What you Need
What to do
Light a candle and wait a short time for the flame to grow, it might help to pour some of the wax out of the candle.
Then hold the gauze/sieve over the candle flame, look in through the top - is anything odd?
Try moving the gauze up and down, and look at the smoke coming out of the candle.
What may happen
The candle flame looks hollow when the gauze is halfway through the candle. Also the smoke from the candle is black when the gauze is high in the flame, but white when the gauze is very low.
Why does it happen?
A candle is made up of a cylinder of wax, which is a fuel (often a hydrocarbon like petrol), this melts then is drawn up the wick by surface tension, it then evaporates and reacts with oxygen in the air. This reaction produces heat, carbon-dioxide and water.
However the reaction can only occur where the air meets the wax vapour. This cannot happen all the way though the flame, just around the outside. So all flames are hollow.
A metal gauze conducts heat away from the flame, this cools the wax and air below the temperature at which they will react, so the flame stops burning.
This means that the combustion is stopped and whatever was in the flame just passes through the gauze and floats upwards. The two visible things coming through the gauze are unburnt wax vapor which condenses to form tiny white droplets looking like white smoke, and partially burnt wax. When wax burns the hydrogen from the hydrocarbon reacts first leaving tiny particles of carbon. When these move through the gauze they form a very black sooty smoke. Inside the flame they are heated to yellow hot are also the reason that a flame gives out so much light.
If you put the gauze near the bottom of the flame there is still lots of unburnt wax but very little soot has had time to be created, so the smoke looks white.
Near the top of the flame there is no completely unburnt wax left but a lot of partially burnt carbon, so the smoke looks very sooty.
How did this save miners lives?
During the nineteenth century there was a huge problem with explosive gasses such as fire damp (methane) building up in coal mines and then getting ignited by the candles the miners were using for light. Sir Humphrey Davy managed to solve this problem by enclosing an oil lamp with a metal gauze, so even if the atmosphere inside the gauze became explosive the flames couldn't move through the gauze making the lamp safe to use. It had the added advantage that the flame would burn differently in different atmospheres, giving the miners warning of problems.
I am a high school chemistry teacher, and I have my students perform a lab every year in which we investigate the mechanics and chemistry of the candle flame. Up until now I have used a number of different tricks to tease out the structure of the flame, including using an index card place just above the wick, removed quickly so not to burn, to look at the annular cross section, and glass tubing placed in the top and near the wick to observe the two forms of smoke. I will be modifying the lab this year to use the gauze instead--it is much easier - and safer. Thank you for this great kitchen science episode, and the wonderful illustrations as well! Dr. Louis G. Casagrande, Sat, 4th Sep 2010