Orcein

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Orcein, also archil, orchil, lacmus, litmus, Citrus Red 2, and C.I. Natural Red 28, are names for dyes extracted from several species of lichen, also called orchella weeds, found in various parts of the world. Commercial archil is either a powder (called cudbear) or a paste. It is red in acidic pH and blue in alkaline pH.

Orcein is approved as a food dye, with E number E121. Its CAS number is [1400-62-0] [1]. Its chemical formula is C₂₈H₂₄N₂O₇. It forms dark brown crystals.

Orcinol is extracted from archil lichen, Rocella tinctoria. It is then converted to orcein by ammonia and air. Orcein is a reddish-brown dye, orchil is a purple-blue dye. Orcein is also used as a stain in microscopy to visualize elastic fibers. It is a mixture of phenoxazone derivates - hydroxyorceins, aminoorceins, and aminoorceinimines.

Other lichen-derived dyes are cudbear and litmus.

α-amino orcein	α-hydroxy orcein	β-amino orcein	β-hydroxy orcein
β-amino orceinimine	γ-amino orcein	γ-hydroxy orcein	γ-amino orceinimine

See also:

• Litmus test
• Ethnolichenology
• Orchil, the poor person's purple

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