What is the rainbow discoloration that happens when steel is subjected to extreme heat?

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What is the rainbow discoloration that happens when steel is subjected to extreme heat?

In: Chemistry

Heating steel creates an oxide layer, and microscopic variations in thickness of that layer reflect different wavelengths (colours) of light, leading to the rainbow of colours. You can see a similar effect in oil on a floor, or the surface of a soap bubble.

If you want to know more, plus other weird and wonderful features of the quantum world, I highly recommend [Q.E.D. The Strange Theory of Light and Matter by Richard Feynman](https://www.goodreads.com/book/show/5552.QED?from_search=true&from_srp=true&qid=BlfE7PIkFA&rank=1).

When you heat the steel it forms a super thin film of iron oxide on its surface. Normally iron oxide is opaque, but these layers are so thin that they allow light through, so you get a similar effect to what you see with coated lenses (which also have a thin transparent layer on the surface)–the refraction of the light from the thin surface layer causes the coloured appearance you see, and the colour you get is determined to some extent by how thick the layer is, so you get different colours from different parts of the metal.

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.