A candle consists of two primary components: a wick, and the solid fuel into which the wick is embedded. The wick must consist of some material which can catch fire quickly and which is also absorbent. In contrast, the fuel must not be able to burn on its own except at high temperatures above the heat of melting and vaporisation. It is vitally important to the operation of a candle that the fuel not be burned directly.
Burning a candle consists of five separate molecular processes.
1. Heat is applied to the wick until it catches fire (oxidises) and itself begins to produce heat.
2. The heat of the burning wick melts the layer of fuel immediately below it.
3. The melted fuel is drawn up through the absorbent wick by capillary action.
4. The melted fuel encounters the greatest part of the wick’s heat at the tip of the wick, and vapourises.
5. The vaporised fuel catches fire and itself begins to produce heat.
Steps 2 through 5 are self-sustaining so long as there remains oxygen, candleflame, and another layer of fuel to melt. After the initial lighting of the candle, the heat of the burning wick is almost entirely replaced by the heat of the burning vapourised fuel.
Not all of the candle’s burning combusts cleanly in a single step. The candle flame consists of at least two distinct parts which reflect two separate combustion processes: a blue area which is the hottest part of the flame, and a yellow or even reddish area. The blue area is where the vapourised fuel is burning. The chemical reaction here is the separation of hydrogen from the rest of the fuel, and the subsequent oxidation of that hydrogen to produce water vapour.
The remaining fuel, including the small portion of wick which is not involved in capillary action, coalesces around the wick as carbon chains, which we know better as soot. These are a direct product of the wick’s burning, and an intermediary product of burning the fuel. Most of these carbon chains are subsequently oxidised in the yellow part of the flame, producing carbon dioxide. The cleaner the fuel burns, the fewer the carbon chains released into the air as a result of incomplete burning, such as the smoke from a guttering candle.
Only the very tip of the wick actually burns. The rest of the wick is involved in capillary action, and is shielded from burning by the vaporisation of the fuel. This has to do with a chemical property called the heat of vaporisation, whereby the fuel absorbs much of the surrounding heat in order to change states from liquid to gas. This vaporisation, in turn, keeps the wick cooler than it would otherwise be.
To continue burning, a candle requires an adequate supply of fuel and oxygen, as well as continued contact of heat (candleflame) with the fuel to be vapourised. The higher the oxygen content of the air, the faster a candle will burn. The less constant the supply of oxygen or cleanly burning fuel, the more carbon chains will be released into the air.
Thus to extinguish a candle, one of the three elements of the candle process must be removed. A candle can be snuffed out by cutting it off from its supply of oxygen. A candle can be blown out by separating the source of heat from the fuel for just too long for the process to self-sustain. Finally, a candle dies when its fuel runs out. After a candle is extinguished, what remains of the unoxidised vapourised fuel solidifies around the wick and the candle holder in a thin layer.
