Tyrian purple ( porphúra; ), also known as royal purple, imperial purple, imperial dye, or simply tyrian, is a reddish-purple natural dye. It is secreted by several species of predatory sea snails in the family Muricidae, rock snails originally known by the name murex (Bolinus brandaris, Hexaplex trunculus and Stramonita haemastoma). The coloured compound is 6,6'-dibromoindigo.
The name Tyrian refers to the city of Tyre in ancient Phoenicia (modern-day Lebanon). In ancient times, extracting this dye involved tens of thousands of snails and substantial labour, and as a result the dye was costly to use and conferred a high social value on those whose clothing bore its color.
History
Biological pigments were often difficult to acquire, and the details of their production were kept secret by the manufacturers. Tyrian purple is a pigment made from the mucus of several species of murex snail. Production of Tyrian purple for use as a fabric dye began as early as 1200 BC by the Phoenicians, and was continued by the Greeks and Romans until 1453 AD, with the fall of Constantinople. In the same way as the modern-day Latin alphabet of Phoenician origin, Phoenician purple pigment was spread through the unique Phoenician trading empire. The pigment was expensive and time-consuming to produce, and items coloured with it became associated with power and wealth. This popular idea of purple being elite contributes to the modern day widespread belief that purple is a "royal colour". The colour of textiles from this period provides insight into socio-cultural relationships within ancient societies, in addition to providing insights on technological achievements, fashion, social stratification, agriculture and trade connections. Despite their value to archaeological research, textiles are quite rare in the archaeological record. Like any perishable organic material, they are usually subject to rapid decomposition and their preservation over millennia requires exacting conditions to prevent destruction by microorganisms.
Tyrian purple may first have been used by the ancient Phoenicians as early as 1570 BC.McGovern, P. E. and Michel, R. H. "Royal Purple dye: tracing the chemical origins of the industry". Analytical Chemistry 1985, 57, 1514A–1522A It has been suggested that the name Phoenicia itself means 'land of purple'. The dye was greatly prized in antiquity because the colour did not easily fade, but instead became brighter with weathering and sunlight. It came in various shades, the most prized being that of black-tinted clotted blood.
Because it was extremely tedious to make, Tyrian purple was expensive: the 4th century BC historian Theopompus reported, "Purple for dyes fetched its weight in silver at Colophon" in Asia Minor.Theopompus, cited by Athenaeus (12:526) around 200 BC. The expense meant that purple-dyed textiles became status symbols, whose use was restricted by sumptuary laws. The most senior Roman magistrates wore a toga praetexta, a white toga edged in Tyrian purple. The even more sumptuous toga picta, solid Tyrian purple with gold thread edging, was worn by generals celebrating a Roman triumph.
By the fourth century AD, sumptuary laws in Rome had been tightened so much that only the Roman emperor was permitted to wear Tyrian purple. As a result, 'purple' is sometimes used as a metonym for the office (e.g. the phrase 'donned the purple' means 'became emperor'). The production of Tyrian purple was tightly controlled in the succeeding Byzantine Empire and subsidized by the imperial court, which restricted its use for the colouring of imperial silks. Later (9th century), a child born to a reigning emperor was said to be porphyrogenitos, "born in the purple".
Some speculate that the dye extracted from the Bolinus brandaris is known as () in Biblical Hebrew. Another dye extracted from a related sea snail, Hexaplex trunculus, produced a blue colour after light exposure which could be the one known as (), used in garments worn for ritual purposes.
Production from sea snails
The dye substance is a mucous secretion from the hypobranchial gland of one of several species of medium-sized predatory sea snails that are found in the eastern Mediterranean Sea, and off the Atlantic coast of Morocco. David Jacoby, citing the 1908 experiment by Paul Friedländer, remarks that "twelve thousand snails of Murex brandaris yield no more than of pure dye, enough to colour only the trim of a single garment." These are the marine gastropods Bolinus brandaris the spiny dye-murex (originally known as Murex brandaris Linnaeus, 1758), the banded dye-murex Hexaplex trunculus, the rock-shell Stramonita haemastoma, 284 pp incl 192 figs. & 32 pls. and less commonly a number of other species such as Bolinus cornutus. The dye is an organic compound of bromine (i.e., an organobromine compound), a class of compounds often found in algae and in some other sea life, but much more rarely found in the biology of land animals. This dye is in contrast to the imitation purple that was commonly produced using cheaper materials than the dyes from the sea snail.
In nature, the snails use the secretion as part of their predatory behaviour to sedate prey and as an antimicrobial lining on egg masses.Because of research by Benkendorff et al. (1999), the Tyrian purple precursor tyrindoleninone is being investigated as a potential antimicrobial agent with uses against multidrug-resistant bacteria. The snail also secretes this substance when it is attacked by predators, or physically antagonized by humans (e.g., poked). Therefore, the dye can be collected either by "milking" the snails, which is more labor-intensive but is a renewable resource, or by collecting and destructively crushing the snails. The dye is collected via the snail-harvesting process, involving the extraction of the hypobranchial gland (located under the mollusk's mantle). This requires advanced knowledge of biology. Murex-based dyeing must take place close to the site from which the snails originate, because the freshness of the material has a significant effect on the results, the colours yielded based on the long process of biochemical, enzymatic and photochemical reactions, and requires reduction and oxidation processes that probably took several days.
Many other species worldwide within the family Muricidae, for example Plicopurpura pansa,; see pp. 406–407. Note: Gould called this species Purpura pansa; it was later renamed Plicopurpura pansa. from the tropical eastern Pacific, and Plicopurpura patulaPlicopurpura patula was originally named Buccinum patulum by Linnaeus in 1758:
World Register of Marine Species (Web site): Plicopurpura patula (Linnaeus, 1758). The genus Plicopurpura was created in 1903 by Cossmann: 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.
Royal blue
The Phoenicians also made a deep blue-coloured dye, sometimes referred to as royal blue or hyacinth purple, which was made from a closely related species of marine snail.
The Phoenicians established an ancillary production facility on the Iles Purpuraires at Mogador, in Morocco. The sea snail harvested at this western Moroccan dye production facility was Hexaplex trunculus, also known by the older name Murex trunculus.In 1758, Linnaeus classified the snail as Murex trunculus:
In 1810, the English naturalist George Perry created the genus Hexaplex:
; see p. 11. World Register of Marine Species (Web site): Hexaplex Perry, 1810
This second species of dye murex is found today on the Mediterranean and Atlantic coasts of Europe and Africa (Spain, Portugal, Morocco).
Background
The colour-fast (non-fading) dye was an item of luxury trade, prized by Romans, who used it to colour ceremonial robes. Used as a dye, the colour shifts from blue (peak absorption at 590 nm, which is yellow-orange) to reddish-purple (peak absorption at 520 nm, which is green). 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: Pliny discusses Tyrian purple throughout Chapters 60–65.The problem with Tyrian purple is that the precursor reacts very quickly with air and light to form an insoluble dye. (Hence Pliny says: "... when [the shellfish] have once discharged their waxy secretion, their juices have no consistency. ...") The cumbersome process that Pliny describes is necessary to reverse the oxidation and to restore the water-soluble precursor so that large masses of wool can be dyed. See: See also: C. J. Cooksey (2001) "Tyrian purple: 6,6'-Dibromoindigo and Related Compounds", Molecules, 6 (9) : 736–769, especially page 761. Indigo, which is chemically very similar to Tyrian purple, behaves similarly. See: http://www.indigopage.com/chemistry.htm
The most favourable season for taking these [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 ] 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 amphorae 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 liquefied 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 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; this special "blackish clotted blood" colour, which was prized above all others, is believed to be achieved by double-dipping the cloth, once in the indigo dye of H. trunculus and once in the purple-red dye of B. brandaris.