What is the rainbow discoloration that happens when steel is subjected to extreme heat?
In: Chemistry
It is the formation of Fe3O4, a “black oxide” layer. (This is as opposed to Fe2O3, which is red oxide, which we commonly call rust.) Fe3O4 is the kind of oxide you would want to coat steel with if you are “bluing” the steel to make it rust resistant. Because it is not significantly expanded vs. iron itself, it stays on the iron, whereas red oxide expands a lot and flakes off the surface and lets moisture and air through, exposing more of the steel to rust. In conjunction with a coating of oil or burned-on oil seasoning, black oxide is a pretty good way to make iron and steel somewhat rust resistant. The color change comes from the fact that it is so thin that it is transparent, but as its thickness changes as the reaction continues, the thickness of the oxide layer interferes with different wavelengths of light differently. By canceling out various different wavelengths of light, the reflected light shows different colors.
This method of “bluing” steel with heat is commonly done to woks and sometimes to carbon steel pans right before seasoning with oil. For example:
Here are videos of carbon steel cookware being blued with heat before seasoning:
* [a De Buyer pan being blued on the stove top](https://youtu.be/X-tcVxIdmmY?t=75)
* [a carbon steel wok being blued](https://www.youtube.com/watch?v=UGXGJD2xTzQ) in a Chinese restaurant
* [a carbon steel wok being blued](https://youtu.be/8WqbnZYltsk?t=41) in a Vietnamese restaurant
* [a carbon steel wok being blued](https://youtu.be/a783vEF2yYA?t=90) in someone’s back yard
* [a carbon steel wok being blued](https://youtu.be/HtFA7hx72xQ?t=97) on the stove top
* [a carbon steel wok being blued](https://youtu.be/AXEtBIjKPII?t=126) on a small table top butane burner
This oxide reaction typically begins to happen at around 600˚F. For items which need to be completely blackened by this process so that they have better rust resistance, such as when gun metal is blued, heat is not enough, and the metal item actually gets dipped in a chemical bath to give it the black oxide coating.
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