How Do They Make the Smoke for a Papal Conclave?

When Catholic cardinals meet to pick a new pope in a papal conclave, they’re sequestered in the Sistine Chapel so their deliberations aren’t influenced by the outside world—and, as moviegoers who saw Conclave (2024) know, it can be a tense meeting. Their ballots are burned after each round of voting so the cardinals’ choices remain secret. Updates from within the conclave come not from press conferences, but the chapel’s chimney.

For a bit more than a century, the cardinals have signaled their progress by sending colored smoke up the chapel chimney. Black smoke signifies a vote didn’t produce a pope, and white smoke means it did.

When the tradition began, the light smoke was produced by the burning ballots and some dry straw, and the darker smoke by the ballots and wet straw. The results weren’t always black and white, though, and sometimes the smoke signal left the outside world confused. During the 1958 conclave, white smoke plumed from the chimney after one vote. The crowds gathered outside the chapel cheered and Vatican Radio announced that the church had a new pope.

Just a few minutes later though, the smoke started to turn dark. The straw that the cardinals added to the fire didn’t take right away, and needed some time to get going.

To avoid this sort of confusion, the cardinals and Vatican officials have tried a few different things to make the two smoke colors, and the election results, more foolproof. They tried smoke bombs for the black smoke in the 1960s. While they left no question about the color, they also filled the room with smoke, sending the cardinals into coughing fits. After that, they tried Italian army flares, and while the color was clear at first, the smoke quickly turned gray, leaving some observers unsure of what was going down.

In 2005, the Vatican went high-tech and introduced an “auxiliary smoke-emitting device” that was fed chemical cartridges that could produce clearly colored smoke for up to six minutes. What was in the cartridges was anyone’s guess. The Vatican was oddly secretive about them and would only say that they were prepared from “several different elements.”

Later, the Vatican revealed its smoke recipe and technique. The smoke device has a compartment where “different colored-smoke generating compounds can be mixed,” the Vatican press office said in a statement. “The result is requested by means of an electronic control panel and lasts for several minutes while the ballots are burning in the other stove.”

The black smoke is made from a mix of potassium perchlorate (an inorganic salt commonly used as an oxidizer in colored fireworks and other pyrotechnics), anthracene (a hyrocarbon component of coal tar) and sulfur. The white smoke is produced by mixing potassium chlorate (a similar compound to potassium perchlorate, used in fireworks and smoke bombs), lactose (the sugar found in cow’s milk), and rosin (a conifer resin).

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A version of this story was published in 2013; it has been updated for 2025.