Fire’s leaping, luminous tongues are familiar to us from fireplaces, campfires, candles and cookstoves. But do you really know what fire is?
Our ancestors didn’t, which is why they made fire central to myth, magic and religion. Many myths speak of a time when humans suffered because they couldn’t warm themselves; then they discovered fire in a lightning-struck tree, or copied the way the wind rubs branches together. (Do you suppose forest fires ever start by — naw, I guess not.) And of course there was Prometheus, who gave us fire and whom Zeus punished by chaining him to a mountain to be eternal eagle food.
Many deities have been associated with fire: Agni, a Vedic god, protected his people against darkness; Baal, the Biblically infamous god of Phoenicia, demanded human sacrifices in fire; the Romans had Vulcan (hence the term “vulcanize”) and the Aztecs had Xuihtecutle. (Gesundheit!)
Fire has been variously viewed as having cleansing qualities, as a representative of the eternal spirit — and, of course, as a means of eternal punishment. Today Yule logs, incense, eternal memorial flames and birthday candles still testify to the power of flames in our imaginations.
During the Middle Ages the four basic elements were considered to be fire, earth, air and water. Fire played a prominent role in the erroneous but catchily titled “phlogiston” theory of matter; but on the positive side, experiments with combustion finally led to the first table of atomic weights and the modern science of chemistry. In 1708 Sir Benjamin Thompson’s experiments with burning revealed evidence of heat as movement of particles, which led to the kinetic theory of gases and the laws of thermodynamics, the theoretical basis of the Industrial Revolution.
But despite all this, the details of flames themselves are still something of a mystery.
Dr. Norman Chigier, William J. Brown Professor of Mechanical Engineering at Carnegie-Mellon University in Pittsburgh (who must have extra-large business cards), defines flame as “A rapid gas-phase exothermic combustion process characterized by self-propagation.” (Uh…thanks. I guess.)
You might prefer the Encyclopedia Americana’s definition, “A clearly defined region that usually emits heat and light and in which combustion or oxidation reactions involving gases occur.”
What this all boils down to (sorry) is that flame is composed of various hot gases and other combustion by-products, all emitting light of various colours. A flame consists of a layer of hot gas surrounding a tapering column of cool, unburned gas (the dark area immediately around the wick in a candle flame). In the brightest part of the flame, still-unburned carbon particles emit light. In the outermost layer the carbon burns, too, resulting in less light but more heat.
A typical flame is around 1,230 to 1,530 degrees Celsius. It forms an upright tongue hot gas rises and because the air above it expands as it is heated, creating a partial vacuum over the flame.
Ash, soot and smoke are all unburned parts of the original fuel. Ash is made of minerals in the fuel that will not burn. Soot is unburned carbon, a result of there not being enough oxygen available to burn all the fuel, and smoke is basically soot suspended in mid air.
To form, fire needs fuel, heat and oxygen. Different fuels kindle at different temperature, but once a flame is produced it’s usually hot enough to kindle just about anything. For example, paper kindles at 232 degrees Celsius, wood at 190 to 266, cotton at 266, wood alcohol at 464 and natural gas at 482 to 632. Recall that a typical flame is over1,200 degrees, and you’ll see why fires keep going until they run out of fuel or oxygen — usually fuel. Once a flame forms, it provides the heat for further combustion itself (or, as Dr. Chigier puts it, it’s characterized by “self-propagation”).
A powerful destroyer but also a valuable tool, fire continues to fascinate us. There have even been songs written about it — but it’s probably a good thing Dr. Chigier didn’t write them.
“Chestnuts roasting on an open rapid gas-phase exothermic combustion process characterized by self-propagation,” I’m afraid, would never have been a Christmas classic.