Photograph by Gregorio Borgia, AP

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White smoke emerging from the chimney on the roof of the Sistine Chapel at the Vatican on March 13 signals the selection of the new pope.

Photograph by Gregorio Borgia, AP

Smoke at the Vatican: How Do They Do It?

Chemistry holds the key to making the white puffs that emerged Wednesday.

White smoke has emerged from the chimney atop the Sistine Chapel at the Vatican, signaling that the cardinals have elected Jorge Mario Bergoglio—a 76-year-old Argentine and archbishop of Buenos Aires—as the new pope. Bergoglio will now be known as Pope Francis.

We talked to Chris Mocella, coauthor of Chemistry of Pyrotechnics, to see if he could shed light on what is done to create that heavenly hue. (Take a look inside the Vatican.)

How do you control the color of smoke when burning something? Isn't all smoke just sort of gray?

There are two ways to create smoke, Mocella explains: combustion, which produces smoke when materials are partially burned, and vaporization, which occurs when solid materials are heated to turn them first into a liquid and then into gas. "The smoke that you see when you burn wood or straw in the fireplace is from a combustion product; the carbon-heavy material in the wood burns, but not completely." This releases carbon dioxide along with "ashy" compounds that appear gray or black. In vaporization, "a solid substance is quickly melted and vaporized from the heat of a pyrotechnic reaction, but not to the point of being burned itself, and recondenses in the air to make a fog—what we would see as smoke." (How Green Was the "Green Pope"?)

What might be used to make white smoke?

"There are certain combustion products that are white or lighter gray based on their nature," says Mocella. He points to certain zinc compounds and elemental phosphorus, which attract moisture when burned and can produce thick white smoke. "These are fairly straightforward reactions for a pyrotechnic chemist," he adds. (Map: The Roman Catholic Diaspora)

Any idea about the Vatican's white smoke methodology?

Vaporization is definitely an option, but it's complicated to master, Mocella says. He suspects that the cardinals use the simpler process of combustion. One easy way to create white smoke is to burn "metallic zinc dust with elemental sulfur, generating zinc sulfide gas that is a thick off-white cloud of smoke when generated."

What about black smoke?

Black smoke is best made by partially burning organic material, like wood, says Mocella. "If we limit the amount of available oxygen in a fast-burning pyrotechnic composition, we'll get a lot of not-completely-burned sooty particles: smoke!"

Did the Vatican once use straw to create black smoke?

"If a certain kind of straw is burned with limited access to the oxygen in the air, that may produce black smoke, but maybe not in the volume and thickness desired. To really make smoke 'black,' it needs to come out thick and dense, which means making a lot of it quickly, which means a chemical pyrotechnic composition, probably not just burning straw."

What other smoke colors are possible? Could the cardinals send up red smoke if they wanted to? How about blue or green?

"Yes, we certainly can make colored smokes," Mocella says. Certain solid dyes can be quickly melted into a liquid, then vaporized into a gas, which then recondenses into tiny droplets in the air, producing a fog of colored smoke. "Careful attention has to be paid in preparing the pyrotechnic materials to be burned to vaporize the dye. Dyes are often made of organic materials that can just as easily burn into gray-black ash if the burning gets too intense."

Can smoke of different colors occur naturally?

"Naturally occurring colored smokes are rare, since the right conditions and materials are needed to produce them. I can't think of any natural colored smokes—on this planet at least!—that aren't aided by humans in some way."

And finally, since you are a pyrotechnics expert after all, could the cardinals send a firework up the chimney instead?

"Yes, the cardinals could certainly send a firework up the chimney instead, if it was a straight chimney and didn't bend or have any blockages. They would need to take that cap off of the top of the chimney pipe, too! Otherwise, the cardinals would have a very close encounter with a firework in the Sistine Chapel, probably not what Michelangelo expected."