You’ve had a long day at work and a tumultuous drive home, punctuated by swearing and honking your horn. You get home for a much-needed sit down and tune in to watch your favourite evening show when unceremoniously the TV goes blank and all the lights go out. You peer out the window to find everyone in your street is experiencing the same disaster. At this point we turn to one of the world’s least celebrated inventions, the candle (or a wind-up torch, but for the sake of this story we’ll pretend they don’t exist).
Although appearing relatively simple, candles are ingeniously designed: candles make use of combustion (burning) to produce light and heat. As soon as the wick – typically made of cotton – is lit, it starts to burn. Heat then rapidly travels down the wick to the wax beneath it, and because candle wax has a very low melting temperature, the solid wax becomes a hot liquid. The wick draws the liquid wax up towards the flame by a process called capillary action. The heat of the flame then vaporises and burns the liquid wax, producing the tall bright flame, and all in less than a second!
The distinctive shape of a candle flame is due to their use of ‘convection currents’. As the flame burns, the air around it heats up, and rises. As this occurs, cool oxygen-rich air moves in at the base, creating a continuous cycle of air moving upwards around the flame. This current of air ensures the flame always points up and therefore gives it its distinctive shape. The colours that you see within the candle flame reflect the type of combustion that is taking place at different heights: at the bottom, the flame is blue where ‘complete’ combustion occurs but higher up it is yellow because the burning is ‘incomplete’.
A flame turns blue where there is lots of oxygen available, meaning that all the components of the wax are burned and turned into gas. Higher up the wick, the flame has already exhausted much of the oxygen in the air, so the wax does not completely combust. The result is soot – which we see as a yellow coloured flame as super-hot smoke streams upwards. Because of the colouring, flames are difficult to see at the base and, due to the shape, this can give the appearance the flame is hovering.
Interestingly, NASA have performed a series of candle-based experiments in space. When a candle is lit in low gravity, the warm air does not move ‘up’ (there is no ‘up’!) and therefore doesn’t create a convection current. The candle flame forms a sort-of sphere, rather than the teardrop shape we see on Earth.
Back on Earth, the upward-pointing nature of candle flames means that it is actually pretty difficult to ‘burn the candle at both ends’ – at least with a straight candle. One way to do it is to make a sort of candle see-saw. In the video below, when the see-saw tips one way, the flame on the lower candle is in direct contact with the wax, causing it to melt extremely rapidly. This reduces the weight on that side of the pivot, and the see saw swings the other way! However, if you are trying to light up your house following a power cut this is not such a good idea, as you will go through your candles faster than you can say “honey we have run out of under-appreciated, soon-to-be-forgotten hydrocarbon-based means of producing light”.
Asked by Damion Perez