In a lot of questions I read that a good approximation of the radiation emitted from a hot piece of steel is the black body radiation.

Than I search for the value of the emissivity of molted metal and I find that they are (for a molten Pure Iron) around 0.4 or less.

I know that the emissivity is the ratio between: the radiant exitance of that surface and the radiant exitance of a black body at the same temperature as that surface, so I expect to find a value which is very close to 1 while I get values of about 0.4.

So my question is: Why?

I hope to not forget something important.

  • $\begingroup$ I don't understand the question. The value that you find and the one that is in the document are both 0.4? What's the problem with that? $\endgroup$ Jun 4, 2019 at 19:50

1 Answer 1


I think you are confusing the fact that molten metal emits radiation with a spectrum that is very much like a black body spectrum with the fact that the material has a certain ability to absorb or emit radiation at all at any given wavelength (the emissivity).

When a physicist says something radiates like a black body, they do not mean that it has an emissivity of 1. What they mean is that spectrum of radiation is similar to that of the black body spectrum.

  • $\begingroup$ Are the spectrum of black body and the one of molten metal equal but different from a scale factor? $\endgroup$
    – Ugo Mela
    Jun 4, 2019 at 20:10
  • $\begingroup$ More or less. A black body is a theoretical construct, so when we say they are well approximated by a black body we don't necessarily mean that there is only a scale factor difference. In this case, when I have done radiation transfer calculations, I always used emissivity x sigma x T^4 as in the Stefan-Boltzmann law and did fine. $\endgroup$ Jun 4, 2019 at 20:25

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