LIGHTING:CANDLES

CANDLES

Candles are an operationally simple device for providing heat and light by means of a controlled flame. A candle is made up of two parts, the fuel made of wax, and the wick made from absorbent string. The wax fuel source is typically paraffin wax that is a heavy hydrocarbon that comes from the refinement of crude oil. Paraffin wax is very flammable and, much like other oils from petroleum refinement, it must be very hot for burning to take place. Once the combustion of the wax has started, it can be very difficult to put out. A candle controls the burning process by allowing only a small amount of the wax to burn. The wick of the candle is made of an absorbent material that absorbs liquid wax and carries it to the end of the wick by capillary action. When a candle is lit, the heat of the flame melts the wax on and near the wick. The wick absorbs the liquid wax, and the heat of the flame vaporizes the wax. It is actually the vaporized paraffin wax being emitted from the wick that produces the flame observed. You may have noticed that you can relight a candle by touching a flame to the smoke leaving a candle just after it has been extinguished. This smoke is actually paraffin vapor that is still hot enough to exist in the gaseous form. Touching a lit match to this stream of vapor allows the flame to run down the vapor trail and relight the wick of the candle. It is also interesting to observe that the wick does not burn significantly. This is because the heat required to vaporize the wax actually cools the wick and protects it from burning. Paraffin wax consists of hydro- carbons with typically between eighteen and thirty-six carbons in their chain. This mixture of chain lengths is actually essential for the success of wax as a candle fuel. Shorter-chained hydrocarbons tend to have lower melting points and boiling points. The shorter-chain hydrocarbons tend to vaporize first and provide the heat necessary to vaporize the longer hydrocarbon chains. In this way, the mixture of chain lengths ensures an easy-lighting candle that burns for a long period of time.