I’ve been learning to blow glass and I noticed when I started to use colored glass, that when you heat it up in the furnaces, the often intense and vivid color of colored glass seems to disappear to be replaced with little but a dull orange glow. However opon cooling the color immediately reappears. Why ? It took me a while to find what I think is the answer. At first I thought it might be the dull glow simply drowning out the, say, cobalt blue but it didn’t make sense to be – the glow is not that intense and the color is often really intense when the glass is cold. Example:

You can see the swirls of colored glass inside this vessel, but they just appear as slightly different intensities of reddish glowing. Once the vessel cools it will be vivdly colourful. Ok, so why ?

Color

Color in glass is created by dissolving various salts in the glass, often of transition metal salts, which have vivid colors. Some examples:

The colors arise because the electrons inside the orbitals of these compounds have quantum energy levels that have the right energetic gaps between them to absorb photons of wavelengths in the visible spectrum. Only photons with exactly the right energy will be absorbed. The absorbtion causes an electron to be promoted to a higher energy state. This electron can then return to its more stable state in a number of ways. It can either fall directly down to the original level creating a photon of the same wavelength as was absorbed previously or it can (if it has appropriate energy levels at it’s disposal) fall down in a number of smaller steps each emitting a photon of longer wavelengths (i.e. smaller) energy, say in the infrared spectrum, hence reducing the amount reflected light of the frequency previously absorbed.

So, a blue compound looks blue beacuse red and green photons are preferrentially absorbed by it. Blue does not get absorbed, only reflected and our eye will detect much more blue photons reflected by the object and hence our brains say “blue!”.

Black Body Radiation

Ok, now when you heat things (anything) it will start emitting radiation. The hotter you make it the shorter the wavelength of the emitted light. Or, in other words, the energy of the photons goes up. This radiation is called (unintuitively) “Black Body Radiation” or (more intuitively) “Thermal radiation”. Maybe sometimes when buying light bulbs or LEDs or flourescent lamps you’ve seen the label 3000K or 4500K or whatever ? That label is indicating that that light source will give off a spectrum of radiation close to what a ideal blackbody radiator will give of if it was heated to that temperature. Blackbody radiation is also what’s responsible for the glowing of the wire in your toaster or the glowing coals in your garden barbecue. The spectrum of the radiation is fairly wide but as things get hotter it moves to higher and higher frequencies. Therefore things first just feel warm (they emit invisible Infrared radiation) then glow red, over yellow to white because as things get hotter and hotter the wavelengths get shorter and shorter (frequencies become higher). In this plot the spectrum is shown for different temperatures.

The sun for example has a surface temperature of 5,778 Kelvin and radiates most strongly in the yellow to green part of the spectrum (note that the reason the sun appears yellow has to do with scattering of light in the athmosphere. In space the sun looks white).

What’s interesting that this general spectral behaviour is independent from the material you’re heating. Doesn’t matter what it is, the shape will be the same (that’s not quite true and we get to that in a minute, but roughly speaking it’s true). This means you can actually tell the temperature of anything glowing red or yellow hot by noting it’s temperature. This is something glassblowers and metal smiths have exploited for thousands of years. It also means that you can relatively accurately measure the temperature of objects by looking at the amount of infrared radiation they emit. This is the basis for contact-less thermometers such as this one:

Ok, back to glass. So what I said above is actually (it turns out!) not quite the truth. What actually happens is that the amount of thermal radiation given off by a compound at a given frequency is proportional to the amount of light usually absorbed by that compound at that frequency. Or, in simpler terms, a blue thing preferentially absorbs red and blue so if I heat it it will also give off red and green preferentially to blue light. The reason is that as you heat things you’re exciting the electrons inside the compound and you promote them to higher energy states. They can then fall down back to their original states giving of photon of the exact energy corresponding to the gap. Those gaps are the same gaps responsible for absorption too! so it makes sense that those two phenomena are related.

And so here we have the answer to the original question. Imagine your’re heating a blue compound. At first blue dominates the reflected spectrum. But as you heat it, it starts giving off red and green preferentially over blue and at some temperature the two match and the compound no longer appears blue to our eyes because red and green have “caught up” with blue. This will happen no matter what the original color was. As you heat the object further though the normal spectrum of the black body radiation will dominate any electronic transitions and the thing will simply glow red–>yellow–>white like anything else.