Pyrotechnics in the Vatican: The chemistry behind the white and black smoke

Approximately 1.4 billion Catholics worldwide are eagerly awaiting a new head of the Church. On Wednesday, 133 cardinals gathered for a conclave to elect a new pope from among their ranks, behind closed doors (extra omnes!).

At the center of attention of the thousands of believers gathered in St. Peter's Square in Rome these days is a rather unspectacular-looking chimney on the roof of the Sistine Chapel. This chimney will be used to announce the outcome of each ballot in the form of a smoke signal.

If black smoke rises, the required majority has not been achieved. However, if white smoke rises, this signal indicates "Habemus Papam," the Church has a new pope. For this purpose, the ballot papers were traditionally burned, mixed with either damp straw (white) or pitch or soot (black).

However, this archaic form of communication hasn't always worked. In 2005, during the conclave to elect Pope Benedict XVI, smoke that was neither clearly black nor clearly white rose after one round of voting—a catastrophe for the impatiently waiting faithful on April 19, 2005.

White smoke was supposed to rise, as Joseph Ratzinger had been elected pope. In fact, the smoke was initially dark gray to brownish, which many observers interpreted as black smoke. Only minutes later did the bells of St. Peter's Basilica ring out, confirming the election of Pope Benedict XVI.

Today, such risks are no longer taken. Instead, chemical cartridges are available that are ignited electrically. These ensure reliable and safe smoke signals.

To produce black smoke, these contain oxidizers such as potassium nitrate (KNO ₃ ), a carbon source (e.g., charcoal), and additives, including sulfur (S), bitumen, or anthracene for soot formation. The dominant components are unburned carbon particles (size: 0.01 to 1 µm) and sulfur compounds that absorb light in the visible spectrum.

Example reaction: 2 KNO ₃ + C + S → CO ₂ ↑ + N ₂ ↑ + K ₂ SO ₄ + soot (C)

To produce white smoke, the cartridges contain an oxidizing agent such as potassium chlorate (KClO ₃ ) and lactose (C ₁₂ H ₂₂ O ₁₁ ) or cane sugar, as well as rosin and a chloride source such as polyvinyl chloride (PVC) or table salt (NaCl). Fine salt particles (NaCl) and water vapor scatter light evenly across all wavelengths (Rayleigh scattering with smaller particles), which is perceived as white.

Example reaction: 8 KClO ₃ + C ₁₂ H ₂₂ O ₁₁ + (PVC) → 12 CO ₂ ↑ + 11 H ₂ O↑ + 8 KCl + (NaCl from PVC)

The smoke colors at the conclave are based on targeted chemical reactions: black smoke from soot and sulfur compounds, white smoke from salt particles and water vapor. Modern technology using pyrotechnic cartridges ensures reliability and precision—an example of applied combustion chemistry in a cultural context.