It’s the world’s greatest disappearing act: when a candle burns, where does the wax go?
At first, it seems that the wax simply melts, puddling at the top or flowing down a taper’s sides. And who hasn’t watched in dismay as a birthday candle drips onto the frosting? But observe a candle long enough, and you’ll notice that as the candle burns low, only a few drops of wax escape and solidify. The rest seems to melt into thin air. In a manner of speaking, that’s exactly what happens.
Despite popular belief, the cotton wick is not the source of fuel for the candle’s flame. Wax is. Though it forms the body of the candle and can be colored or scented, wax’s true purpose is to catch fire. By itself, a piece of string would easily burn up in seconds. Wax, however, burns much more slowly, allowing candles to flicker for hours—even days.
Once a wick is lit, the candle flame heat quickly melts the wax at the wick’s base. The wick soaks up the melted wax. Upon meeting the intense heat of the nearby flame, the liquid wax is vaporized, transformed into hot gas. The gas molecules then react with oxygen in the air and ignite. This is why solid wax doesn’t simply catch on fire: only wax vapor mixed with oxygen can burn. To become vapor, solid wax must first melt, then encounter the extreme heat of a wick’s flame.
As the wax vapor burns, it allows the process to continue: the steady flame ensures a constant puddle of melted wax, which continuously flows up the wick, vaporizes, and catches fire.