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!