When the College of Cardinals assembled at the Vatican’s Sistine Chapel this March to choose a new pope, there was no clear front-runner and the outcome was very much in doubt. But the method that would be used to communicate the result was not: The cardinals would rely on a much-adapted system of duct­work and solid-fuel combustion equipment that has been evolving at least since the papal conclave of 1903.Traditionally, just-counted ballots from each inconclusive round of papal voting are burned with damp straw, which limits combustion and produces thick black smoke. The announcement of a successful vote — and...

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