Tyrian Purple

Tyrian purple (Greek, πορφύρα, porphyra, Latin: purpura), also known as royal purple, imperial purple or imperial dye, is a purple-red natural dye, which is a secretion produced by certain species of predatory sea snails in the family Muricidae, a type of rock snail by the name Murex. This dye was possibly first used by the ancient Phoenicians. The dye was greatly prized in antiquity because the colour did not easily fade, but instead became brighter 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 meant that purple-dyed textiles became status symbols, and early sumptuary laws restricted their uses. The production of Tyrian purple was tightly controlled in Byzantium and was subsidized by the imperial court, which restricted its use for the colouring of imperial silks, so that a child born to a reigning emperor was porphyrogenitos, "born in the purple", although this term may also refer to the fact that the imperial birthing apartment was walled in the purple-red rock known as porphyry.

The dye substance comprises a mucous secretion from the hypobranchial gland of one of several medium-sized predatory sea snails that are found in the eastern Mediterranean. These are the marine gastropods Bolinus brandaris the spiny dyemurex, (originally known as Murex brandaris (Linnaeus, 1758)), the banded dye-murex Hexaplex trunculus, and the rock-shell Stramonita haemastoma. The dye is an organic compound of bromine (i.e., an organobromine compound), a class of compounds often found in algae and some other sea life, but much more rarely in the biology of land animals.

In Biblical Hebrew, the dye extracted from the Bolinus 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.

Many other species worldwide within the family Muricidae, for example Plicopurpura pansa (Gould, 1853), from the tropical eastern Pacific, and Plicopurpura patula (Linnaeus, 1758) from the Caribbean zone of the western Atlantic, can also produce a similar substance (which turns into an enduring purple dye when exposed to sunlight) and this ability has sometimes also been historically exploited by local inhabitants in the areas where these snails occur. (Some other predatory gastropods, such as some wentletraps in the family Epitoniidae, seem to also produce a similar substance, although this has not been studied or exploited commercially.) The dog whelk Nucella lapillus, from the North Atlantic, can also be used to produce red-purple and violet dyes.

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. David Jacoby remarks that "twelve thousand snails of Murex brandaris yield no more than 1.4 g of pure dye, enough to colour only the trim of a single garment."

Recent research in Organic electronics has shown that Tyrian Purple is an ambipolar organic semiconductor. Transistors and circuits based on this material can be produced from sublimed thin-films of the dye. The good semiconducting properties of the dye originate from strong intermolecular hydrogen bonding that reinforces Pi stacking necessary for transport.