Background Essay: On Fire

The anatomy of a candle flame is complex, and both physical and chemical reactions are involved in producing it. In a room with still air, a flame hardly flickers. It burns at a fixed distance above a pool of melted wax. Everything is in balance, as wax is consumed, light emitted, and heat produced. Enclosed within the flame's bright yellow tongue is a dark cone that rises from below the curl of the wick. At the bottom of the flame -- around the wick but not quite touching it -- is a blue region of lesser brightness that seems to envelop the base of the dark and yellow regions above it.

As a flame burns, heat radiates equally in all directions and melts the candle's solid wax into a hot liquid pool of usable fuel. Where the melted wax meets the heat of the flame, it evaporates, forming a gas. This vaporized fuel floats up from the wick and out to the edges of the dark zone. Here, wax vapor meets oxygen molecules from the air. This is the main reaction zone, where combustion takes place. Measuring about 1,400 degrees Celsius, this glowing blue region is the hottest part of a flame. The products of this combustion flow up into the yellow tongue of the flame, which is made up of carbon particles heated to incandescence.

Fuel molecules that don't burn up right away -- because not enough oxygen is present -- combine to form particles called soot, which swirl around inside the body of the flame without actually burning. Eventually, though, most of the soot enters the reaction zone and burns blue like the rest of the fuel. But if the reaction zone is not very efficient, the soot may escape the flame without burning. Outside the flame, soot cools quickly and drifts upward as black smoke.