The world's attention will soon turn to a chimney on the roof of the Sistine Chapel as cardinals inside use a once-mysterious recipe to create smoke that signals their progress in choosing a new pope.
During the conclave to elect a new pope by secret ballot set to begin in Rome on May 7, cardinals inside the sealed-off Sistine Chapel will send out a column of smoke to tell onlookers whether they have chosen Pope Francis' successor.
If the smoke is black, it means the vote wasn't conclusive. If the smoke is white, it means a new pope has been selected by a two-thirds majority. There are 135 cardinals eligible to vote, according to the Vatican.
Ballots are burned in the process, but the smoke seen by the world is created using another stove that combines chemicals, the Vatican Information Service reported in 2013 as the last conclave began:
"This device stands next to the ballot-burning stove and has a compartment where, according to the results of the vote, different coloured-smoke generating compounds can be mixed. 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 process has been refined over the years. Before 2005, recipes included pitch and wet straw.
During last conclave in 2013, which ended in the election of Pope Francis, the black smoke was a mixture of potassium perchlorate, anthracene (a component of coal tar) and sulfur. The white smoke was produced using potassium chlorate, lactose and chloroform resin.
The once-mysterious recipe for the smoke was unveiled in 2013, the New York Times reported then. The cardinals' method for creating the smoke evolved into the use of cartridges containing the chemicals after previous mishaps. In 1958's conclave, when the black smoke was made by adding damp straw to the burned ballots, the straw's failure to ignite caused several false alarms when the smoke first appeared white, the Times reported.
After that fiasco, they tried smoke bombs, which made a clearer color, but filled the chapel with smoke, NBC News reported. They also tried army flares and chemical additives that sickened the cardinals. The cartridges were first used in 2005.
NBC also noted the tradition of white or black smoke isn't particularly ancient and dates back to the early 1900s.
During the conclave, cardinals gather at the Sistine Chapel and will be sequestered from communication with the public. On the first day of the conclave, one vote will be cast. If no pope is chosen that day, the cardinals will meet for two daily sessions until one candidate has two-thirds of the votes.
The cardinals fold their ballots twice and drop them into a chalice, according to the United States Conference of Catholic Bishops. The votes are counted by cardinals selected to record the votes. If they are inconclusive, the ballots are placed into a cast-iron stove and burned. Another stove sends up the puffs of black or white smoke.
At about noon and 7 p.m. each day of the conclave, the smoke will be sent out indicating the conclave's progress. Black means the votes were inconclusive. White means there is a new pope.
Francis died on April 21 at age 88 after 12 years in his position. The former Argentine cardinal was the first pope from the Americas and the first Jesuit. With their conclave votes, the College of Cardinals will decide whether his successor will continue his legacy of bucking norms or hearken back to more traditional aspects of the Catholic Church.
Contributing: Greta Cross, USA TODAY; Reuters