ARCHIL (a corruption of "orchil," Ital. oricello, the origin of which is unknown), a purple dye obtained from various species of lichens. Archil can be extracted from many species of the genera Roccella, Lecanora, Umbilicaria, Parmelia and others, but in practice two species of Roccella - R. tinctoria and R. fuciformis - are almost exclusively used. These, under the name of "orchella weed" or "dyer's moss," are obtained from Angola, on the west coast of Africa, where the most valuable kinds are gathered; from Cape Verde Islands; from Lima, on the west coast of South America; and from the Malabar coast of India. The colouring properties of the lichens do not exist in them ready formed, but are developed by the treatment to which they are subjected. A small proportion of a colourless, crystalline principle, termed orcinol (a dioxytoluene), is found in some, and in all a series of acid substances, erythric, lecanoric acids, etc. Orcinol in presence of oxygen and ammonia takes up nitrogen and becomes changed into a purple substance, orceine (C7H7NO3), which is essentially the basis of all lichen dyes. Two other colouring-matters, azoerythin and erythroleinic acid, are sometimes present. Archil is prepared for the dyer's use in the form of a "liquor" (archil) and a "paste" (persis), and the latter, when dried and finely powdered, forms the "cudbear" of commerce, a dye formerly manufactured in Scotland from a native lichen, Lecanora tartarea. The manufacturing process consists in washing the weeds, which are then ground up with water to a thick paste. If archil paste is to be made this paste is mixed with a strong ammoniacal solution, and agitated in an iron cylinder heated by steam to about 140° F. till the desired shade is developed - a process which occupies several days. In the preparation of archil liquor the principles which yield the dye are separated from the ligneous tissue of the lichens, agitated with a hot ammoniacal solution, and exposed to the action of air. When potassium or sodium carbonate is added, a blue dye known as litmus, much used as an "indicator," is produced. French purple or lime lake is a lichen dye prepared by a modification of the archil process, and is a more brilliant and durable colour than the other. The dyeing of worsted and home-spun cloth with lichen dyes was formerly a very common domestic employment in Scotland; and to this day, in some of the outer islands, worsted continues to be dyed with "crottle," the name given to the lichens employed.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)