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Orange flame and blue flame: what causes the difference? metal vapors?

  1. Feb 19, 2013 #1
    What causes flame color? I know light color corresponds to photon energy which is related to electrons relaxing from a high to low energy state.

    I'm not sure how to conceptualize this situation though.

    My professor was heating a 0.8 wt% carbon steel rod, in austenite phase, with a blow torch until it started glowing. I assume it was still in the austenite phase.

    During the heating, the flames from the blow torch were blue until the metal started glowing orange. At this most of the flames changed color to orange. There were still some blue flames at the outlet of the blow torch but those that were past the piece of metal were all orange.
  2. jcsd
  3. Feb 19, 2013 #2
    Re: Orange flame and blue flame: what causes the difference? metal vap

    it may be that metal atoms or clusters are being vaporized from the surface by the hot gas flow across it, at which point they'll start emitting light at their characteristic frequencies. this is actually an analytical technique for trace metals in organics or solutions called atomic emission spectroscopy.
  4. Feb 19, 2013 #3
    Re: Orange flame and blue flame: what causes the difference? metal vap

    Blue means hotter than orange flame. That is why blue stars are hotter than yellow/orange stars just like Betelgeuse (red giant) which is located in the orion constellation. The cause of the blue flame is just that is coming out really hot due to the introduction of more hydrogen or oxygen atoms (I think), you can tell by the color of the flame. Also remember from the spectrum, visible blue light has shorter wavelengths than red and thus the energy is higher. photons with shorter wavelengths have more energy than photons with longer wavelengths. That is why radio waves don't kill you, but Gamma rays and x-rays are dangerous.
    Last edited: Feb 19, 2013
  5. Feb 20, 2013 #4
    Re: Orange flame and blue flame: what causes the difference? metal vap

    I also think the orange glow is due to admixtures originated from heated steel surface. It may be any solid particles such as ferrum oxide, or may be soot. Moreover Fe atoms have a lot of emission spectral lines within the visible range, and there may be some other admixtures, such as Na atoms which have very intense orange emission lines.

    Different metals give different colors to the flame. For example Cu gives green or blue color (I dont remember exactly), Co - blue, and Cr - green.
  6. Sep 20, 2013 #5
    In this case I am pretty sure that the color of the flame is due to black body radiation. The blue flame is hot, so it glows blue. Once the hot gas passes the steel, it transfers a lot of it's heat to the steel so it is cooler and glows orange.

    The flame only started glowing orange once the steel was glowing because hot air and cold air don't like to mix, so their was a thick boundary layer of cool air around the steel that prevented the flame from transferring heat efficiently.

    Iron boils at 3000K which is white hot. I don't think you would see any sort of electron relaxation light until the steel got really close to that point. Carbon is even hotter.
  7. Sep 20, 2013 #6
    The blue flame is not blue because of black-body regulation. That is a misconception.

    A black body at the temperature of a typical blowtorch flame (1500-2000 C) is reddish-orange. There are no black bodies in the typical blowtorch or Bunsen burneflame, which is why it isn't reddish-orange.

    The blue color of the flame comes from the emission spectra of various hydrocarbon radicals. See, for example, this spectrum.

    The orange color of the flame after passing the bar probably comes from the emission spectrum of iron atoms. It doesn't take very many to produce a very bright color. If you put the bar in the oxygen-rich part of the flame, you may also get sparks as larger particles of iron burn. Of course, there is also a ton of junk on the surface of the iron bar that could provide its own color to the flame.

    If the "metal emission spectrum" theory is correct, then doing it with a copper rod instead of an iron rod should produce a bluish-green flame beyond the bar (at least after all the extraneous junk has been burned away). If this happens, it can't be explained by any theory of blackbody emission, which would disprove that theory.
  8. Sep 20, 2013 #7


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    no that is incorrect

    I agree with chill_factor and eigenperson on this one

    the flame is initially blue primarily because that is the emission spectra of the gas/gas mixture used to create the flame any other material iron, copper etc will colour the flame according to its emission spectra

    Last edited: Sep 20, 2013
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