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Soot: Giver and Taker of Light

The complex structure of soot greatly influences the optical effects seen in fires

Christopher Shaddix, Timothy Williams

From earliest childhood, people are fascinated with the flickering yellow glow of candle flames and burning logs. However, few of us realize, even in adulthood, that soot—a material that epitomizes blackness—is behind that warm light. The 19th-century physicist Michael Faraday put it well when he said, "You would hardly think that all those substances which fly about London, in the form of soot and blacks, are the very beauty and life of the flame." Indeed, if it weren't for the presence of clouds of tiny soot particles within these fires, they would appear blue, like those on a well-operating gas stove, which give off considerably less visible and thermal radiation.

Figure 1. Where there's smoke, there's fireClick to Enlarge Image

We can vividly illustrate this phenomenon in the laboratory. Ethylene, for example, burning in air yields plenty of soot and a bright yellow flame, whereas the same fuel diluted with nitrogen to suppress the formation of soot gives a much dimmer blue flame. The cerulean color arises from the highly excited products of the combustion reaction. These molecules emit light in discrete spectral bands that correspond to the excitation levels of their electrons. One of the primary molecules is of a class called radicals because they have unpaired electrons in their outer shells. This radical is made from a single carbon and a single hydrogen, denoted CH*, which emits light at a wavelength of about 431 nanometers—squarely at the blue end of the visible spectrum.

In contrast, the light emitted from soot extends across the visible wavelength range and into the near-infrared. Cool soot looks black, but while it's in a flame, the heat liberated from the surrounding combustion reactions makes these particles incandesce like so many tiny light-bulb filaments. Gas molecules cannot absorb or emit such large amounts of energy across a range of wavelengths. As Faraday correctly surmised, it is the solid nature of soot that gives most flames their distinctive luminescent qualities.

A combination of factors explains why the radiation from soot typically appears yellow. Part has to do with way the human eye responds to the spectral variation in soot emissivity (the ratio of the light given off to the light radiated by a perfect black body). Another reason has to do with the typical temperature of soot (around 1,400 degrees Celsius). Hotter soot appears whiter and cooler soot will be redder, just as is true for heated metal. Thus soot often provides some pleasing optical effects. But these radiant abilities can also lead accidental fires to grow swiftly out of control. That is, although soot is the offspring of flame, it alters the nature of its creator.

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