Why would different metals glow red at different temperatures?

Question

According to everything I've been taught about incandescence and black-body radiation, and some quick Googling to confirm I'm not crazy, just about everything, regardless of composition, should start glowing red at about the same temperature- 798K, the Draper point, where sufficient power in the black-body radiation curve crosses into the visible spectrum to be visible.

I have just been informed by a metallurgist friend, however, that different metals in his experience begin to glow red at wildly different temperatures; typically, just below their melting points. For example, apparently aluminum glows red at much lower temperatures than steel.

My hypothesis so far: The metals in question are far from perfect black bodies (reasonable, since most metals are shiny), and differing levels of emissivity in the low end of the visible spectrum require different temperatures to raise total emission in that range to visible levels. This, however, does not explain why there should be any connection between glow-point and melting point.

Am I close to correct? Is there another better explanation? Or is my friend simply crazy?

Answer 1

All materials have some color (i.e. are not perfectly black or white bodies). So, even at incandescent temperatures, if they are illuminated, there is reflected light with color. A good example of a non-black-body would be glass; softening a glass rod in a Bunsen burner flame, it does glow red, but there's little optical coupling of the transparent material with visible light, so it's likely you will see a yellow sodium glow in the Bunsen burner's flame when the glass rod starts to melt, rather than the red of the hot glass. Optical pyrometers work best when peeking through a hole into a dark oven chamber. No matter how inefficient the light emission is, when the oven's content is at a uniform high temperature, the black-body curve is the color emitted, because non-blackness means you see reflection or transmission, of light from other (equally hot) surfaces in the oven.

Answer 2

Different metals would to glow at different temperatures because of their different abilities to hold on to electrons. Some metals hold their electrons very weakly and some hold it very tightly. The temperature at which it glows is dependent on the strength of this force. Things glow either because of absorption or emission spectrum. The lesser the force with which the nucleus holds the electron, the lesser is the energy required to make it glow. Actually, to get a more satisfying answer, you must post this question in chemistry stack exchange.

Edit: The ability to remain solid is dependent on this force too, hence the connection between melting point and glowing red.

Answer 3

There is no direct relation between melting point and colour of light produced, it's just that some heat energy is used in breaking intermolecular forces and a part of it is transferred to atoms. So for a higher melting point, a bigger part of energy is used in breaking intermolecular attraction and to change its state of matter.

The rest is explained by black body radiation's explanation by Max Plank. When energy is given to atom, it's valence electron gets excited and jumps to higher energy levels and return back to original shell by emitting electromagnetic rays of different wavelength like first low energy radiations like that of infrared, red and so on as the heat increases.

And your question of different light in different elements is explained by law of conservation of energy. neglecting energy lost in breaking intermolecular forces, same energy given to two different atoms produce same light no matter which element that it is.