This ancient factory helped purple reign

The most prized pigment of antiquity was processed not from a tangle of root or the frothy extract of a weed, but by drawing out a slimy secretion from the mucus glands behind the anus of murex sea snails - "the bottom of the bottom-feeders," historian Kelly Grovier has written. The common name of the dyestuff, Tyrian purple, derives from the habitat of the mollusks, which the Phoenicians purportedly began harvesting in the 16th century BC in the city-state of Tyre in present-day Lebanon.

Because each snail yielded little more than a drop of the discharge, some 250,000 were required for an ounce of dye, by some accounts. Purple was labor-intensive, but so widely produced that piles of shells discarded millenniums ago are now geographical features in the region. The dye was also so pricey — worth more than three times its weight in gold, according to a Roman edict issued in...