What Makes Campfire Flame Yellow – Carbon Nanoparticles or Sodium Salts?

In numerous articles posted on the Internet, there are two opinions on why campfire flame is yellow. Some publications say that the yellow color occurs due to the emission of sodium atoms; the others suggest glowing carbon nano-particles are the cause.

The project's goal is to clarify the contradiction experimentally and get the correct answer. To achieve this, I measured the visible radiation spectrum of the flame, which helped to explain the observations. 

Four objectives were set:

·        Design and manufacture a spectrometer, and then perform its calibration. 

·        Carry out the necessary experiments. 

·        Explain the obtained results.

·        Publish the work results. 

The spectrometer was designed and manufactured to operate in the range 400-700 nm with a resolution of 0.3 nm. Replaceable optical slits with a width of 50, 100, 200, and 300 microns were used. 

A diffraction grating with a step size of 740 nm was made from a DVD-R disc.

Replaceable optical slits with a width of 50, 100, 200, and 300 microns.

The spectrum was recorded with an SLR camera with a large lens aperture, a large sensor, and manual settings. The device was mounted in a sturdy case. 

The spectrometer was calibrated before each series of experiments on the known spectrum of a compact mercury fluorescent lamp.

Processing of measurement results (measurement of the wavelengths of the investigated spectrum) was carried out as follows:

The spectrum of the calibration lamp and the spectrum under study were combined in one frame. Knowing the location of the mercury lines, the desired wavelength was determined by measurements and subsequent calculations. The measurements were carried out with an accuracy of one pixel of the camera sensor matrix, corresponding to 0.1 nm. Three pixels were required for a reliable registration of the spectral lines. The width of half of the spectral line was 0.3 nm; therefore, the spectrometer resolution is no worse than 0.3 nm. Given that the location of the centers of the lines can be determined with an accuracy of 1 pixel, the wavelength was established with an accuracy of 0.1 nm.

Typical homemade spectrometers, information about which can be found on the Internet, have a resolution of more than an order of magnitude lower - from one to several nanometers. They are not suitable for such measurements. 

Software from the Spectral Workbench website was used to construct the spectral curves. 

To ensure the correct operation of the spectrometer, the following measurements were carried out. The spectral emission of the Sun and its absorption lines were recorded. The spectral lines of three lasers were measured, with wavelengths of 405 nm, 532 nm, and 650 nm. Experiments were performed to measure the concentration of different solutions. The spectrum of the gas burner flame and the spectra of lamps of various types were measured. 

The study of the campfire flame was the main experiment of the project.

Measurements were carried out with birch wood in the fireplace. 

Two bright lines were registered on a background of a continuous spectrum.

The first novel result was obtained in the flame of a campfire. Two bright yellow lines were registered, with wavelengths of 589.0 nm and 589.6 on a background of a continuous spectrum. Both lines were measured with an accuracy of 0.1 nm. According to the NIST database, these are the spectral lines of sodium. 

The second and more meaningful novel result was a measurement of the behavior and appearance of the aforementioned lines. While the campfire was heating up, the spectral lines of sodium were not present [1]. As coals began to form and the intensity of the campfire flame increased, the lines appeared [2] and their brightness was steadily growing [3]. During this process, the intensity of the continuous background emission did not change. The color of the campfire remained the same. 

The spectral emissions of the campfire ash were also measured. Samples were placed in ethanol, and the ethanol ignited. In the spectrum of this flame, the same spectral lines were observed, i.e. the bright lines in the flame spectrum are spectral lines of sodium, which was present in the ash.

The appearance of the sodium lines can be explained in the following way. Sodium salts were present in the birch and after combustion they remained in the ash. In the process of combustion the quantity of ash increased resulting in the increasing amount of sodium in the campfire flame. Therefore, the intensity of sodium spectral lines increased with time.

In the campfire flame, the carbon atoms form particles of soot up to 100 nm. They provide the black-body radiation with a maximum frequency in the infrared range, and the intensity of emission falls in the visible spectrum with the decrease of the wavelength. Such spectrum is perceived as yellow light. 

Consequently, the color of the flame, which the brain interprets as yellow, is composed of two completely different types of emission: (1) continuous black-body radiation of the carbon particles and (2) sodium spectral emissions.

I could not find such spectra and similar conclusions on the Internet.

In the process of analyzing the experimental results an interesting hypothesis was presented. The fact that our eyes perceive the constant spectrum of glowing carbon nanoparticles and the two bright lines of sodium as the same yellow color may not be a coincidence, but an evolutionary trait that came about in the course of almost a million years that had passed since humanity began relying on firelight at night. Similarly, humans adapted to seeing the sun as a source of white light. In support of this hypothesis, the eyesight of other mammals may have developed differently. But this is a question which is still being explored by relevant papers on the evolutionary development of eyesight and perception of color.

To test the hypothesis, I have conducted the following experiments: alcohol, alcohol with ash, and alcohol with soot were placed in three different vessels. I lighted them simultaneously. Visual observation: Alcohol burns with a blue flame, soot and the ash burn with a yellow flame with the same shades of yellow. The ash spectrum is a double sodium line while the spectrum of soot is a continuous spectrum of carbon particles. So the nature of these spectra is entirely different.

It is known that a person can distinguish up to a million shades; hence the probability of coincidence is 1/1,000,000. Even if I could distinguish 1,000 colors, I would still have a 1/1,000 chance of a random coincidence. Such a small probability could be evidence that this is not a random coincidence but was the result of evolution after humans began to use the campfire.

Archaeologists have determined that humans have been using a campfire for over a million years. My experiment proves — has proven that either the evolution of color vision took place over a million years or it lasted longer time, and man has been using a campfire for well over a million years. 

Conclusion: The campfire's flame played an important role in the evolution of color vision.

In the course of multiple experiments it has been determined that (1) in the campfire flame there are two absolutely different and not connected with each other mechanisms which provide the black-body radiation of carbon particles on the one hand and the monochrome sodium salts emissions on the other hand.

The colors of both are interpreted by a human brain as yellow (2). 

The probability of a coincidence in the perception of these different kinds of emission can be estimated by percent fractions.

However, this coincidence can be believably explained by the two types of evolution – an eye structure changing and the adaptation of information processing by brain.

In addition to that, the hypothesis on campfire flame influence upon the evolution of human color vision could also explain some other well-known facts: the trichromatic vision appearance, the asymmetric (namely: 430 nm, 530 nm и 560 nm) location of the three types of cones perception maxima in the retina.

Besides, this hypothesis could answer the following questions: why the color of the campfire flame does not change while burning? Why the perceived fire brightness is maximal as well as why color vision works both during the sunshine and the light from the fire though the spectra of these light sources are different?

I wanted to answer the question: "Why is the campfire flame yellow?" —  and I have got interesting new results in engineering, physics and biology.

1. I have designed and manufactured a spectrometer with characteristics that are unique to DIY instruments.

2. I measured the spectrum of a campfire and found that a person perceives yellow color due to two completely different reasons: black-body radiation of carbon particles and sodium radiation at two wavelengths.

3. I came up with and made simple and illustrative experiments: burning soot and ash simultaneously in different vessels, I registered the same colors. In fact, I split the yellow of the campfire into two different parts.

4. I hypothesized that the campfire influenced the adaptive evolution of color vision and substantiated this hypothesis using the theory of probability.

As far as I know, no one has made such experiments and similar conclusions yet, so I have published the results.