The science of candles
by Chris Woodford. Last updated: May 30, 2014.
Imagine if there were no electricity and you had to survive up to 12 hours of darkness each night by candlelight! It sounds wonderful in our age of cold, sterile, fluorescent light. But if you had to live that way all the time you'd find it an awful lot of bother, especially if your house had many candles, all burning at once. You'd not only have to keep the wicks burning brightly, you'd also have to ensure they weren't going to tip over and cause a fire. Drawbacks aside, candles will always be a symbol of romance. Look more closely and you'll also find they're classic examples of ingenious technology. Let's take a closer look at how they work!
Photo: Who says science isn't romantic... or romance isn't scientific? Candles are a great example of how science adds an extra dimension to the beauty of the natural world—a point the brilliant American physicist Richard Feynman was fond of making. Listen to him discussing the question: Can a scientist really enjoy the beauty of a flower?
How candles use combustion
Candles make light by making heat, so they're crude examples of what we call incandescent lamps (old-fashioned, electric filament lamps, pioneered in the late 19th century by Thomas Edison, are a much more sophisticated version of the same idea). All the light a candle makes comes from a chemical reaction known as combustion in which the wax (made from carbon-based chemicals typically derived from petroleum) reacts with oxygen in the air to make a colorless gas called carbon dioxide. Water is also produced in the form of steam. Since the wax never burns perfectly cleanly, there's also a little smoke produced. The smoke is an aerosol (tiny particles of solid, unburned carbon from the wax mixed in with the steam) and it often leaves a black, carbon deposit on nearby walls or the ceiling above where the candle's burning. The steam is made in the blue part of a candle flame, where the wax burns cleanly with lots of oxygen; the smoke is made in the bright, yellow part of the flame, where there isn't enough oxygen for perfect combustion to take place.
Artwork: How a candle works: A candle is a miniature chemical factory that converts the hydrocarbons (molecules based on the atoms hydrogen and carbon) in wax into carbon dioxide and water (steam) through the chemical reaction we call combustion. Oxygen is pulled in at the bottom, fuel is drawn up the wick, and heat is given off at the top where the hot air rises.
How a candle wick works
Candles may look simple but they're remarkably ingenious. Set fire to the wick (the little string poking up at the top) and heat travels rapidly downward toward the wax body of the candle beneath. The wax has a low melting point so it instantly turns into a hot liquid and vaporizes, funneling straight up around the wick as though it's rushing up an invisible smokestack (chimney). The wax vapor catches light and burns, sending a flame high above the wick. Heat from the flame travels in three directions at once by processes called conduction, convection, and radiation. Conduction carries heat down the wick to melt more wax at the top of the candlestick. Convection draws hot wax vapors out from the wick and sucks oxygen from the surrounding air into the base of the flame. The flame also gives off invisible beams of heat in all directions by radiation. The candle continues to "feed" on the wax underneath it until it's all burned away—until all the potential energy locked away in the wax is converted to heat, light, and chemical waste products.
Which part of a candle flame is the hottest?
Here are some approximate temperatures for the different parts of a candle and its flame. Note that the exact temperatures vary quite a bit depending on all kinds of different factors, notably the type of wax from which the candle is made but also the ambient (air) temperature, and how much oxygen is present. Please don't take these values as absolutely definitive ones that apply in all cases—they're just a rough guide.
- Wick: 400°C (750°F).
- Blue/white outer edge of the flame (and also the blue cone underneath flame where the oxygen enters): 1400°C (2550°F).
- Yellow central region of the brightest part of the flame: 1200°C (2190°F).
- Dark brown/red inner part of the flame: 1000°C (1830°F).
- Red/orange inner part of the flame: 800°C (1470°F).
- Body of the candle: 40-50°C (104-122°F).
- Melted pool of wax on top of the candle: 60°C (140°F).
Perhaps surprisingly, the brightest part of the flame is not the hottest. The blazing part of the flame gives off three quarters of its energy as light and only a quarter as heat (so you can see a candle is, at best, around 75 percent efficient as a lamp). The hottest parts of a candle flame are actually the blue, almost invisible area near the base, where oxygen is drawn in, and the blue/white part around the edge, where the flame meets the oxygen-rich air all around it. The flame gets progressively cooler as you move in from the outside edge toward the wick. Cooler areas are darker and colored orange, red, or brown. Most of the flame's heat is delivered toward the tip, where a large volume of gas is always burning and convection is sweeping hot gases constantly upwards. If you want to heat something with a candle, hold it near the tip.