Burnt out
What controls the rate at which candles burn?
Combustion reactions require a supply of fuel and oxidant. In the case of a burning candle, it is the supply of molten wax to the wick that limits the rate of burning. Molten wax is sucked up into a candle wick by surface tension and capillary forces, the same forces that cause water to flow up into a dry piece of tissue paper.
This flow rate can be predicted by the Washburn equation, which states that the flow rate will increase with wax surface tension, decrease with increasing wax viscosity and decrease as the pore size of the wick decreases. This explains why, after some time, the wick of a burning candle reaches a 鈥減seudo-steady-state鈥� condition in which the wick remains at constant length. This is the maximum distance over which wax can flow toward the flame before being combusted. Beyond this length, the wick becomes dry and the wick fabric itself starts to burn.
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Simon Iveson
Cleveland, Queensland, Australia
鈥� A candle burns by operating as a self- sustaining capillary-action wax pump.
The wax itself will burn only as a vapour, not as a solid or a liquid. After the flame is established on the wick, the heat of the flame vaporises the liquid wax in the pores of the wick, as well as liquefying part of the solid wax of the candle below. The vaporised wax then acts as a protective barrier around the wick, with the vapours diffusing out through the flame while the oxygen diffuses in. This is why the wick does not burn up straight away.
This also explains why candle flames have different colours in them. At the base they are clear or blue as the wax vapour burns to completion, while further inside the flame, where oxygen is scarce, the reaction does not go to completion, resulting in hot carbon atoms which glow orange like natural flames. These carbon atoms rise through the flame and those not further reacted by the heat at the top of the flame cool down to become smoke.
As the solid candle wax is melted and drawn up into the wick, the wick embedded in the candle is progressively exposed. But the distance the liquid wax travels up the wick before becoming hot enough to vaporise does not change and so the top of the wick loses its cushion of insulating vapour, causing it to char and burn.
Adam Wray
Perth, Western Australia
The Chemical History of a Candle was one of the topics of Michael Faraday鈥檚 famous series of lectures given at the Royal Institution in the mid-19th century. Here is an edited extract where Faraday explains how you can see the hot rising air that shapes a candle flame by casting a shadow of the flame. Thanks to reader Robert Davidson for drawing our attention to Faraday鈥檚 lectures which can be found at or 鈥� Ed
鈥� 鈥淭here is a current [of air] formed, which draws the flame out; for the flame which you see is really drawn out by the current, and drawn upward to a great height. You may see this by taking a lighted candle, and putting it in the Sun so as to get its shadow thrown on a piece of paper. How remarkable it is that that thing which is light enough to produce shadows of other objects can be made to throw its own shadow on a piece of white paper or card, so that you can actually see streaming round the flame something which is not part of the flame, but is ascending and drawing the flame upward. Now I am going to imitate the sunlight by applying the voltaic battery to the electric lamp. You now see our Sun and its great luminosity; and by placing a candle between it and the screen, we get the shadow of the flame. You observe the shadow of the candle and of the wick; then there is a darkish part, and then a part which is more distinct. Curiously enough, however, what we see in the shadow as the darkest part of the flame is, in reality, the brightest part; and here you see streaming upward the ascending current of hot air, which draws out the flame, supplies it with air, and cools the sides of the cup of melted fuel.鈥�
Frost-free freezing
I鈥檝e just bought a new freezer that never needs defrosting. I understand how, in old freezers, condensation introduced each time the door is opened leads to surfaces frosting up. But I can鈥檛 work out how this is avoided in the new non-frosting models.
鈥� In conventional freezers, water in the humid air introduced by opening the door condenses and freezes on cold surfaces. Because these surfaces stay cold as long as the freezer is operating, the frost accumulates continually and can only be removed by scraping or raising the temperature inside the freezer by switching the freezer off.
The component in a freezer which absorbs heat from the surroundings is the evaporator. In a conventional freezer it is located in the food compartment with cold air circulating by convection. In a frost-free freezer the evaporator is located in a separate compartment and the cold air is circulated to the food compartment by a fan. The humid air drawn over the evaporator by the fan deposits its condensation on the evaporator and the resulting dry air is circulated to the food compartment, avoiding frost on food and other cold surfaces.
The frost that builds up on the evaporator gradually reduces its efficiency and must be removed. This happens automatically by means of a system that switches off the compressor (to stop the evaporator cooling), switches off the fan (to stop the warming air from circulating to the food compartment) and switches on a heater (to raise the temperature of the evaporator over 0 掳C).
The water produced from melting ice is usually collected in a trough and fed outside the freezer via a tube to drip onto the top of the still-hot compressor, where it evaporates in the atmosphere. After a predetermined time, the heater is switched off and the compressor and fan are switched back on, restoring the circulation of cold, dry air to the food compartment.
Graham Bailey
Cold Business Director Invensys Appliance Solutions Weingarten, Germany
This week鈥檚 questions
Who needs men?
Why is the ratio of men to women roughly equal? As a man can impregnate a woman more quickly than a woman can make a baby, the human race could easily survive on a ratio of, say, 50 women to one man, so is there another reason for the equality?
Julian Harrow
Southampton, Hampshire, UK
A long meal
How long does it take to digest food? And how do different foodstuffs vary?
Sylvia C
Los Angeles, California, US