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.
Answer
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.
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