- Also called:
- orchil
archil
What is archil?
How is archil dye prepared?
What is the chemistry behind archil dye?
archil, a purple (or reddish purple) dye obtained from some lichens by fermentation. It is also the term for any lichen that yields archil, such as Roccella, Lecanora, Ochrolechia, and Evernia species, and refers to any color obtained from this dye. The active coloring component of archil is orcein, a complex mixture of several phenolic compounds.
Archil in antiquity
The original archil lichen is Roccella tinctoria, a species of rock lichen found widely in the Canary Islands and other Macaronesian islands. Other lichens of commercial interest include the North American species Umbilicaria mammulata and Lasallia papulosa, the pantropical species Parmotrema tinctorum, and the cosmopolitan species Evernia prunastri, Lasallia pustulata, Ochrolechia pallescens, Parmelia perlata, and Umbilicaria vellea.
In the classical period purple dyes obtained from lichens were known in the Mediterranean region and were described by several writers, including Greek scholars Theophrastus and Dioscorides and the Roman author Pliny the Elder. Archil is mentioned in the Stockholm Papyrus—a 4th-century-ce Greek codex considered to be one of the earliest treatises on dyes and colors. Because it could be prepared from lichens that grew in relative abundance, archil offered a more accessible way to achieve purple tones in the ancient world, though the color was less durable and faded more easily in light. As a result it likely served as a practical alternative for everyday dyeing, for enhancing other colors, or as a base for more prestigious and costly dyes such as Tyrian purple. Besides being used to dye silk and wool, archil was also used to color parchment. The dye has been identified in European manuscripts dating from the 6th to the 9th centuries, but knowledge of it is thought to have been lost thereafter. According to accepted accounts, archil was reintroduced to western Europe in the early 14th century by a Florentine merchant who had learned the art of processing the dye from the eastern Mediterranean region historically known as the Levant. Archil was widely traded across Europe until the introduction of synthetic dyes in the 19th century.
Preparation of the dye involved drying and crushing lichens and placing them in vessels with an ammonia-rich liquid—traditionally stale urine. Other chemicals such as rock salt, saltpeter (potassium nitrate), arsenic, and potash were also added. The mixture was left to ferment for several weeks while being periodically stirred. During fermentation, the colorless compounds in the lichen chemically transformed into purple pigments. The resulting dye was used to color natural fibers directly without adding a mordant (chemical fixer).
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Archil chemistry
Diverse species of lichens found around the world contain precursors of the coloring substance in archil and similar red-to-purple dyes. These substances are natural chemicals in lichens called depsides and depsidones, such as lecanoric acid. During processing, these compounds break down into simpler substances, first forming orsellinic acid and then the colorless compound orcinol (also called orcin). When orcinol is exposed to oxygen in the air and ammonia, it reacts to form orcein, the purple coloring substance in archil and related dyes such as cudbear.
