The preparation of Litmus is almost exclusively carried on in Holland, the details being kept a secret. About nineteen kinds seem to be there, varying very much in value. The lichens are coarsely ground with pearlashes, and macerated for weeks in wooden vessels in a mixture of urine, lime and potash or soda, with occasional stirring. In fermentation the mass becomes red and then blue, and is then moulded into earthy, crumbling cakes of a purplish-blue colour. The scent is like violets and indigo and the taste is slightly saline and pungent.
Indigo is mixed with inferior kinds to deepen the colour.
Blue Litmus Paper is prepared by steeping unsized white paper in an infusion or Test Solution of Litmus, or by brushing the infusion over the paper, which must be carefully dried in the open air. Red Litmus Paper is similarly prepared with an infusion faintly reddened by the addition of a small percentage of sulphuric or hydrochloric acid. Vegetable red , much used in colouring foods, is a sulphonated derivative of orchil. The lichen contains a brown resin, wax, insoluble and lichen starches, yellow extractive, gummy and glutinous matters, tartrate and oxalate of lime and chloride of sodium. The colouring principles are acids or acid anhydrides, themselvescolourless but yielding colour when acted upon by ammonia, air and moisture.