In the ninth century, Daoist alchemists in Tang China searching for an elixir of immortality stumbled onto something far more consequential than eternal life: a black powder that burned with startling force when sulfur, charcoal, and saltpeter were combined. For centuries the formula stayed almost entirely a Chinese secret, refined from incendiary fire-arrows into fire lances, iron bombs, and by the late thirteenth century, metal-barreled hand cannons. The knowledge most likely crossed into the Islamic world through the upheaval of the Mongol invasions of Persia and Mesopotamia in the mid-1200s — the Syrian engineer Hasan al-Rammah wrote down over a hundred gunpowder recipes between 1240 and 1280, tellingly calling saltpeter “Chinese snow,” and his contemporaries in Mamluk Egypt and Syria turned the same powder toward rockets and early hand-held guns. By the early fourteenth century the technology had reached Europe — Roger Bacon's 1267 writings hint at it, a 1326 English manuscript shows the first drawing of a European cannon, and within twenty years gunpowder weapons were roaring at the Battle of Crécy. What changed from region to region was never really the chemistry; it was the proportion and the purpose: a little saltpeter made a slow-burning incendiary useful for fire-arrows and rockets, while a lot of it, ground fine and later packed into dense grains, made an explosive violent enough to end the thousand-year age of the castle wall.