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Science of Papal Smoke Colors

smoke from campfire

This is a repost of an experiment I wrote in 2005, for the last Papal selection. I was quoted on CBS news ( ) about the science, so I thought I should repost the experiment.

The question was about the problems that the Vatican has been having with smoke.  At each vote, if there is no new pope, they are supposed to send black smoke up the chimney.  When a new pope is elected, they send up white smoke.  In the past, they have had problems with the smoke color.  If things don't burn properly, they can get gray smoke, which confuses everyone.  To understand about smoke colors, you will need:

  • a lighter or lit candle
  • a sheet of typing paper
  • a rubber band
  • tongs or tweezers
  • a large container of water
  • (optional) a metal strainer or a piece of wire window screen

WARNING!!!  This experiment uses fire and makes BAD smells.  Do this one outside, with an adult to help.

First, set the container of water on the ground.  Use a large bowl or tub.  You are going to work over this, so any burning bits will fall into the water instead of on something that could burn.

Light the lighter or candle.  Look at the flame.  Do you see any smoke?  No.  There is fire, but you should not seem smoke.

Roll the sheet of typing paper into a tight roll.  Holding one end, light the other end on fire.  If you rolled it tight enough, the flame should go out, leaving glowing embers and producing white smoke.  If not, you may have to blow out the flame to get the smoke.  When you are done, dip the paper into the water, to be sure the fire is out.

Next, hold the rubber band in the tongs, and place it over the flame.  When the rubber catches fire, hold it over the container of water.  It will produce dense, black smoke.  It will also make a bad smell.  Be sure that you keep it over the water, as drops of melted, burning rubber can drip from it.  Again, put this into the water to extinguish the fire.

OK, so we made white smoke and black smoke.  But, what does that tell us?  First, what is smoke?  Smoke is made of tiny particles or droplets in the air.  This is usually a sign that the fire is not completely burning the fuel.  If the fuel was burning properly, then there would not be any smoke, as we saw with the flame of the lighter. 

Once we get smoke, the color will depend on the chemicals that make up the particles.

In the black smoke, the particles were tiny clumps of carbon.  Carbon is the black stuff that was left behind when you burned the paper.  With the rubber, the carbon went up as smoke instead of being left behind.  That gave us the nice, black smoke.

With paper, the smoke particles are more complex.  They are a combination of water droplets (formed from the burning cellulose in the paper) and a variety of chemicals produced as the cellulose got hot enough to break apart.  This process of using heat to break chemical apart is called pyrolysis.  The combination of water drops and unburned fuel particles give the smoke a white color.  Did you notice that while the paper had a flame, it did not make much smoke?  While there is a flame, the fuel particles burn instead of becoming part of the smoke.

You can even test this with the candle itself. Light the candle, and notice that it is not producing any smoke. Being careful not to burn yourself, bring a piece of wire screen down to the top of the flame. Lower the screen slowly into the flame until it starts to give off black smoke. That smoke is from unburned carbon particles inside the flame. The metal screen absorbs enough heat to let them escape without burning.

Remove the screen, and blow out the candle. Now it gives off white smoke instead. That white smoke is unburned fuel, vaporized wax.

There are other ways to produce white smoke. Burning magnesium produces tiny bits of magnesium oxide instead of carbon, so it produces white smoke in the same way that the rubber band produced black smoke. Pouring water on a campfire produces billows of white smoke, but in that case, the smoke is mostly tiny droplets of water, vaporized by the heat and condensed into the fog-like smoke.

In the past, the Vatican has tried smoke bombs, military flares, and a variety of other things.  Some smelled so bad that they were ruled out.  Others make poisonous fumes, so they would never be used.  So far, I have not been able to find out which chemicals they are using this time.  If anyone finds out, please let me know. 

Have a wonder filled week.