What is the rainbow discoloration that happens when steel is subjected to extreme heat?
In: Chemistry
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.
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