed oil. It’s in the refrigeration section because it goes rancid so easily. Shake it before you use it.

    #1. Heat the pan on a burner for 30 seconds until it’s just warm to the touch, then put it on a paper towel, pour a teaspoon of flaxseed oil on it and rub it on.

    #2. Now rub it all off! Rub it off with paper towels or a cotton cloth until it looks like there’s nothing left on the surface. The pan should look dry. Put the pan upside down on a square of aluminum foil in a cold oven.

    #3. Turn the oven to a baking temperature of 500°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 don’t open the oven door. Let it cool down with the pan inside for two hours.

    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 — like a half dozen — so repeat #1 to #3, five more times.

  21. Hi Sheryl,
    is there a way to use your seasoning technique without the use of an oven (maybe a campfire) and get similar results, as we don’t want to smoke our house out?

  22. Sharyl, if there is some curing came off from the middle of my Grishwold pan, does it mean I have to restart seasoning from the scratch, or there is a room for a compromise? I seasoned it first time two weeks ago, it is possible I didn’t do enough layers, but used my skillet gently. I mainly cook with coconut oil and a beef tallow, is it possible the choice of cooking oils prevented better build-up of layers of seasoning?

    Thank you, Sheryl for your excellent summery. I just recently got hooked on the seasoning of cast iron and cooking with it. Cooking is my hobby, so far I managed with what I got(mainly stainless still) by keeping food pieces dry, heating pans to the right temperature and using enough oil could be quite helpful while using almost any pans. However, trying to equip my son for the cooking on his own in the University inspired me to explore more equipment options. As a result my son got Adcraft naked cast aluminum 10″ pan (totally fool-proof after initial period) from a restaurant supply store, and the Country Cabin #3 thick modern cast iron skillet (for stakes), and I got my first piece of Grishwold #6 and a very neat light cast iron Taiwan skillet with milling marks 8″ upper diameter(perfect for paper-thin pancakes). I still get a lot of use from my stainless-still with copper bottom pans when crust is not an issue and with some acid dishes. I feel like I have to be super careful with my smooth high quality cast iron, and it sometimes interferes with convenience.

  23. What about looking at the process for Parkerizing a gun? Sounds similar (inducing a rougher, durable surface, which, mostly, holds oil.)

    Browning a gun? (An earlier process than bluing…)

  24. i was thinking of parkerizing as well! i have done it on other things.
    also.. electrolysis process for rust removal will produce a coating of magnitite which might work as well.

  25. I just prepared strawberries for canning in my enabled cast iron pan. The left the pan covered over-night and the next day found dribbles of black along the sides of the white enable and small deposits of black in the strawberry mixture. I think it’s rust. I scooped out the patches of black, cleaned the sides of the container, reboiled and would like to freeze the mixture for later consumption but am not sure if it is safe to eat?

    Is black rust toxic?

    I think this qualifies as a scientific inquiry, hence, I turn to you.

  26. Hi Sheryl,

    I seasoned my new pan like you said, it looked great but I had a lodge pan. No one said anything about the rough surface on the in side. I was reading a web site that, this lady had her husband sanded down the inside of the pan. That did it, I seasoned it with flaxseed oil like you said. What a great pan, thank you.

  27. Every weekend i used to go to see this web page, because i wish for enjoyment, for the reason that this this web site conations genuinely good
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  28. I followed these steps to the letter, at least a best I could. My result was very pretty, but disappointing for cooking. I have only cooked on it once, and the eggs suck to it like I hadn’t seasoned at all. After cleaning it, which was very difficult with all the stuck food, I was left with that dull silver/grey iron I had before the first coat. What did I do wrong? Do I need to start over, or can I try to add more coats without cleaning off this “bad” seasoning?

  29. Your website is fantastic! Sheryl you manage to combine the two things I love…intellectual curiosity and cool cooking. I grew up with my mom teaching me to cook in a huge Wagner with a nasty black crust on the bottom but smooth as glass inside. I never understood (until your site) the science behind it, but often wondered why my cast skillet stuck and hers didn’t. Now I have a similarly slick Griswald #8 that my wife and I enjoy cooking eggs on and watching them slide around as good or better than non-stick. You did that with the information you gave me to take it down to metal and then re-season it to slick and pretty. The flax seed oil worked like a charm. Thanks.

  30. I tried the flax on my cast iron and it worked great. I thought I would mention that one way to avoid smoking you house, is to heat it on your propane grill. Worked great for me.

  31. I am closing this post for further comments. Everything has been asked and answered many times now. If you have a question (or comment), please read the original article and the comments already posted. It’s most likely been asked and answered already. You also can refer to the previous post on cast iron seasoning, Chemistry of Cast Iron Seasoning: A Science-Based How To.

    I’ll close on a link to a pure flax oil that works, since many people have had problems from using flax oils that contain other ingredients to retard oxidation (most flax oils on the market do). Someone commenting in the post previous to this one said he found that Barleans flax oil worked well. That’s because it’s 100% flax oil and contains no other ingredients. Here’s a link to buy it:

    Barlean’s Flax Oil (from Amazon)
    When you first click on this it goes to the large 32oz size, but it also comes in a 16oz size.

    I suspect that many of the people who’ve been posting about sticking problems expect that an initial seasoning will result in a non-stick surface like teflon where oil during cooking is not needed. That’s not how it works. This article and the previous one describe how to do an INITIAL SEASONING; it improves over time. That’s the nature of cast iron seasoning. You still need grease when cooking.

    Happy seasoning.

    P.S. Please do not email me with seasoning questions.

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