Tyrian purple
The spiny dye-murex snail, known scientifically as Bolinus brandaris, secretes the precursor to Tyrian purple from its hypobranchial gland. This medium-sized predatory sea snail lives in the eastern Mediterranean Sea and off the Atlantic coast of Morocco. Another species, Hexaplex trunculus, also produces a similar substance that turns into an enduring purple dye when exposed to sunlight. The rock-shell Stramonita haemastoma contributes to this biological family, though it is less commonly used than the first two. These marine gastropods use their secretion as part of their predatory behavior to sedate prey or as an antimicrobial lining on egg masses. When attacked by predators or physically antagonized by humans, the snails secrete this substance as a defense mechanism. David Jacoby notes that twelve thousand snails of Murex brandaris yield no more than 1.4 grams of pure dye. That tiny amount is enough to color only the trim of a single garment.
Pliny the Elder described the cumbersome process required to reverse oxidation and restore the water-soluble precursor for mass-producing wool. The most favorable season for taking these shellfish was after the rising of the Dog-star or before spring. Once collected, the vein containing the hypobranchial gland had to be extracted with advanced knowledge of biology. Salt was added at a ratio of about twenty fluid ounces per hundred pounds of juice. The mixture was left to steep for three days, and then set to boil in vessels made of tin or lead. Every hundred amphorae were boiled down to five hundred pounds of dye using moderate heat. The process took several days, involving reduction and oxidation reactions that likely lasted until the tenth day when the contents liquefied. Archaeological data from Tyre indicates snails were collected in large vats and left to decompose, creating a hideous stench mentioned by ancient authors. The Talmud specifically granted women the right to divorce any husband who became a dyer after marriage due to this pervasive smell. Hands of the dyer reeked like rotting fish according to the Egyptian Papyrus of Anastasi.
Production of Tyrian purple for use as fabric dye began as early as 1200 BC by the Phoenicians. The name Tyrian refers to the city of Tyre in ancient Phoenicia, which is modern-day Lebanon. The pigment spread through the unique Phoenician trading empire, much like the modern-day Latin alphabet of Phoenician origin. The Phoenicians established an ancillary production facility on the Iles Purpuraires at Mogador in Morocco. This western Moroccan dye production facility harvested Hexaplex trunculus, also known by the older name Murex trunculus. Traces of this once very lucrative industry are still visible in many Punic sites such as Kerkouane, Zouchis, Djerba, and Carthage itself. According to Pliny, Meninx produced the best purple in Africa, ranking second only after Tyre's. Accumulations of crushed murex shells from a hut at Coppa Nevigata in southern Italy may indicate production there from at least the 18th century BC. Archaeological evidence from Slaves' Hill site dates back to the late 11th or early 10th centuries BC.
The fourth-century BC historian Theopompus reported that purple for dyes fetched its weight in silver at Colophon in Asia Minor. Because it was extremely tedious to make, Tyrian purple became expensive status symbols restricted by sumptuary laws. The most senior Roman magistrates wore a toga praetexta, which was white with a stripe of Tyrian purple. The even more sumptuous toga picta consisted of solid Tyrian purple with gold thread edging, worn by generals celebrating a Roman triumph. By the fourth century AD, sumptuary laws in Rome had tightened so much that only the Roman emperor was permitted to wear Tyrian purple. Purple is sometimes used as a metonym for the office, such as the phrase donned the purple meaning became emperor. A child born to a reigning emperor was said to be porphyrogenitos, or born in the purple. The production of Tyrian purple was tightly controlled in the succeeding Byzantine Empire and subsidized by the imperial court.
Recently, archaeologists found surviving wool fibers dyed with royal purple in the Timna Valley in Israel. This find, dated to 1000 BC, constituted the first direct evidence of fabric dyed with the pigment from antiquity. Archaeological data from Tyre indicates snails were collected in large vats and left to decompose. Accumulations of crushed murex shells from a hut at Coppa Nevigata in southern Italy may indicate production there from at least the 18th century BC. Evidence of dye use in pottery appears on the upper part of ceramic basins where reduced dye-solution was exposed to air. Recently, substantial numbers of Murex shells on Crete suggest the Minoans may have pioneered extraction centuries before the Tyrians. Dating from collocated pottery suggests the dye may have been produced during the Middle Minoan period in the 20th to 18th century BC. Findings from Slaves' Hill site include evidence of purple dye found in stains used on pot shards.
The colored compound is 6,6'-dibromoindigo, an organic compound of bromine often found in algae and some sea life. In nature, the snail uses the secretion as part of its predatory behavior to sedate prey or as an antimicrobial lining on egg masses. Research by Benkendorff et al. in 1999 investigated the precursor tyrindoleninone as a potential antimicrobial agent against multidrug-resistant bacteria. Recent research in organic electronics shows 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 originate from strong intermolecular hydrogen bonding that reinforces pi stacking necessary for transport. Variations in colors relate to indigo dye, 6-bromoindigo, and red 6,6'-dibromoindigo. Additional changes can be induced by debromination from light exposure or heat processing. In 1998, a process for dyeing with Tyrian purple was rediscovered after a lengthy trial and error process.
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Common questions
What is the scientific name of the spiny dye-murex snail that produces Tyrian purple?
The scientific name of the spiny dye-murex snail is Bolinus brandaris. This medium-sized predatory sea snail lives in the eastern Mediterranean Sea and off the Atlantic coast of Morocco.
How many Murex brandaris snails are needed to produce 1.4 grams of pure Tyrian purple dye?
David Jacoby notes that twelve thousand snails of Murex brandaris yield no more than 1.4 grams of pure dye. That tiny amount is enough to color only the trim of a single garment.
When did production of Tyrian purple for use as fabric dye begin by the Phoenicians?
Production of Tyrian purple for use as fabric dye began as early as 1200 BC by the Phoenicians. The name Tyrian refers to the city of Tyre in ancient Phoenicia, which is modern-day Lebanon.
Who was permitted to wear Tyrian purple in Rome during the fourth century AD?
By the fourth century AD, sumptuary laws in Rome had tightened so much that only the Roman emperor was permitted to wear Tyrian purple. A child born to a reigning emperor was said to be porphyrogenitos, or born in the purple.
What is the chemical compound responsible for the color of Tyrian purple?
The colored compound is 6,6'-dibromoindigo, an organic compound of bromine often found in algae and some sea life. Variations in colors relate to indigo dye, 6-bromoindigo, and red 6,6'-dibromoindigo.