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

I have read this question:

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? – niels nielsen 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. – Árpád Szendrei 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? – Árpád Szendrei 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

## 1 Answer

Clear glass does not glow all that much in the visible. The illustration seems to show quite thick glass, and it is impossible to see from the image whether it is clear when cooled down. The temperature would be quite high (1000 °C or more), an opaque object at the same temperature would radiate more.

Edit: an image from https://youtu.be/FXXwCNSiAFY

• If you search for "molten glass" on YouTube, you'll find that even quite small pieces of glass, when just taken out of the furnace, do glow visibly. That's not as much glowing as I think steel would have, but that's easily visible. – Ruslan Oct 8 '20 at 8:09
• Just out of the crucible, the mass of glass has an even higher temperature. But it becomes easy to look through clear glass at the working temperature, also when the background is not very brightly lit. youtu.be/FXXwCNSiAFY?t=37 – Pieter Oct 8 '20 at 11:48