Together, the current answers to Is the visible light spectrum from "red-hot glass" at least close to Blackbody Radiation? explain that while we can not necessarily call a heated sample of good quality transparent glass a blackbody emitter, the shape of the thermal emission spectrum may still approximately follow Planck's_law if the emissivity is nonzero and relatively constant over the visible wavelength range.
That question shows several examples of apparently transparent glass (at least when cool) glowing brightly when it is hot enough.
I have two interrelated questions about this:
- Hot glass during glass blowing gets quite bright. I have no measurements but it certainly seems brighter than one would expect if the emissivity were as low as the absorptivity. Does heated glass or do transparent covalent crystals (e.g. quartz, diamond) glowing yellow-white become less transparent by the same mechanism that allows it to radiate light, or does it remain just as transparent and the visible radiation is actually as low as one would calculate from Kirchhoff's_law $a = \varepsilon$?
- What electronic mechanisms produce visible thermal radiation transparent covalent glasses and crystals? Reading @JohnRennie's answer I have a hunch that thermal excitation first moves electrons to higher bands allowing them to radiate, which suggests that yellow-white hot glass would also be darker (more absorbing) than room temperature glass, but I'm just grasping at straws.