Tyrian purple (Greek, πορφύρα, porphyra, Latin: purpura), also known as royal purple, imperial purple or imperial dye, is a purple-red dye, which is extracted from sea snails, and which was first produced by the ancient Phoenicians. This dye was greatly prized in antiquity because it did not fade, rather it became brighter and more intense with weathering and sunlight.
Tyrian purple was expensive: the 4th-century-BC historian Theopompus reported, "Purple for dyes fetched its weight in silver at Colophon" in Asia Minor. The expense rendered purple-dyed textiles status symbols, and early sumptuary laws dictated and forbade their use. The production of shellfish purple was tightly controlled in Byzantium and subsidized by the imperial court, which restricted its use for the coloring of silks for imperial use, so that a child born to a reigning emperor was porphyrogenitos, "born in the purple", although this may also have to do with the fact that the Imperial birthing apartment was walled in Porphyry.
The dye substance consists of a mucous secretion from the hypobranchial gland of one of several medium-sized predatory sea snails found in the eastern Mediterranean. These are the marine gastropods Murex brandaris the spiny dye-murex, (currently known as Bolinus brandaris (Linnaeus, 1758)), the banded dye-murex Hexaplex trunculus, and the rock-shell Stramonita haemastoma.
In Biblical Hebrew, the dye extracted from the Murex brandaris is known as argaman (ארגמן). Another dye extracted from a related sea snail, Hexaplex trunculus, produced an indigo colour called tekhelet (תְּכֵלֶת), used in garments worn for ritual purposes.
In nature the snails use the secretion as part of their predatory behaviour and as an antimicrobial lining on egg masses. The snail also secretes this substance when it is poked or physically attacked by humans. Therefore the dye can be collected either by "milking" the snails, which is more labour intensive but is a renewable resource, or by collecting and then crushing the snails completely, which is destructive.
The fast, non-fading dye was an item of luxury trade, prized by Romans, who used it to colour ceremonial robes. It is believed that the intensity of the purple hue improved, rather than faded, as the dyed cloth aged. Vitruvius mentions the production of Tyrian purple from shellfish. In his History of Animals, Aristotle described the shellfish from which Tyrian purple was obtained and the process of extracting the tissue that produced the dye. Pliny the Elder described the production of Tyrian purple in his Natural History:
The most favourable season for taking these fish [i.e., shellfish] is after the rising of the Dog-star, or else before spring; for when they have once discharged their waxy secretion, their juices have no consistency: this, however, is a fact unknown in the dyers' workshops, although it is a point of primary importance. After it is taken, the vein [i.e., hypobranchial gland] is extracted, which we have previously spoken of, to which it is requisite to add salt, a sextarius [about 20 ounces (567 grams)] about to every hundred pounds of juice. It is sufficient to leave them to steep for a period of three days, and no more, for the fresher they are, the greater virtue there is in the liquor. It is then set to boil in vessels of tin [or lead], and every hundred amphoræ ought to be boiled down to five hundred pounds of dye, by the application of a moderate heat; for which purpose the vessel is placed at the end of a long funnel, which communicates with the furnace; while thus boiling, the liquor is skimmed from time to time, and with it the flesh, which necessarily adheres to the veins. About the tenth day, generally, the whole contents of the cauldron are in a liquified state, upon which a fleece, from which the grease has been cleansed, is plunged into it by way of making trial; but until such time as the colour is found to satisfy the wishes of those preparing it, the liquor is still kept on the boil. The tint that inclines to red is looked upon as inferior to that which is of a blackish hue. The wool is left to lie in soak for five hours, and then, after carding it, it is thrown in again, until it has fully imbibed the colour.
Archaeological data from Tyre indicate that the snails were collected in large vats and left to decompose. This produced a hideous stench that was actually mentioned by ancient authors. Not much is known about the subsequent steps, and the actual ancient method for mass-producing the two murex dyes has not yet been successfully reconstructed.
The Roman mythographer Julius Pollux, writing in the second century BC, asserted (Onomasticon I, 45–49) that the purple dye was first discovered by Heracles, or rather, by his dog, whose mouth was stained purple from chewing on snails along the coast of the Levant. Recently, the archaeological discovery of substantial numbers of Murex shells on Crete suggests that the Minoans may have pioneered the extraction of Imperial purple centuries before the Tyrians. Dating from collocated pottery suggests the dye may have been produced during the Middle Minoan period in the 20th–18th century BC.