The science behind the smoke that signals selection of new pope
A papal conclave is the special gathering of the Catholic Church’s College of Cardinals to select a new pope. Only cardinals under the age of 80 are allowed to vote. While any baptized Catholic male can technically be chosen as pope, since the 1300s, every pope has been selected from within the College of Cardinals. To be elected pope, a candidate must receive two-thirds of the votes cast.
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VATICAN CITY – All eyes are on the Sistine Chapel and a chimney installed on the roof that will produce smoke signals at the selection of a pope.
Steeped in Catholic tradition, smoke has been used since at least the early 1900s to signify either a successful election or the need for another vote if no one has reached the required votes.
The smoke is produced from burning the ballots used by the College of Cardinals during a papal conclave, which is tasked with electing the next pope.
VATICAN CITY, VATICAN - MAY 02: Firefighters install the chimney on the roof of the Sistine Chapel on May 02, 2025 in Vatican City, Vatican. The chimney used to communicate the choosing of a new Pope is erected on the roof of the Sistine Chapel today, ahead of Conclave beginning on the 7th May. Cardinals vote for their chosen candidate, and if there is no majority, the ballots are burnt with additives giving off black smoke. Once there is a majority, the votes are burnt without additives, giving off the white smoke signal that a new Pope is chosen. (Vatican Media via Vatican Pool/Getty Images / Getty Images)
According to the Vatican, only cardinals under the age of 80 are eligible to participate in the selection process, and voting can be conducted several times a day.
If no candidate receives a two-thirds majority, the ballots are burned with a chemical mixture that produces black smoke. When a new pope is elected, a different chemical mixture is used to make the smoke appear white.
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Smoke is produced because when paper burns, it undergoes what is known as "incomplete combustion."
During incomplete combustion, the amount of available oxygen is insufficient to fully oxidize the fuel, releasing chemicals that make the exhaust visible.
The process is similar to what happens during a wildfire, though on a much smaller scale, where insufficient oxygen prevents full combustion.
According to the U.S. National Park Service, which manages many wildfires, at least 16% oxygen is required in the air for flames to ignite.
Human-made materials such as charcoal and coke burn with minimal smoke due to their production in low-oxygen environments, leaving behind carbon-rich solids.
The Catholic tradition would not be possible if cardinals used charcoal or a similar substance, as that would result in what's known as "complete combustion."
The smoke signals have not always been as black and white as the colors suggest.
In past conclaves, confusion sometimes arose over whether the smoke was white or black, prompting Vatican officials to make enhancements.
Since the 2013 conclave that elected Pope Francis, two stoves have been used in the Sistine Chapel: one to burn the ballots and another to add chemical compounds that enhance the color of the smoke.
"The black smoke was produced by a mixture of potassium perchlorate, sulfur and anthracene, a chemical found in coal tar. The white smoke on the other hand was derived from a mixture of potassium chlorate, lactose, and some pine resin known as Greek pitch," Canada's McGill University detailed in a 2017 explainer.
To further eliminate any doubt about the color of the smoke, bells around St. Peter’s Basilica are rung at the same time as the appearance of white smoke.
Soon after the initial signals, a cardinal appears on the balcony of St. Peter's Basilica and announces the famous words "Habemus Papam" - Latin for "We have a pope."
Sometimes the selection of a pope is not straightforward. The Vatican reports the longest conclave lasted nearly three years, with the shortest only lasting a few hours.