During the papal conclave in the Sistine Chapel, progress will be measured in black and white -- the colors of the smoke that will emerge after the voting from a copper flue jutting from the chapel roof. But just how those colors are created remains a bit of a mystery.
Vatican officials closed the chapel last week for preparations, including the installation of the flue and two gray stoves that connect to it by the main entrance, across the chapel from the altar. One of the stoves, built in 1938 and bullet shaped, is used to burn the paper ballots. The second, built in 2005, is used to create the proper smoke -- white if a new pope has been elected by the 115 cardinals, black if not -- which mixes with smoke from the ballots as it travels up the flue.
Traditionally, after an unsuccessful vote officials would add damp straw to the ballots to make sooty black smoke. But confusion during the 1958 conclave, when there were several false alarms -- apparently because the straw failed to ignite -- led the Vatican to find a more foolproof system using chemicals. In 2005, officials started using cartridges, including one marked "fumo bianco" that created the white smoke announcing the election of Benedict XVI. The cartridge was meant to produce smoke for six and a half minutes.
Even so, there was some confusion that year about whether black or white smoke poured from the flue, and bells were rung at St. Peter's Basilica to confirm the election. The Vatican will ring bells again this year, as a reinforcement. As to what chemicals the cartridges contain, a Vatican spokesman, the Rev. Thomas Rosica, said only that the product was prepared by technicians "from several different elements."
Ben Baxter, director of Pea Soup Limited, a smoke-machine supplier in Ingleby Barwick, England, said the principal chemical was most likely potassium chlorate, which ignites easily -- a 9-volt battery will do -- and produces fine white particles as it burns. "That's what we sell in our smoke pellets and smoke grenades," he said.
A black cartridge probably uses potassium chlorate too, he said, along with a dye to coat the particles. "It's less nasty than anything that would create black smoke in the olden days," he said.
Mr. Baxter said that the most likely procedure would be to place a cartridge in the newer stove, close the door and then ignite it by sending current through a wire. Providing the flue is properly installed, no smoke should escape into the chapel to bother the cardinals -- or the priceless Michelangelo frescoes on the ceiling.
For the Vatican officials, it will just be a matter of choosing the proper cartridge. "Let's hope somebody labeled them right," Mr. Baxter said.
Daniel J. Wakin contributed reporting from Vatican City.
This article originally appeared in The New York Times.