# Why does hot (molten) glass glow, while diamond does not?

Why doesn't diamond glow when hot?

This is because of Kirchhoff's law of thermal radiation. The corollary from it is that emissivity of a material is equal to its absorptivity.

Does any material glow, under appropriate conditions?

a body must absorb and emit identically at a given wavelength. Transparent is a terrible emitter.

These answers specifically explain that though every objects glows in a sense when heated (just some not in the visible range), in the case of diamond, heating it will not cause it to simply emit visible wavelength photons.

Thus diamond will not emit visible light, since it is remaining transparent when heated, and can only emit little number of visible wavelength photons.

Though, this explanation does not work for glass. If you heat glass, it emits visible (glows), meaning either of two things:

1. it absorbs visible and re-emits visible

2. it absorbs all wavelength and re-emits visible (in cascades or for other reasons changes the re-emitted photons' wavelength relative to the absorbed ones)

Question:

1. Why does hot (molten) glass glow, while diamond does not?
• Diamond has an extraordinarily high thermal conductivity: diamond-materials.com/EN/cvd_diamond/thermal_properties.htm (exceeding that of Cu by a factor of $5$!)
– Gert
Aug 1 '20 at 16:56
• who says diamond does not glow when heated to the same temperature as molten, glowing glass? Aug 1 '20 at 17:10
• @nielsnielsen those answers I believe, as I understand the graphs. Glass melts around 1400-1600F, and diamond does not glow even then. It says diamond is transparent even at 1000C which is 1800F. Aug 1 '20 at 18:39
• @Gert thank you, though can you please elaborate on that, how extreme thermal conductivity leads to diamond not emitting (and I guess not absorbing) visible when heated? Aug 1 '20 at 18:43
• I can't. I don't know the explanation. But silicon is quite similar: very high $k$. I've tried to make a piece of pure $\text{Si}$ glow with a propane burner: it just won't!
– Gert
Aug 1 '20 at 19:12