# Why does melted glass glow in yellow? **URL:** \ **Category:** Factual Questions\ **Created:** [October 29, 2011, 8:20pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222 "2011-10-29T20:20:05Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Walrus12389](https://avatars.discourse-cdn.com/v4/letter/w/f6c823/32.png) [@Walrus12389](https://boards.straightdope.com/u/Walrus12389)\ **Post date:** [October 29, 2011, 8:20pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/1 "2011-10-29T20:20:05Z")
how come it’s transparent when it’s cool and opaque when it’s been heated… then it turns transparent again when cooled? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 29, 2011, 8:28pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/2 "2011-10-29T20:28:55Z")
**Why does melted glass glow in yellow?** Umm… because it’s really freakin’ hot? Look at this [photo](http://artbyky.com/wp-content/uploads/2011/08/Gather-of-molten-glass.jpeg). The glass is still transparent when liquid. It’s just that when it’s really, really, hot the glow obscures your view through it. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [October 29, 2011, 9:21pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/3 "2011-10-29T21:21:09Z")
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. ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [October 29, 2011, 10:43pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/4 "2011-10-29T22:43:52Z")
That’s the sodium doublet at 588.9950 and 589.5924 nm. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [October 30, 2011, 5:59am UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/5 "2011-10-30T05:59:19Z")
Is this an example of “black body radiation?” (Even if cold glass isn’t “black?”) Trinopus ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [October 30, 2011, 7:04am UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/6 "2011-10-30T07:04:06Z")
> [@Trinopus](#): > > 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. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [October 30, 2011, 7:39am UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/7 "2011-10-30T07:39:08Z")
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](http://smartflix.com/vidcaps/lvidcap_305.jpg?1297877504), 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 ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [October 30, 2011, 1:18pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/8 "2011-10-30T13:18:51Z")
> [@Bill\_Door](#): > > That’s the sodium doublet at 588.9950 and 589.5924 nm. 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. ---
**Author:** ![johnpost](https://avatars.discourse-cdn.com/v4/letter/j/f17d59/32.png) [@johnpost](https://boards.straightdope.com/u/johnpost)\ **Post date:** [October 30, 2011, 1:50pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/9 "2011-10-30T13:50:17Z")
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. ---
**Author:** ![bizzwire](https://avatars.discourse-cdn.com/v4/letter/b/958977/32.png) [@bizzwire](https://boards.straightdope.com/u/bizzwire)\ **Post date:** [October 30, 2011, 9:57pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/10 "2011-10-30T21:57:16Z")
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! ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [October 30, 2011, 11:26pm UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/11 "2011-10-30T23:26:31Z")
> [@Francis\_Vaughan](#): > > 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](http://smartflix.com/vidcaps/lvidcap_305.jpg?1297877504), 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 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. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [October 31, 2011, 2:53am UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/12 "2011-10-31T02:53:13Z")
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. ---
**Author:** ![Trinopus](https://avatars.discourse-cdn.com/v4/letter/t/2bfe46/32.png) [@Trinopus](https://boards.straightdope.com/u/Trinopus)\ **Post date:** [October 31, 2011, 5:06am UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/13 "2011-10-31T05:06:14Z")
> [@Trinopus](#): > > Is this an example of “black body radiation?” (Even if cold glass isn’t “black?”) > [@njtt](#): > > Why would you think that? . . . ??? I’m asking, dude. I don’t know, and would like to. Trinopus ---
**Author:** ![Ruken](https://avatars.discourse-cdn.com/v4/letter/r/f475e1/32.png) [@Ruken](https://boards.straightdope.com/u/Ruken)\ **Post date:** [October 31, 2011, 11:06am UTC](https://boards.straightdope.com/t/why-does-melted-glass-glow-in-yellow/601222/14 "2011-10-31T11:06:45Z")
> [@Trinopus](#): > > ??? 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. > **[Atomic emission spectroscopy](https://en.wikipedia.org/wiki/Flame_emission_spectroscopy)** > > Atomic emission spectroscopy (AES) is a method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line in the emission spectrum gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element. The sample may be excited by various methods. > Atomic Emission Spectroscopy al... > **[Emission spectrum](https://en.wikipedia.org/wiki/Emission_spectrum)** > > The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to electrons making a transition from a high energy state to a lower energy state. The photon energy of the emitted photons is equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different rad... 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.