Orchil, a red or violet dye extracted from certain lichens, has an etymology as obscure as the lichens from which it is derived. The word enters English from Old French orcheil, but the French word's own origin is uncertain — proposals have included Arabic, Late Latin, and pre-Roman Mediterranean sources, none conclusively established.
The dye itself is extracted from lichens of the genera Roccella, Ochrolechia, and several others that grow on coastal rocks in Mediterranean, Atlantic, and tropical regions. The extraction process involves treating the lichen with ammonia (historically provided by fermented urine) and allowing the mixture to ferment for weeks, producing a purple or crimson dye. The chemistry involves the conversion of lichen acids into orcein and related colorants.
The orchil trade was one of the significant luxury industries of medieval and early modern Europe. Purple and crimson dyes were enormously valuable — Tyrian purple from murex snails was the most famous, but orchil provided a less expensive alternative that could produce rich violet and reddish-purple tones. The Canary Islands, Cape Verde, and various Mediterranean coasts were major sources of dye-producing lichens.
The most celebrated connection between orchil and history involves the Rucellai family of Florence. According to a persistent tradition, the family's fortune — which helped finance major architectural and artistic commissions during the Florentine Renaissance — derived from the orchil trade. The family name itself may be connected to oricello, the Italian form of orchil. The Rucellai Palace, designed by Leon Battista Alberti, and the Rucellai Chapel in San Pancrazio stand as architectural monuments to wealth built on lichen
In chemistry, orchil played an important role in the development of acid-base indicators. Litmus — the indicator that turns red in acid and blue in base — is derived from orchil lichens. The word litmus comes from Old Norse, but the substance itself is closely related to the orchil dyes that medieval dyers produced.
The orchil industry declined with the development of synthetic dyes in the mid-19th century, which could produce consistent colors at lower cost. Today, natural orchil dye is a curiosity of historical chemistry and traditional dyeing, preserved mainly by textile historians and craft practitioners.