Many lichens contain compounds from which colouring matters can be obtained by appropriate treatment; the colouring matters known as cudbear, archil and litmus were formerly much more extensively used in dyeing than at present: like so many natural colouring matters they have been replaced by coal-tar dyes.

The lichens which produce colouring matters live upon trees and rocks, chiefly on the sea-shore; the colouring matters are thus generally manufactured in places near the coast. The species principally used for this purpose are Lecanora, Rocella and Usnea.

The lichens contain colourless compounds, which by treatment with acid or alkalis produce orcinol. In contact with moist air and ammonia, orcinol is decomposed into water and orcein, which is the colouring matter. The lichens are treated so as to produce the largest quantity of orcinol, which is then converted into orcein. There is not space here to enumerate and describe the many compounds obtained from lichens, only the most important can be mentioned; these are erythric, lecanoric and usnic acids. Lecanoric acid produces, under certain conditions, orsellic acid, which is again changed to orcinol.

Pure orcinol, C₇H₈O₂. H₂O, is a crystalline substance of sweet taste; it is soluble in ether, alcohol, water, etc. In damp air containing ammonia, orcinol gradually acquires a fine red colour, being converted into orcein, C₇H₇NO₃, according to the following reaction:—

C₇H₈O₂ + NH₃ + 3O = C₇H₇NO₃ + 2H₂O.

When pure, orcein is a brownish-red amorphous powder, soluble in alcohol to a scarlet solution and in alkalis to a purple solution. On the addition of water to the ammoniacal solution or of salt to the alkaline solution, orcein is again separated; this procedure may be used to completely purify the colouring matter.

Archil is principally obtained from the lichens Lecanora and Rocella. These lichens have a wide geographical distribution; archil is made in Sweden, Italy and Spain. It is obtained in a simple manner: the powdered lichen is mixed with decomposing urine to a paste, which is allowed to lie in the air until the colour has changed through red to violet; on the average fourteen days are required for the transformation. More recently the process has been made less disgusting by replacing urine by ammoniacal gas liquor. When the mixture is violet all through the process is finished; the wet mass is packed into barrels, which are well closed to prevent it from drying.

The presence of alkalis accelerates the formation of orcein; in many districts it is customary to add a little lime to the mixture of lichen and urine; alum is also added to retard putrefaction, which readily occurs in warm climates.

Instead of using lichens to produce the colouring matter, they may be extracted by boiling water, the solution considerably evaporated and then exposed to the air with the addition of ammonia, until the violet colouration appears. Slight additions of sulphuric acid give a more purple tint; soda produces a deeper violet.

French Purple is a compound of the archil colouring matter with lime, i.e., a lime lake. This beautiful substance is obtained by treating the lichen for some minutes with ammonia, pressing out the liquid and neutralising it exactly with hydrochloric acid. The precipitate obtained consists of the lichen acids; it is again dissolved in ammonia and the solution exposed to the air until it has become cherry red. When this is the case the liquid is quickly boiled and placed in large shallow dishes, which are kept at a temperature of 75° C. until the colour is purple-violet. Calcium chloride is then added; a garnet red precipitate is produced, which, when washed and dried, constitutes French purple.

Archil and French purple produce shades of great beauty and purity, but they have little durability and are quickly altered by exposure to light. They are, therefore, seldom used as pigments. The same is the case with cudbear and litmus.

Cudbear is a dirty purple powder with an ammoniacal odour and salt taste. It is made on a large scale in Holland. It differs from archil only in containing the colouring matter in the solid form.

Litmus is generally made from lichens of the genus Variolaria, though varieties of Rocella and Lecanora are often used. It is not made in quite the same manner as archil and cudbear. Together with the ammonia, potash is added to the powdered lichen. The mixture is left until it has become violet, when urine, potash and lime are added at intervals to continue the decomposition until the colour is changed to blue and the whole mass has become a paste. Gypsum and lime are then added, the mixture formed into rectangular plates, and brought into the market, described by a number, according to the amount of gypsum added.

Pure litmus should leave little solid residue when dissolved in water, the solution should be a fine violet. On account of its want of stability this colouring matter is not used in dyeing. It can be used for colouring foods since it is innocuous; but indigo carmine has much greater colouring power and at the same time gives fine shades.

The only use of litmus at the present time is as an indicator to decide whether a solution is acid, neutral or alkaline. In a quite neutral solution litmus is violet. Acids turn the solution to red and alkalis to pure blue. In preparing litmus tincture regard must be given to the fact that the aqueous solution of the colouring matter is always alkaline; acid must be cautiously added until the colour is violet, and the least addition of acid or alkali suffices to change it to red or blue.