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How a Candle Burns

May 25th, 2009

how-candles-burn.jpgThe most important thing to remember about how a candle burns is that it requires steady fuel as well as a container. Brilliantly, the wax of a candle acts as both fuel and container, and while some fuel leaks from the edges of a freestanding candle, the rest is still the primary component in fueling the flame.

So what role does the wick play? Consider it a wax conduit. Usually comprised of braided cotton, it needs to be very absorbent in order to soak up all of the liquid wax. The braiding in the cotton creates a thin tube, and the tension in this tube leads to capillary action — the tendency of a small tube to give liquid nowhere else to spread but along it. This same process is what allows a sponge to soak up water. In a candle, the wick itself barely burns at all.

While the flame melts the wax so the wick can absorb it, the flame’s heat also turns the melted wax to vapor. This gives the wick’s capillary action process room to bring more wax upward to the flame. It’s the wax vapor, rather than the liquid wax, that burns; in fact, if melted wax was immediately flammable, holding a lit match to a pool of wax would create an explosion! Liquid wax requires much more heat to produce combustion than is typically available around a candle flame.

The candle’s process keeps the heat high enough to continue melting the wax, bringing it to the flame, and turning it into vapor to keep the heat high enough, thus creating a self-sustaining cycle. This constant cycle continues until one of the components (the wax, the flame or the wick) is taken out of the equation, either by running out of wax, blowing out the candle, or breaking the wick off.

You’ve probably noticed that when you blow out a candle, white smoke with a very characteristic “candle smell” continues to emit from the tip of the wick for a while. The white part of this smoke is the wax vapor. This vapor continues to be emitted until the tip of the wick loses the amount of heat required to vaporize the wax. Since the flame is gone, this doesn’t take long — just long enough to stink up a room a little bit.

There’s a reason a burning candle is often used as a meditation aid: stare at one long enough, and its function looks like some kind of magic. This isn’t only due to the flame. Look closely (mind your eyebrows!) at the base of the wick in a candle that’s been burning long enough to create a small pool of wax there, and you may notice some extra movement: the debris in the pool moves toward the wick, then away from it, then back to the wick again in an undulating motion.

The debris, which is mostly comprised of little flecks of burned wick, is drawn in under the surface with the motion of heated wax, then lifted up and pushed away with the motion of cooled wax, like helpless little swimmers in an undertow or current. This ties in with why the wick itself barely burns — it’s cooled by the process of the wax vaporizing, just as is a small amount of the liquid wax. Turning the liquid to vapor requires energy, and as the heat energy is used up by the transformation, it is taken away from the surrounding area (the wick and the liquid wax), resulting in slight cooling.

While it does not take place in a candle, it is possible to create fire with heat, wax, and no wick, but it requires a third ingredient called liquid soda for its carbonation. Yes, this does sound like an interesting research topic, but remember: trying this is a dangerous experiment that should only be performed with small quantities of ingredients, full sobriety, and the close supervision of mature, informed adults.


Currie Jean holds a B.A. in creative writing and is an experienced writer with work published both in print and online. Native to Ontario, she works full time in technical support and writes as much as she can in her spare time.


2 Responses to “How a Candle Burns”

  1. comment number 1 by: mary

    oh yeah im doing this for a sciencefair project but could you make this page shorter I DONT OR ANYONE ELSE WANTS TO READ IT CUZ ITS SOOOOOOOO LONG but thanks for the information !:)

  2. comment number 2 by: lila

    same as mary you could take out the last two paragraphs we dont need them

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