Book excerpt: The ancient industry of imperial purple dyeing was eradicated during the sack of Constantinople in 1453 and its secrets, purposefully kept mysterious, were lost with it. This booklet attempts a practical explanation of the micro-biological processes that could transform, as if by magic, the liquid from marine molluscs into an insoluble purple pigment. The methodology is accompanied by ancient and medieval references to the dye which was used to clothe Alexander the Great, Roman emperors and Christian rulers.