Natural · Antiquity
The main colourant of Tyrian purple is 6,6′-dibromoindigo — the indigo skeleton carrying a bromine atom on each outer ring. The heavy bromine substituents shift absorption so the compound looks reddish purple rather than blue. Real snail dye is a mixture: depending on the species, it also contains monobromoindigo and plain indigo, and the balance of these decides whether the result leans red-violet or blue-violet. Like indigo it is insoluble and must be reduced before it can dye.
The dye comes from a gland of predatory sea snails — the spiny dye-murex, the banded dye-murex and the rock-shell of the Mediterranean. The gland's colourless precursors turn purple within minutes of exposure to air and sunlight. Production was concentrated on the Levantine coast around Tyre, but excavations have found Bronze Age workshops elsewhere, including one on Aegina with thousands of crushed shells. Because each snail yields very little, the colour was among the most costly goods of antiquity, and Roman law eventually reserved it for the emperor.
Ancient writers prized Tyrian purple for not washing out and even brightening with wear and sun, which matches its chemistry: dibromoindigo is a very stable, insoluble pigment within the fibre. Light does change it, though — it can remove bromine from the molecule, shifting the colour from reddish towards bluer purple, and the same photochemistry was apparently exploited to obtain blue shades. Surviving archaeological textiles are rare, so most of what is known about its ageing comes from chemistry and from the few fragments and dye residues that have been analysed.
Chemical identification rests on liquid chromatography: the proportions of dibromoindigo, monobromoindigo, indigo and indirubin distinguish shellfish purple from plant indigo and even hint at which snail species was used. Detecting bromine by XRF gives a quick, non-destructive pointer, since plant dyes contain none. Chromatography also exposes cheaper imitations, such as purples made from lichen dyes or from a red dye over indigo, which can look similar to the eye.
Nothing replaced Tyrian purple directly; after the fall of Byzantium the industry disappeared and purples came from lichens, mixtures and, from 1856, synthetic dyes such as mauveine and later violets. 6,6′-Dibromoindigo can now be made in the laboratory and is sold in tiny amounts for research and reconstruction. Modern purple textiles use synthetic dyes with none of its bromine chemistry, and none of its cost.
FactStrong evidence
6,6′-Dibromoindigo is the predominant component of Tyrian purple, and like indigo it is insoluble in water.
Source: A new synthesis of Tyrian purple (6,6'-dibromoindigo) and its corresponding sulfonate salts
FactStrong evidence
HPLC of residues from a 16th-century BC workshop on Aegina found high monobromoindigotin alongside dibromoindigotin, pointing to the banded dye-murex, with thousands of crushed shells in a single waste pit.
Caveat: One site; dye composition varies with species and processing.
FactLimited evidence
Light can debrominate the dye, shifting its hue from reddish purple towards blue, and the colour was reputed to brighten rather than fade with weathering.
Caveat: Supported here only by a general reference; the ancient 'brightening' reputation is literary rather than measured.
Source: Tyrian purple — Wikipedia