The European madder-plant, a native of Greece, belongs to the tribe Galieæ, of the order Rubiaceæ; it is the Rubia tinctorum of Linnæus. Several other species of this genus are used or grown in India for the sake of the red dye they afford. Among such species, Rubia cordifolia (Linn.) and R. sikkimensis (Kurz.) may be named, but the European madder is also cultivated extensively in India. Much madder was formerly grown in the Levant, in Holland, and in the south of France; but the manufacture by artificial means from the anthracene of coal-tar of its two chief colouring matters, alizarin and purpurin, has almost entirely extinguished the cultivation of the madder-plant in Europe. We shall have something to say presently concerning the artificial products above named.

The root of madder contains a much larger proportion of the colouring matters (or, it would be more correct to say, colour-making substances) than the other parts of the plant. They occur dissolved in the yellow cell-contents of the soft tissue of the root. The finest madder was grown in the 'Palud,' a chalky valley near Vaucluse. But the cultivation of this plant was carried out in great perfection in Zeeland during the eighteenth century.

The colouring matters obtained from madder exist in the plant in the form of glucosides. These glucosides are resolved by the fermentation, brought about by a peculiar ferment in the plant itself, and by many chemical agents, such as mineral alkalies and acids, mainly into glucose on the one hand, and on the other into the several colouring principles. Of such colouring principles the glucosides in madder yield at least three, of which the most important are these two:

1. Alizarin, C14H8O4.

2. Purpurin, C14H8O5.

Both alizarin and purpurin are now manufactured artificially from anthracene. This compound, which occurs in coal-tar, is a crystalline fluorescent hydrocarbon, C14H10. By a series of processes this substance gives rise to alizarin and purpurin, which are in all respects identical with these colouring matters as derived from the madder plant itself. The artificial alizarin of commerce contains several other colouring matters, two of which are better known than the others; these are anthrapurpurin (C14H8O5) and purpuro-xanthin (C14H8O4). Purpuroxanthin is also present in the natural pigments derived from madder, but it exists in small proportion. Of all these compounds alizarin is the most important and the best known, and yields lakes having various hues of crimson, rose, purple, violet and marone, according to its purity, its concentration, and the nature of the base (alumina, aluminium phosphate, iron oxide, manganese oxide, copper oxide, or lime with alumina) with which it is associated. The purpurin and anthrapurpurin resemble one another closely, and give pigments which are generally characterized by more orange or red hues than those obtained with alizarin.

The rose and pink madders and the madder carmines of commerce are generally so manufactured as to include, for their colouring constituents, much alizarin and very little purpurin. A few indications of the ordinary methods of preparing these lakes may first be given.

The material used is often that called 'madder flowers,' which consists of the finely ground dried root after it has been submitted to the action of dilute sulphuric acid and washed. Four pounds of this madder are taken and warmed for two or three hours on a steamer, with a solution of 1 pound of pure alum in 1 gallon of water. The mixture is placed in a filter-press, and the liquor obtained (which must be perfectly clear) precipitated by the gradual addition of a solution of sodium carbonate. The first portions of madder lake which fall, being the best, should be collected apart. All the precipitates should be thoroughly washed with rain or distilled water till the wash-waters are no longer troubled on the addition of barium chloride solution; they are then moulded into small cones, drops, or discs, and carefully dried at a moderate temperature. Another process for preparing madder lakes is a modification of the above. Four pounds of madder-root in powder, after having been fermented and then washed with a weak solution of sodium sulphate, are boiled for fifteen minutes with 4 gallons of a 10 per cent. solution of pure alum, the whole is filtered, and at a temperature of 45° partially neutralized with a solution in water of about 8 ounces of pure sodium carbonate.

The liquor is now brought nearly to the boiling-point; the madder lake which is then deposited is to be thoroughly washed and then dried: it is much denser than that produced by the preceding process. In the manufacture of alizarin lakes it is customary to introduce a small quantity of a preparation known as Turkey-red oil or sulphated castor oil. This is made into a soap and added to the alkaline solution employed to precipitate the lake.

By the employment in various proportions of solutions of alum and calcium chloride, by the substitution of sodium phosphate for the carbonate, and by choosing various qualities of madder-root, a number of hues and tints of rose and pink madder may be obtained when one or other of the methods above described is adopted. The oxides of iron, manganese and copper, when used in association with more or less alumina as a base for receiving the various colouring matters of madder, give other hues, including madder purple and madder brown.

But occasionally the pigments sold under these names are mixtures. For instance, burnt sienna and copper ferro-cyanide have been found in samples of madder brown; the presence of copper in madder brown seems, however, to be usual, but it arises from the employment of copper sulphate in its preparation along with alum.

From alizarin and from purpurin (either natural or artificial) lakes may be readily prepared by dissolving these substances in the smallest necessary quantity of an alkali, such as ammonia or sodium carbonate, and then adding a solution of a pure aluminium salt or some pure freshly precipitated and thoroughly washed aluminium hydrate. Another and more recent process consists in dissolving the colouring matter in a solution of sodium aluminate, and then precipitating the 'lake' by adding dilute sulphuric acid or, better, a solution of alum.