Every year candle fires can be attributed to over 100 civilian deaths, as well as nearly 900 fire fighter injuries. They represent nearly 4% of civilian home fire deaths (NFPA stats). Candles are by far a serious and not to be misunderstood beast. On the days after where many observed Earth Hour, thousands would have lit these beasts, and many just might not understand the complexity behind them.
The complexity of a lit candle is profound when you think about it, and its natural to understand why they can cause so much damage when you understand what the science is behind a lit candle. I find that they are useful teaching aids to illustrate the complexity of a flame. So this entry today explores this science in a really basic way (but more so that you the reader do not explore this on your own and become part of the above statistic).
Lets consider an ordinary candle. The candle includes a simple wick at the center surrounded by (parraffin) wax. When the candle is lit by a match, it sets off a complex number of reactions which produce a visible ‘fire’ that appears ‘clinging’ to the top of the wick.
Candles have been studied for centuries, and profoundly by individuals such as Michael Faraday (see my recent paper which discusses him). I love the quote provided by Crookes to the start of Faraday’s six lecture Chemical History of a Candle book;
“…Surely, among the millions of fire-worshippers and fire-users who have passed away in earlier ages, some have pondered over the mystery of fire; perhaps some clear minds have guessed shrewdly near the truth. Think of the time people have lived in hopeless ignorance: think that only during a period which might be spanned by the life of one, has the truth been known “
I think its profound when you apply it to even our fire engineering practice today and the amount of knowledge generated in the last 60 or 70 years of fire dynamics. Even though it was intended to be apply to the Victorian mind. But Faraday was onto describing (rather teaching in an effective way) science behind fire.
For instance if you were to estimate the temperatures of the visible flame with the candle, what would you think the temperatures are? If you crudely use a thermocouple to measure temperature, you see well over 1000C near the candle’s perimeter. If you are careful, you may also observe a temperature dip near the center of the ‘flame’. More strikingly (if your eyes do not blind from the brightness – like mine do when studying flames), you can see distinct regions in of different ‘colors’ or rather ‘luminescence’ of the flame. Maybe above the candle you may see some ‘smoke’. And if you lit it for a festive occasion (upon which house fires caused by candles are most predominate) you may question what is causing all of these ‘behaviors’.
When one ‘lights’ a candle you melt the wax on the surface of the wick and also near the base of the wick. The wax becomes a liquid and eventually gets hot enough that it becomes a gas. The gas emitted mixes with the surrounding oxygen. Your heat source ‘ignites’ this gas. While this is on going, the wick, through a ‘straw like’ capillary action, draws up more molten wax, which turns to vapor and continues the combustion process. Fueled by an abundant amount of oxygen at the base of the flame and perimeter, you typically see a blue hue, or very dark black hue. Its an efficient combustion there. The wax is mostly breaking down into carbon dioxide and water. But above that dark hue of a flame, is a bright yellow redish zone. That flame is a sign that soot is being created, emitting visible light. On the outside of the flame you get a very efficient combustion process as you have a good amount of oxygen, but as you move into the visible flame, there is less oxygen available and the reaction is limited by the amount of oxygen that can diffuse into the flame. Even more so, the wick as these processes continue – bends – rather curls. As the wick bends its tip emerges to the outside of the flame where it now receives abundant amounts of oxygen. It too begins to degrade and the wick length becomes controlled (self regulating) in the flame.
Of course my above remarks are so “watered down” to the complexity of just what science and chemistry is going on here in a candle and one could argue my observations are so simplified that they don’t even come close to describing the science which is going on in the candle as it burns off its waxy fuel.
I think one of the interesting observations of a candle is to actually blow it out and then heat the smoke that emerges above (dont try this at home). The result is the ‘smoke’ is none other that the fuel (gas wax) mixed with oxygen, which when conditions are just right with an introduction to flame, can re-ignite the wick of the candle (see my above video – so you do not need to try this). As Faraday began his lectures most appropriately:
“There is no more open door by which you can enter into the study of natural philosophy than by considering the physical phenomenon of a candle”.
And I believe that to be true.
Of course I also believe, that if one cannot understand or rather acknowledge the complexities associated with a simple candle – how can one hope to understand the complexities of a fire in a house or even a building which is infinitesimally more complex?