Colors observed as an heating effect on steel [duplicate]

Question

On steel utensils there are certain colour combination observed, why is so? May be it is due to heating, but how can heating create such kind of colours ? Description : certain group of blending colours like violet blue etc.

Answer 1

It's an example of thin-layer interference. The same is observed in soap bubbles or oil films in water puddles. Light enters a thin transparent layer and is partially reflected both upon first entering the layer and on the inside of the layer. The two rays of light have slightly different path length (twice the thickness of the layer, if shone head-on, slightly more depending on angle otherwise) and will thus interfere with each other. The resulting interference may either be constructive or destructive depending on wavelength, i.e. color. If the light bounces inside the layer multiple times before escaping, particular wavelength will be singled out with a higher fidelity resulting in a monochrome reflection, an effect exploited in the so-called Fabry-Perot interferometer.

In the particular case of iron/steel, the thin layer is actually an oxide layer. Heating can facilitate the formation of an oxide layer but the layer will persist even after cooling, so it's not strictly a heating effect. The colors are known as tempering colors and can be used to infer (mind the pun) the maximum temperature experienced by the iron. Theoretically, as the oxide layer grows even further, the colors would repeat over and over again as higher-order interference maxima shift into the visible part of the spectrum. In practice however, the oxide layer looses its transparency at some point, resulting in a dark gray color.