I was learning about black body radiation and the explanations given by Max Planck and Albert Einstein when a thought crossed my mind.

When we heat an iron piece, its color changes gradually from red, orange, yellow to bluish white. Yet such a change is not visible in a glowing piece of charcoal obtained from wood. Why is it that, wood charcoal is not able to glow in colors of higher frequencies?

  • $\begingroup$ This isn't directly an answer, but it will probably help your understanding to know that the blue color of a gas stove's flame is not caused by blackbody radiation, but Swan emission bands. The blackbody peak of a typical gas flame is in the infrared. $\endgroup$
    – zwol
    Oct 12, 2019 at 16:05

3 Answers 3


Any black body emits radiation of all frequencies. So a hot charcoal definitely emits some blue light. It doesn't appear blue, because most of its light is red, orange, or yellow. To shift the spectrum toward perceiving the white light, you must increase the temperature to the temperature of the Sun (5,778K), but this would destroy your furnace, as we don't have materials capable of sustaining this temperature. The highest recorded melting point is 4,215K of tantalum hafnium carbide. To make charcoal blue, you would need to increase its temperature even more, which is unrealistic in any existing furnace.

Hot iron doesn't become bluish white, but only yellowish, because it melts at 1,811K. You may see bluish colors on it after it cools down. They are caused by thin layers of iron oxide reflecting light differently for different wavelengths.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Chris
    Oct 11, 2019 at 7:50
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    $\begingroup$ Should add that you don't need something that can hold those temperatures, if you only have a small region that is heated compared to the area where it held. Halogen (or incandescent) lamps provide light as black body radiation. - Well above the melting point of the glass container. $\endgroup$
    – paul23
    Oct 11, 2019 at 14:10

Iron is supposed to glow blue-white at around 1600 degrees C. page 7.

Would carbon have that color at that temperature?

Spectral lines of iron

spectral lines of iron

Spectral ines of carbon

spectral lines of carbon

Maybe not. Maybe those bright yellow lines would add too much. Carbon might have to be hotter.

Do we have examples of carbon heated very hot? How about a carbon arc lamp? Pass a whole lot of electricity between two pieces of carbon. Some carbon vaporizes, and the hot carbon vapor gets very bright.

carbon arc lamp

Does this look bluish-white to you? Maybe it's possible, and maybe the charcoal has to be vaporized to do it. Can you find a way to get charcoal to do that by burning it with pure oxygen? I don't know. Can you get that color with solid charcoal, or would the carbon vaporize first? Carbon's sublimation point is 3642 C, so maybe.

Sorry I can't give a more definite answer.

(Here's a tip if you want to look for yourself. If you do an image search that includes "white-hot" or "blue-white-hot", be sure to set your censor. For reasons I didn't think of ahead of time, these searches produce a lot of porn.)

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    $\begingroup$ What do spectral lines have to do with the black body spectrum? $\endgroup$ Oct 10, 2019 at 11:57
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    $\begingroup$ Maybe they have nothing to do with it. My thought was that you get some light at the spectral lines -- like you see bright spectral lines from the sun -- and so they would contaminate the color from the heated carbon. An ideal black body would not have any spectral lines, because it would not be made of carbon or any other element. $\endgroup$
    – J Thomas
    Oct 10, 2019 at 12:02
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    $\begingroup$ Ah, the internet... it just wouldn't be the same without porn resulting from the most innocent queries. More on-topic, spectral lines are the results of electrons jumping between their possible positions in their respective "shells". Bleeding off heat through planck-spectrum/black body radiation has nothing to do with that. The only places where spectral lines are important in a macroscopic setting I could name right away is lasers. Probably TCL lighting as well, but I'd have to look that up. $\endgroup$
    – Gloweye
    Oct 10, 2019 at 13:13
  • $\begingroup$ @JThomas You actually see the opposite in the sun's spectrum: absorption at elemental spectral lines. One reason you don't get spectral line effects from solids is also because the energy levels are smeared out compared to lone atoms $\endgroup$
    – llama
    Oct 10, 2019 at 15:04
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    $\begingroup$ @Gloweye Spectral lines are also important in gas discharge tubes, which includes neon signage and fluorescent lighting. $\endgroup$
    – Hearth
    Oct 10, 2019 at 15:09

The glowing color of an object is based on its temperature. Wood Charcoal probably won't get hot enough to look blue. The sun is around 5500 degrees and emits all colors of em radiation. Wood charcoal will take a lot of help from the user (pumping air on the charcoal) to approach that required temperature and it will probably turn to ash before the temp is even reached. To glow with a blue color, the required temperature is probably greater than 8000 degrees.


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