We know the Cleopatra’s sails were dyed with Tyrian purple. We know that Roman emperors dyed their new robes Tyrian purple as a sign of wealth. What we don’t know, exactly, is what this legendary purple looked like. But we do know where it comes from—namely, “snail milk.”

Snails on near-opposite sides of the world have produced the most famous purples in history. Learn how to milk a snail, and why the snail “milk” was one of the most sought-after chemicals in the world for over 3000 years.

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Tyrian purple was created by the Phoenicians, whose dyeing process is lost to history. One thing everyone agrees on, though, is that it starts from hypobranchial gland of one of the snails of the family Muricidae, or Murex snails. These snails, found on near-opposite sides of the world, have a mildly poisonous secretion that it uses to ward off enemies and protect its eggs. That secretion is white. When processed the right way, it turns to the spectacular purple of legend.

Unfortunately, we don’t exactly know how to treat it the right way, though. Archaeologists can climb over mountains of shells from the Bolinus brandaris and the Hexaplex trunculus, but they don’t know in what proportion they were used. They can stand over and measure waist-deep dying vats, but they don’t know exactly what went on in them. Some experts think that it was relatively simple process of applying the snail juice to the fabric, which is technically more painting than dyeing.

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Others think the process was more complicated, or at least became so over time. Purple pigment was incredibly expensive; snails had to be harvested from the sea, and each yielded only a drop or two of liquid. The Tyrians probably used potash and salts to reduce the dye to a colorless liquid, which the Phoenicians dipped fabric into. When the fabric was exposed to the air, the dye was reoxidized, which turned the pigment purple. (No matter what, everyone agrees that the dying process was smelly as hell. Dyeing facilities were always on the very edges of town, and contemporary sources agree that textile workers were smelly.)

But the Tyrians weren’t the only civilization to make glorious purple garments. As early as the 1500s, Spanish travelers in the New World marveled at the deep purple garments worn by the Mixtec. The color itself was impressive, and the fact that the garments seemed to get more vivid when washed or exposed to light and air was more so. Across the globe, another civilization had figured out how to make this extraordinary purple.

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But it wasn’t technically Tyrian purple. The Mixtec used a different kind of Muricidae, named Plicopurpura pansa. This snail lives along the coast from Costa Rica up to Mexico; it’s a dark, unassuming little thing, but it does make a beautiful dye. Some of the early explorers complained of a slight fishy smell, but unlike Tyrian there was no unrelenting wall of stink anywhere nearby. This may be because, unlike the Phoenicians, the Mixtecs didn’t extract the dye from killing the snail. They milked it.

The snail uses the raw material of the dye to defend itself, so it excretes the white “milk” whenever it gets alarmed. People through history described lots of ways to milk a snail. Early travelers mentioned dyers squeezing the snail’s shell. Others talk about poking it with a stick, or tickling it with a knife. One woman, visiting in the early 2000s, reported that modern dyers do nothing more than gently blow on the snails. Then, like the Tyrians, they rub the textile against the snail and watch the dye turn yellow, then neon green, and then darken in the sun to a deep purple.

There is still a dying trade going on in coastal Mexico. It has a rocky history. In the mid-1980s, a foreign firm got the rights to use the snails to dye clothing. Like the Phoenicians they killed the snails instead of milking them, and P. pansa numbers dropped precipitously in just five years, until local environmentalists got the contract revoked. Today, the dyeing industry is extremely modest. There are cheaper and easier ways to make purple. But you can still buy, if you know where to look, the closest possible thing to the legendary Tyrian purple made from Muricidae.

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Top Image: Daniel Baise.