Conclave smoke: a mystery recipe

Guys, during the papal conclave in the Sistine Chapel, alternating puffs of black and white smoke emerge after the voting, "coming from a copper flue jutting from the chapel roof." But nobody outside the Chapel seems to know how those colors are created, and the New York Times is on it. Guesses range from damp straw to potassium chlorate. Papal Purple Nurple, or any other known strain of sweet-ass bud, is unfortunately not on the list. [NYT]