Few institutions are more steeped in tradition than the Roman Catholic Church, and few of its traditions arouse more curiosity in Catholics and non-Catholics alike than the secretive selection of a new pope. When that process concludes, the news first reaches the world through a decidedly ancient means: a plume of white smoke.

After the death of a pope, cardinals from around the world assemble in a meeting called a “conclave.” The Vatican’s Sistine Chapel, which first served as the gathering site as early as 1492, became the permanent site for conclaves in the 1870s.

By tradition, the cardinals are locked inside the building and must remain there until they reach a decision, a practice that dates back to the late 13th century. The goal was to expedite the process after cardinals in 1271 took nearly three years to elect the man who became Pope Gregory X.

The cardinals conduct their deliberations in strict secrecy from the outside world. They also vote by secret ballot, conducting as many as four votes a day until one candidate has received the necessary two-thirds majority. At that point, the faithful who have gathered in St. Peter’s Square are alerted by the tell-tale white smoke signal.

The smoke signal is the result of burning the cardinals’ paper ballots in a stove after they have been counted. The tradition of burning the ballots dates back to at least the year 1417—and probably well before then, according to historian Frederic J. Baumgartner.

The use of white smoke to herald a new pope is a far more recent innovation. In his 2003 book Behind Locked Doors: A History of the Papal Elections, Baumgartner traces it to 1914, with the election of Pope Benedict XV.

“Perhaps the principal reason for this development,” he writes, was “Pius X's mandate that all papers relating to the election be burned, not just the ballots themselves, thus producing a great deal more white smoke for the final ballot and making it truly visible.” (Pope Pius X was Pope Benedict XV’s immediate predecessor.)

Black smoke means the cardinals failed to arrive at a decision with their latest vote. White signifies that they have chosen the new pope. The two colors are achieved by mixing different chemicals with the smoke from the burning ballots.

For chemistry nerds, McGill University reported in 2017 that the black smoke is created with “a mixture of potassium perchlorate, sulfur and anthracene, a chemical found in coal tar,” while the white smoke involves “a mixture of potassium chlorate, lactose and some pine resin known as Greek pitch.”

The smoke billows from a special chimney installed on the roof of the Sistine Chapel for that purpose. It’s connected to the stove where the ballots are burned as well as to a second stove “designed to ignite flares electronically which, depending on their composition,” send out “either white or black smoke,” according to the McGill report. Both stoves feed into the same copper flue.

Having any sort of chimney to the outside is also a fairly recent development, given the church’s long history. Baumgartner writes that the first one was installed only in the 18th century.

The burning of the ballots is a ritual in its own right. Three officials called scrutineers are chosen by lot each day from among the assembled cardinals. Their task is to count, read aloud and record each vote. Once that has been completed, they, along with other officials known as the secretary of the conclave and the masters of ceremonies, oversee the burning of the ballots.

The ceremonial burning not only provides the smoke signals that people have come to expect. It also ensures that the cardinals’ individual votes will forever remain a secret.

In the days before mass media and widespread literacy, smoke signals were a simple, visual way to give the keenly interested public—at least a hint of what was happening in the sealed-off chapel. But the final, single column of white smoke isn’t the only way the Vatican informs the world that the cardinals have made their decision. It is also announced by the ringing of bells and proclaimed verbally by a senior cardinal, who appears on the balcony of St. Peter's Basilica to declare "Habemus papam," meaning "We have a pope."

After more than 600 years, the smoke signals have become an important part of a deep-rooted Catholic tradition, where fire and faith still speak louder than words. While they may serve little functional purpose in today’s world of almost-instant electronic communication, a Tweet, Facebook post or TikTok video announcing the new pope would hardly be the same.