Stand next to a Roman emperor in his magnificent purple robes and you would notice something the marble statues leave out: he smelled intensely of rotting fish. This episode follows Tyrian purple from the defensive mucus of Mediterranean murex snails to the strictly policed wardrobes of Rome and Byzantium, through an 800-year blackout after the sack of Constantinople, and on to a surprising second life in modern organic electronics.

The conversation walks through Pliny the Elder's recipe, in which extracted glands were salted, steeped for three days and boiled for ten in lead or tin vats, with roughly 12,000 snails yielding about 1.4 grams of dye. It explains the alkaline reduction and oxidation chemistry that made the color so colorfast, the double-dipping technique behind the prized shade of clotted blood, and the sumptuary laws that rese ...