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.