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Ever Wonder About Candlelight?

At a rare, candle-lit dinner with family, my ten-year-old wondered all kinds of things about the candle I couldn’t explain, so I sought enlightenment on the internet.

Over a hundred years ago, the famous British scientist Michael Faraday delivered an illuminating talk on the science of candles. Candles are no longer such an integral part of everyday life, but much of what he said still holds true. A candle is still made of wax, which consists of hydrocarbons—strings of carbons and hydrogen atoms. And it still has a wick, which is absorbent. 

When you light the wick, the heat from the flame radiates outward and is carried through conduction down the wick, so the nearby wax melts in a little puddle. If the candle is made properly and the air currents are not too erratic, over time, a symmetrical cup forms at the top of the candle.

The liquid wax travels up the wick by capillary action. The heat of the flame adds enough energy to the liquid wax so it turns into gas and rises with the heat. The fuel continues to rise for a moment after you put out a candle. If you light the lingering wisp of smoke, the unburned fuel in it can relight the candle. 

Air enters the flame from the bottom. The hydrocarbons from the now gaseous wax break into hydrogen and carbon atoms. When heated up, atoms give off blue light, which is what you see at the bottom of the flame. The hydrogen and carbon atoms react with oxygen from the air and combine to form water vapour and carbon dioxide, a reaction that gives off heat. This is the hottest part of the flame, about 1400°C. About 3/4 of the energy from a candle flame is light and 1/4 is heat.

The optimal mix of oxygen and fuel occurs in an outer curtain, so the flames are hollow. Inside there is fuel vapour without oxygen. When fuel reacts without enough oxygen, some carbon atoms get left behind. These carbon atoms collect to form particles of soot. The carbon atoms can take on different arrangements. These include teeny-weeny diamonds, but no one has figured out how to collect them yet. When the particles of soot heat up, they glow, resulting in the bright yellow light in the top part of the flame. They give off heat in the neighbourhood of 1200°C, a little cooler than the blue flame. In the microgravity of space, a flame is round and mostly blue because the hotter gases do not rise.

Gradually, the wax gets used up and the candle gets shorter and shorter. Yet the same amount of wick sticks out the whole time. Only the tip of the wick gets enough oxygen to burn. It glows and burns up while the rest of the wick in the middle of the flame doesn't burn because it doesn’t have access to enough oxygen.

So the next time you have a candlelit dinner, never mind the romance...enjoy the science!

Are you interested in the science of light? Drop by before January 4 for our feature exhibition LIGHT: Illuminating Science and Art and do some of your own experiments with light and colour.