Orcein, also Archil, orchil, lacmus, litmus, Citrus Red 2, and C.I. Natural Red 28 is a dye extracted from a lichen. 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 . 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 microscopical stain. It is a mixture of phenoxazone derivates - hydroxyorceins, aminoorceins, and aminoorceinimines.

Other lichen-derived dyes are cudbear and litmus.

Image:alpha-aminoorcein.png|α-amino orcein Image:alpha-hydroxyorcein.png|α-hydroxy orcein Image:beta-aminoorcein.png|β-amino orcein Image:beta-hydroxyorcein.png|β-hydroxy orcein Image:beta-aminoorceinimine.png|β-amino orceinimine Image:gamma-aminoorcein.png|γ-amino orcein Image:gamma-hydroxyorcein.png|γ-hydroxy orcein Image:gamma-aminoorceinimine.png|γ-amino orceinimine

See also:

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

Natural dyes | Food colorings | Staining dyes | pH indicators

Orcein