Orcein is the name given to the purple dye from lichens. In the 20th century
orcein was used as a food colouring and as a microscopical stain. Unlike
other ancient dyes, the chemical structure was not appreciated until the
More about that later. How is it formed from the lichen? Precursors
in the lichens are esters, depsides or depsidones which are hydrolysed
to orcinol by ammonia. Have a peek at some of the
structures involved. In the presence of air, ammonia and orcinol react
to give a series of compounds with a phenoxazone structure.