how come it’s transparent when it’s cool and opaque when it’s been heated… then it turns transparent again when cooled?
Why does melted glass glow in yellow?
Umm… because it’s really freakin’ hot?
Look at this photo. The glass is still transparent when liquid. It’s just that when it’s really, really, hot the glow obscures your view through it.
glass at its molten temperatures is very hot. at those temperatures the atoms in the glass emit colored light, lots of yellow, those colors travel through the glass.
That’s the sodium doublet at 588.9950 and 589.5924 nm.
Is this an example of “black body radiation?” (Even if cold glass isn’t “black?”)
Trinopus
Why would you think that?
I think Bill Door nailed it. There is lots of sodium in regular glass (soda glass) and sodium has two strong spectral emission lines in the yellow. It is the same yellow that we used to get from those sodium vapor street lights.
Yes, soda glass emits a yellow lot of light due to the sodium present. Much more than you would expect from a black body. This is enough to be an issue for glass blowers, and they wear didymium glasses to specifically cut the emission.
The counterpoint is borosilicate glass, which doesn’t contain sodium. Googling about it is hard to find a definitive picture, perhaps this, which is much more the colours one would expect from a simple black body.
ETA. Borosilicate glass is worked a significantly higher temperatures, and thus there is good reason for a red glow, whilst a soda glass remains pretty quiet from a black body point of view in comparison when being worked
This suggests that glass would also be less transparent at those wavelengths, even when cool. I guess you wouldn’t notice a couple narrow color notches in a general scene.
soda glass emits so much light that you are blinded (at least for any detail in the molten glass area) without filtering lenses. this for lampwork (torch).
in a furnace glass is close to white hot.
This is the “D”-line of the sodium emission spectrum. This is why, when you’re making pasta and the pot boils over, the stovetop flame turns from blue to yellow.
Flamework (torch) glass-blowers wear “didynium”-doped glasses which absorb at precisely this frequency, allowing them to see through the yellow glare. True story!
Maybe you can get borosilicate without sodium in it, but all of the glassware I’ve worked gives off a D-line. I think it’s usually Corning 7740 or equivalent, but it’s been a while.
Yeah, according to Corning, 7740 is 4% NaO - so it will still have some Sodium glow. I was under the impression that they avoided sodium, but clearly some is useful.
??? I’m asking, dude. I don’t know, and would like to.
Trinopus
Blackbody radiation is an idealized emission that should be independent of material, so that’s not what we have here. I think this would be an example of flame-induced atomic emission.
However, these are ions here. It’s not a situation like AAS where you’re atomizing the sample. I remember that not really mattering for the emission, but it does matter for absorption; glass transmits well around 589 nm (ZenBeam). But maybe the spectrum is too squishy and blurs out the gap? Not unusual for a solid. I haven’t looked at this stuff since undergrad, so hopefully someone a bit fresher on the details will stop by.