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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.

enter image description here

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?
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  • $\begingroup$ Diamond has an extraordinarily high thermal conductivity: diamond-materials.com/EN/cvd_diamond/thermal_properties.htm (exceeding that of Cu by a factor of $5$!) $\endgroup$
    – Gert
    Commented Aug 1, 2020 at 16:56
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    $\begingroup$ who says diamond does not glow when heated to the same temperature as molten, glowing glass? $\endgroup$
    – niels nielsen
    Commented Aug 1, 2020 at 17:10
  • $\begingroup$ @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. $\endgroup$
    – Árpád Szendrei
    Commented Aug 1, 2020 at 18:39
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    $\begingroup$ Within one of the related Q&A topics you linked to, someone asked in a comment: "Does heating glass make it absorb more light?" — i.e. is it possible that glass changes its emissivity in a way that diamond doesn't? Is that idea included in this question? $\endgroup$
    – natevw
    Commented Oct 7, 2020 at 20:16
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    $\begingroup$ @natevw thank you, yes, that could be an answer, if this includes an explanation why glass changes its emissivity during heating, while diamond does not do the same, there must be some difference maybe in the lattice structure of the two materials, or some QM level difference? $\endgroup$
    – Árpád Szendrei
    Commented Oct 7, 2020 at 20:53

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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 Hot glass, Noomoon Pictures

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    $\begingroup$ 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. $\endgroup$
    – Ruslan
    Commented Oct 8, 2020 at 8:09
  • $\begingroup$ 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 $\endgroup$
    – user137289
    Commented Oct 8, 2020 at 11:48

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