The Science of Candles

The Science of Candles

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, but 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!

How Candles Use Combustionhow a candle burns
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.
Candles don't burn all by themselves. It takes energy to kick-start the chemical combustion reaction that makes the wax burn. The initial energy you need to start a chemical reaction is called activation energy. You can provide it using a burning match.

What is Wax Anyway?
 candle waxYou might think wax is just... well, wax, but it's actually quite a tricky thing to define.
The word "wax" is a bit like the word "plastic": it refers to a collection of different substances with similar properties. Just as we should talk about "plastics" (because there are many different ones), so we should talk about waxes.

Waxes are mainly defined by their physical properties, not their chemical properties. For our purposes, we can think of wax as a complex mixture of fatty organic chemicals that has waxlike properties:

  • It has a relatively low melting point above room temperature (50°–90°C) and melts without decomposition above 40°C
  • It has relatively low viscosity just above the melting point
  • It has no viscoelasticity (deforms and gradually returns to shape after a force is applied)
  • It can be polished (buffed) and becomes plastic above 20°C with slight pressure
  • It burns with a sooty flame (the characteristic property of a candle)
  • It's a poor conductor (of both heat and electricity)

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 (it also carries down into the solid base of the candle, which gets noticeably warm to the touch). 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.

  1. Wick: 400°C (750°F)
  2. Blue/white outer edge of the flame (and also the blue cone underneath flame where the oxygen enters): 1400°C (2550°F)
  3. Yellow central region of the brightest part of the flame: 1200°C (2190°F)
  4. Dark brown/red inner part of the flame: 1000°C (1830°F)
  5. Red/orange inner part of the flame: 800°C (1470°F)
  6. Body of the candle: 40–50°C (104–122°F)
  7. Melted pool of wax on top of the candle: 60°C (140°F)

candle burn tempPerhaps 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, an 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.

Do Candles Make Pollution?
Yes, but some make more than others. What you have in a candle is a flame burning a mixture of hydrocarbons in the air (oxygen and nitrogen), so, apart from making steam and carbon dioxide, it will also make small amounts of carbon monoxide, nitrogen dioxide, and give off quite a lot of "particulates" (those sooty unburned carbon particles). In one anecdotal study reported by The Times newspaper, a scented candle rapidly produced twice the level of fine particulates (known as PM2.5s) recorded at a bus stop in a busy street in central London. Air-quality researchers have also found that paraffin-based candles give off toxic chemicals such as benzene (a known carcinogen) and toluene (which has numerous adverse effects if you breathe enough of it); technically these are known as volatile organic compounds (VOCs), because they evaporate readily, and polyaromatic hydrocarbons (PAHs), because of their chemical structure. Scented candles can also produce formaldehyde, another known carcinogen. What matters with air pollution is generally your exposure (how much of a pollutant you breathe in, in total, over a period of time). If you light candles individually and only once in a while, they're not likely to cause enough indoor pollution to worry about; other sources of air pollution, both in your home and outside it, are likely to be far more significant.

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