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without turning red.

Metal light emission depends almost entirely on blackbody radiation [0] which depends only on temperature. Aluminum melts at lower temperatures than iron and that's why iron glows before it melts and aluminum doesn't.

Blackbody radiation is also the principal source of sunlight (and all starlight). It was one of the first physical phenomena that required the quantum theory of energy to explain.

[0] http://en.wikipedia.org/wiki/Black-body_radiation




Indeed, though the iron crucible I use gets hot enough to be red hot. Whether the molten aluminum has a faint red glow about it or not I cannot say; I don't usually melt it at night.




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