Re: Why do candles burn quicker in warmer environments?

Dear Katie and Julie:
It is not a surprise that you have had difficulty getting an answer to your 
candle question; it turns out that candles are quite complex.  And getting 
a reasonable answer is going to take a bit of explaining.  A famous 
physicist, Michael Faraday, (after whom Faraday's constant is named) said 
"There is no better, there is no more open door by which you can enter into 
the study of natural philosophy (science) than by considering the phenomena 
of a candle..." (Faraday, M.: Faraday's Chemical History of a Candle. 
Chicago Review Press, Chicago, IL, 1988.)  In fact, to understand 
combustion better, researchers at NASA have been burning candles on the Mir 
Space Station to see the effects of weightlessness on combustion.  At the 
web site, http://mgnews.msfc.nasa.gov/db/combustion/combustion.html there 
is a description of some of NASA's research on candle combustion.  The text 
is pretty complex, but the photos are absolutely great!!!  A candle in a 
weightless environment produces a spherical blue flame, with almost no 
yellow.  I didn't see any measurements, but you can bet money that a 
weightless candle will not burn as quickly as one on earth.

Now, to your question.  You state that cold candles in your experiment burn 
more slowly than those at room temperature.  So, that is data we can use.  
You suspect that the Zeroth Law of Thermodynamics might have something to 
do with the slower burning.  The web site  http://library.thinkquest.org/3659/thermodyn/ presents a pretty good, quick 
review of the laws of thermodynamics.  The Zeroth law states (essentially) 
that if two objects are at one temperature, and a third object is at the 
same temperature as one of the two objects, then it must be the same 
temperature as the second object. Pretty neat, but it really doesn't say 
anything useful about candles and combustion.  So we can rule that one out. 
 
So, lets examine the process by which a candle combusts and then see if we 
can make an educated guess on what might be temperature sensitive steps.  
The following takes place as a candle burns 
(http://encarta.msn.com/find/Concise.asp?z=1&pg=2&ti=034EA000 is a pretty 
good explanation of this process,  http://microgravity.grc.nasa.gov/expr/cfm.htm is another good reference, 
even for obscure Faraday quotes!):

1.  Heat from the flame maintains a pool of wax at the base of the wick.
2.  Liquid wax travels by capillary action from the liquid pool up the 
wick.
3.  Heat from the flame causes the wax to evaporate or vaporize away from 
the wick.
4.  As the vapor travels away from the wick and nearer the combustion zone 
of the flame, it heats up and begins to decompose.  The wax, which is a 
long chain hydrocarbon, begins to break down into smaller and smaller 
molecules as it approaches the combustion zone of the flame.
5.  At the same time, air begins to mix into the vaporized and decomposing 
wax, providing oxygen for combustion.  This air also begins to heat up as 
heat is provided from the combustion zone.
6.  The hydrogen and very small hydrocarbons begin to combust with the 
oxygen in the air, giving off heat.  This initial combustion region is 
colored blue.
7.  More air mixes with the flame, and with the hydrocarbons, and the 
larger hydrocarbon molecules burn.  Since some of the hydrogen has been 
stripped away from the molecules earlier on, the remaining molecules have a 
lot of carbon which causes the flame to burn with a bright yellow flame.  
If too much hydrocarbon is delivered to the flame, (if the wick is too long 
or too large in diameter), soot or unburned carbon leaves from the tip of 
the flame as a black smoke.

Now if you look at the effects of temperature on these processes, we can 
make the following educated guesses.  The pool of liquid candle wax in 
contact with the solid candle wax is probably close to the melting point of 
the wax.  As the liquid wax gets closer to the wick, it will heat up 
because it is getting closer to the flame.  When you heat up any 
hydrocarbon, its viscosity (or resistance to fluid flow) decreases.  So 
lower viscosity wax will more easily travel up the wick, feeding the flame.

When you cool down the surroundings around the candle, more heat is 
transferred away from the pool of melted wax around the wick.  You end up 
with a smaller pool of melted wax.  The distance that the molten wax 
travels from the edge of the pool to the wick is shorter, so there is less 
opportunity to heat up the wax.  Therefore, the viscosity of the wax at the 
base of the wick is higher, the resistance to traveling up the wick is 
greater, so there is less wax fed up the wick.  The flame in a cold 
environment would, for that reason alone, be expected to be smaller.  Also, 
the fact that you are adding cooler air into the decomposing wax would also 
tend to slow down that part of the overall combustion process.

So, I would submit that the lower temperatures cause an increase in wax 
viscosity at the base of the wick, slowing the amount of wax which travels 
up the wick, and causing the candle in a low temperature environment to 
burn more slowly.

Let me provide you with one other piece of potential evidence for my 
theory.  In step 7 of our combustion process, I stated that wicks which 
were too long or too thick cause soot to form because more hydrocarbon is 
being delivered to the flame than can burn cleanly.  Scented candles are 
becoming more popular.  The scented candles contain not only wax but oils 
which are liquid at room temperature.  Candle manufacturers also sometimes 
add petroleum jelly or vegetable oil to their candles to improve their 
manufacturing process.  These materials which are oils at room temperature 
will decrease the viscosity of the wax so that more hydrocarbon is carried 
up the wick.  Not surprisingly, scented candles and candles that contain 
oils or petroleum jelly produce much more soot.  So you would expect that a 
candle which contained oils would burn at a faster rate than one made only 
with wax.  A lot of insurance and heating/air-conditioning companies are 
receiving complaints about soot building up on the walls and ceilings in 
homes which burn a lot of scented candles.

There might be alternate explainations for why cold candles burn slower.  
Steps 4 and 5 where the hydrocarbons vaporize and break down into smaller 
molecules might be slowed down by colder air being mixed into this region 
of the flame, producing a smaller flame and less heat, which causes less 
hydrocarbon to be vaporized, which produces a smaller flame (it is a 
feedback loop).  If this is the case, then changing the composition of the 
candle by adding oil to the wax may have a fairly small effect upon the 
rate of combustion, particularly if the temperature of vaporization for 
the wax and oil are close to the same.

So it might be interesting to make a couple of candles, one with straight 
paraffin wax, and the second with maybe 20 to 30% vegetable oil, and see if 
they burn at the same rate.  It might also be interesting to see if you can 
get the oil-containing candle to burn at 50 degrees at the same rate as the 
pure wax candle burning at 70 degrees by changing the amount of oil in the 
candle.

Thanks for asking such a good question.  Please be careful if you try to 
make a candle or two; hot wax can burn and ruin anything it spills on.