I remember well what happened to me during basic training in the U.S. Army at Fort Dix, New Jersey in the winter of 1959: “You, Green, will spend all night for a week in the basement of the Captain’s cabin making certain to keep the coal stove fire going—or else.” My crime was having gotten into my bunk, after lights out, with my boots organized right on the left and left on the right. Well, keeping that fire going was no easy task and a dirty job at that, but one thing I did learn on those lonely cold nights (the basement fire’s heat was mostly sent upstairs to the Captain’s quarters) was that there was a great deal of blue in that coal fire, especially when I stirred the coals. I compared that to the wood fires I made as a Boy Scout by rubbing two sticks together. Those fires were mostly yellow. I wondered why.
I seem to be connected to fire. When I went into the Army in 1959, they learned that I had a college degree with a major in chemistry. They had the perfect MOS (military occupational specialty) for me: flame-thrower operator. There’s plenty of chemistry in carrying a heavy tank on your back and spraying flames on people carrying guns. I got out of that job, which is another story.
Fires are oxidation, which means oxygen in the air is necessary to combine with some substance (which we call flammable). The flammable material, such as wood or gasoline, usually contains carbon (C) and hydrogen (H). If there is enough energy, if the temperature is high enough, the oxygen in the air can pull the carbon and hydrogen atoms out of the flammable substance and combine with these atoms to make carbon dioxide (CO2) and water (H2O), which are extremely stable molecules. Like a stable marriage, once these molecules are formed it is very, very difficult for other atoms to pull them apart to form different molecules. The carbon and hydrogen atoms in the flammable substances are not in such stable arrangements. When oxygen, which is a violently active element, comes near, it can tear their former atomic relationships apart. So in a fire, we are seeing atoms move from less stable arrangements to more stable arrangements. When an arrangement of atoms that is less stable is changed to one that is more stable, heat is released—the molecular world becomes less energetic, calmer, as if a storm has passed.
The released heat makes the fire hot. And hot a fire can be, reaching around two thousand degrees Fahrenheit for a wood fire. The molecules in the air around the fire, the nitrogen and oxygen, absorb that heat, and the energy they gain from the heat causes the molecules to move faster and faster, bombarding our skin and transferring that kinetic energy to us, warming us. It is the very heat released by the formation of CO2 and H2O that supplies the energy that can ignite something flammable near the fire—like the trees near the edge of a forest fire, or even your clothing or yourself if you get too close. More CO2 and H2O are formed releasing more heat, causing more flammable material to ignite and keeping the fire going.
This kind of transfer of heat is called convection and requires contact with a fluid medium, air or water. But the effect of a fire can be felt in another way, conduction, which is the mechanism leading to the burn you get when you pick up something that has been on a hot stove. In conduction, the heat is transferred through a solid material not by molecules moving through the material but by molecules jumping and spinning around in one place. This agitation spreads through the molecules in the material and eventually to you when you touch the material, the pot on the stove. Because both convection and conduction require molecules to transfer heat among one another, the process is not instantaneous. It takes time, which is the reason you can pass your hand quickly through a flame or touch something hot and rapidly pull away without getting burned.
Understanding the third way the heat from a fire reaches its surroundings helps to answer the question of where the color comes from.
All substances are always undergoing vibrations at the atomic and molecular level, but not like guitar strings. These vibrations involve charged particles, positive protons and negative electrons. The nature of the vibrations of these charged particles does not produce sound but rather something different called electromagnetic radiation. In scientific terms, light is electromagnetic radiation, and there is a spectrum of this radiation. We see only a very tiny part of that spectrum—the visible region of the spectrum, the colors we see. We feel other parts of the spectrum as heat—the infrared region. We use other parts of the spectrum to communicate by cell phones or listen to the radio. The dentist uses other parts to X-ray our teeth.
The flame from a wood fire has a lot of soot in it, and the vibrating charged atoms in the soot emit electromagnetic radiation—a large proportion of it in the infrared region, which we feel as heat. But also emitted from the black soot is electromagnetic radiation from another part of the spectrum, which we perceive as the color yellow, the source of the color of the wood fire.
The particles (soot) in the flame belong to a distinguished family of matter called “black bodies.” Black bodies are of great importance to science. To take just two examples: the sun emits black-body radiation; and the age of the universe is estimated by measuring black-body radiation from particles present at the time of the Big Bang.
The flame on a gas stove has very little soot in it, meaning there is not much black-body radiation. But that flame is hot enough to cause fragments (pieces) of molecules called free radicals to form alongside the CO2 and H2O. The free radicals emit their own special electromagnetic radiation. This is the source of the blue color, which comes from highly energetic fragments of molecules in the flame.
Something interesting to close this story is that we combine the C and H in the food we eat with the O we breathe to form the CO2 and H2O we breathe out, like a fire but without the flame. The energy released during this process warms us and gives us the energy to sustain all we do—keeping us alive. Each of us, one might say, is a fire under control.