It is a gas and it is neither.
A fresh surface of iron is shiny.
Dull surface is a better absorber. A shiny surface would better reflect than absorb.
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dull
It is in fact the best radiator. Dull black is the best absorber and emitter. Then shiny black, followed by white, then silver. While the best reflector goes the other way around (silver's the best, then white, followed by shiny black, then dull black)
A dull surface will absorb infrared best, but it needs to be dull at infrared wavelengths. Do not judge the infrared behavior by the visible performance.
Most glass is shiny but can be made to look dull.
dull
dull
dull
dull