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 reserved solid purple for emperors. It also covers the Phoenician trade empire, the independent discovery of a similar snail dye in Mexico, and the 1998 reconstruction of the lost process.

• The myth of Heracles’ dog discovering the dye, later painted by Rubens
• Why the Talmud allowed women to divorce husbands who took up the dyeing trade
• Toga praetexta, toga picta, and how usurping the throne became known as donning the purple
• How the stench served as a certificate of authenticity for Mexican nobility according to Zelia Nuttall’s research
• Why 6,6′-dibromoindigo’s hydrogen bonding and pi stacking make it a promising organic semiconductor