Custom fireworks and standby firefighters: How the Vatican makes its smoke signal

When the Catholic Church elects a new pope, the world watches not for a press conference or social media post, but for rising smoke from a small chimney atop the Sistine Chapel.

If the smoke is black, no new pope has been chosen. If it is white, a decision has been made: Habemus Papam - we have a pope. It's high drama, broadcast live to millions.

But what viewers don't see is the centuries-old ritual's hidden complexity: the carefully built chimney, the engineered stove and the precise chemical recipes, each part painstakingly designed to ensure that a wisp of smoke carries a clear message.

Experts told the BBC that the process requires "two custom fireworks", smoke test rehearsals and Vatican firefighters on standby. It is meticulously organised by a team of engineers and Church officials working in unison.

Pope Francis died on Easter Monday aged 88 and with the funeral now over, attention has turned to the conclave - a private meeting through which a new pope will be chosen.

The Vatican has confirmed that cardinals will meet at St Peter's Basilica on 7 May to celebrate a special Mass before gathering inside the Sistine Chapel, where the complex vote will commence.

The tradition of burning the cardinals' paper ballots dates back to the 15th Century and became part of conclave rituals aimed at ensuring transparency and preventing tampering, particularly after earlier papal election delays had led to public frustration and unrest.

Over time, the Vatican began using smoke as a way to communicate with the outside world while preserving the strict confidentiality of the vote.

And today, despite countless advances in communication, the Vatican has chosen to preserve the tradition.

"From antiquity onwards people have seen rising smoke - of animal and grain sacrifices in the Bible, or of burning incense in tradition - as a form of human communication with the divine," Candida Moss, a theology professor at the University of Birmingham, told the BBC.

"In Catholic tradition, prayers 'ascend' to God. The use of smoke evokes these religious rituals and the aesthetics of wonder and mystery that accompany them."

Prof Moss also says that the rising smoke allows people gathering in St Peter's Square "to feel included - as if they are incorporated into this mysterious and secretive affair".

The reasons are symbolic, but making it work in the 21st Century requires real-world engineering.

Inside the Sistine Chapel, two stoves are temporarily installed specifically for the conclave: one for burning ballots, the other to generate the smoke signals.

Both stoves are connected to a small flue - a pipe within a chimney that allows smoke to escape - that leads up through the chapel roof to the outside. On Friday, fire crews were seen on the roof, carefully securing the chimney top into place, while workmen erected scaffolding and constructed the stoves inside.

The Sistine Chapel, which was built more than 500 years ago, is home to one of the most famous ceilings in the world. Adorned with Michelangelo's frescoes, it is not exactly designed for smoke signals, and the chimney needs to be installed discreetly and safely.

It's a complex process. Technicians either use an existing opening or create a temporary hatch through which the flue - typically made of a metal such as iron or steel - is inserted. The pipe runs from the stoves to the outside, emerging through the tiled roof above St Peter's Square.

Every joint is sealed to prevent leaks and every component is tested. Specialists rehearse smoke tests in the days before the conclave begins, ensuring the chimney draw works in real time. Even Vatican firefighters are involved; on standby in case of malfunction.

"This is such a precise process because if one thing goes wrong, it's not just a technical failure - it becomes an international incident," Kevin Farlam, a structural engineer who has worked on heritage properties, told the BBC. "It's not like putting a pipe on a pizza oven. Every part of the system has to be installed without damaging anything."

This setup is constructed days before the cardinals arrive and is dismantled once a pope has been chosen.

To ensure the signal is visible, Vatican technicians use a combination of chemical compounds.

"What they're essentially building here is two custom fireworks," Prof Mark Lorch, head of the department of chemistry and biochemistry at the University of Hull, told the BBC.

"For black smoke, a mix of potassium perchlorate, anthracene, and sulfur is burned - producing thick, dark smoke.

"For white smoke, a combination of potassium chlorate, lactose, and pine rosin, is used, which burns clean and pale.

"In the past they tried to burn damp straw to create a darker smoke and dry straw to make lighter smoke - but this caused some confusion because sometimes it appeared grey."

He explained that these chemicals are "pre-packed into cartridges and ignited electronically" so there's no ambiguity.

The addition of bell ringing - introduced during Pope Benedict XVI's election - now serves as confirmation and is used alongside the smoke signal.

Over the years, there have been suggestions to modernise the system: coloured lights, digital alerts, or even televised votes. But for the Vatican, the ritual is not just a communication tool - it's a moment of continuity with centuries of tradition.

"This is about tradition and secrecy, but it has real theological heft to it as well," Prof Moss said.

"Plus 'Catholic Church' and 'cutting edge' are far from synonyms - innovation is almost antithetical to ritual."