Did you ever wonder how and why a candle actually burns? Why is it that a tiny piece of wick, that would normally turn into ashes in a few seconds if it wasn't at the heart of a paraffin block, can sustain a flame for hours and hours? Why won't paraffin burn without a wick?
Candles have take nsuch a place in our household universe that we don't even wonder how they burn anymore. And yet, the process that keeps the flame burning is an interesting and a fascinating one!
Two main components work together in a candle:
The wick needs to be naturally absorbent, like a towel, or it needs to have a strong capillary action (as in glass fiber wicks used in oil lamps).
If you buy a length of unwaxed wick (see article "Why and how prime your wicks?") at a craft store and play with it, you will notice that it feels like soft string and absorbs water very well. This absorbency is important in a candle because the wick needs to absorb liquid wax and move it upward while the candle is burning.
Paraffin wax is a heavy hydrocarbon derived from crude oil.
When you light a candle, you melt the wax in and near the wick. The wick absorbs the liquid wax and pulls it upward. The heat of the flame vaporizes the wax, and it is the wax vapor that burns.
You can prove that it is wax vapor, rather than liquid wax, that is burning with two experiments:
If you place one end of a metal or glass tube (shaped like a thin straw, 4 to 6 inches long) into a candle's flame at a 45-degree angle, you can then light the upper end of the tube.
The paraffin vapor flows up the tube and is the fuel for this second flame.
When you blow out a candle, you notice a stream of white smoke leaving the wick. This stream is paraffin vapor that has condensed into a visible form. It continues to form as long as the wick is hot enough to vaporize paraffin. If you touch a lit match to the stream, a flame will run down it and re-light the wick.
The reason the wick does not burn is because the vaporizing wax cools the exposed wick and protects it. You may have seen the camping trick of boiling water in a paper cup. The cup does not burn because the water inside cools it. The liquid wax does the same thing for the wick.
Paraffin wax will burn on its own (see article "Safety first" about this topic), but it is like cooking oil, motor oil and coal in that you have to get it very hot for combustion to begin. This is what the miracle of candles is all about: only the tiny amount of wax on the wick is hot enough to vaporize and burn!
As simple as it may look, a candle flame is actually a tiny world of wonder and complexity!
The combustion processes taking place in a candle flame are also extremely fascinating, not only from a scientific point of view but also because they give candlemakers precious informations that can help them make better quality candles. Knowledge is power.
Look carefully at the picture above. Area 1 points towards melted paraffin wax. You will notice that, at the base of the wick (area 3 on the picture), the flame is almost transparent. It is the "coldest" zone of the flame (1100°F). This can be explained by the fact that for combustion to happen, we need both a fuel and the presence of enough oxygen. Oxygen is naturally present outside the flame but most of it gets used before it reaches the area near the wick, which explains the relatively low temperature.
It is also in that zone, poor in oxygen, that most of the fragmentation and rearrangement processes of paraffin vapor molecules take place.
Remember that paraffin is an hydrocarbon. One of the main reactions is the separation of the hydrogen atoms ffrom the carbon chains.
Some of these carbon chains fragment to form gaseous (diatomic) carbon (C2, see below) and small molecules and molecular fragments.
Atoms of hydrogen separated from the wax molecules combine with oxygen atoms (coming from the air in the room) to form water (H2O) molecules.
Carbon atoms also combine with oxygen atoms to form carbon monoxide (CO) and carbon dioxide (CO2) but before this happens several of these carbon atoms combine together to create large (from a molecular point of view) pieces of a carbon-rich substance called soot.
If there is enough oxygen and not too much paraffin vapors (hence the importance of using the correct wick size), most of those soot particles are consumed in the flame and all the candle produces is light, heat, water and carbon dioxide.
If the flame is too "rich", some of the produced soot escapes the flame unharmed, so to speak, and ends up in the room.
On the picture above, you can see that the lower outside part of the flame is colored blue (area 2). That blue light is a result of the molecular emission of the gaseous (diatomic) carbon, C2, I was talking about earlier.
Just above the wick, the flame is orange (zone 4 on the picture). There as well, oxygen does not easily get through. The temperature there ranges from 1475 to 1800°F.
Above that is the combustion zone: the flame colors yellow (zone 5 on the picture). It is in that area that soot particles burn like charcoal and produce light, just like the filament of a light bulb does. The temperature in this area averages 2200°F. The closer we get to the outside of the flame, the more oxygen is present and available and the higher the temperature.