When a polished piece of metal (or steel in particular) is heated to incandescence, how do its reflective properties change?

Given a mirror-like surface, would the object temporarily cease to act like a mirror at a certain temperature? Would the specular reflection of light turn into diffuse reflection; in other words, would the surface temporarily become matte/dull?

I am not asking about permanent changes to the object such as melting, but rather can there be a temporary change in the reflectivity only while the the object is hot, that disappears when the object is cooled down?

Background of the question: I have a laser scanner that usually has trouble scanning shiny surfaces with high specular/ low diffuse reflectivity; and it seems to work a lot better for hot steel pieces than for cold ones. Glowing steel also looks less shiny to me than cold steel, but of course that is hard to tell with all the glowing


1 Answer 1


Assuming the surface of the metal remains smooth, the reflection from it will be specular and the metal will look shiny regardless of the temperature. However the amount of light metals absorb, instead of reflecting, generally increases with increasing temperature because you get more scattering of the conduction electrons by lattice vibrations. So the metal remains mirror-like as it is heated, but the reflection gets gradually darker. However the decrease in reflectivity will generally be small.

I suppose it's possible that a phase change in the metal might cause the surface to become rough, which would make the metal surface look dull. However this would only happen if a relevant phase transition existed, and isn't likely to be reversible so it doesn't really fit with your question.

Also the metal surface might oxidise and become dull as it's heated, but again I don't think this is in the spirit of your question.

If you're interested in taking this further Googling for metal reflectivity temperature returns lots of related articles.

  • $\begingroup$ Phase transitions may indeed be relevant for the case in question, if the temperature of the steel passes ~1000 K, where it turns into en.wikipedia.org/wiki/Austenite . Added nickel can stabilize the FCC structure of the austenite during cooling, but otherwise, it will change again, with proportions of different crystals depending on composition and cooling schedule. I don't know whether/how(/ to what extent) reflectivity is affected, though, but it would not be a big surprise to find that it is. $\endgroup$ Commented Mar 13, 2023 at 7:22

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