Alizarine, the coloring principle of madder, so called from alizari, the name by which that plant is known in the Levant. Madder is the root of several species of rubia, among which the rubia tinctorvm is the most valued for its dyeing properties. This grows in Asia Minor, Holland. Alsace, and in the south of France and of Russia, and has been cultivated to some extent in Delaware and Ohio. A species native to England is the rubia peregrina. This belongs to the order rubiaceae, the native members of which, as the galiums, are mostly inconspicuous wild plants. Some of the foreign species are, on the contrary, important plants, such as the cinchona, ipecacuanha, and coffee plants, and these are distinguished for the number and variety of the peculiar principles which they yield, as quinine, cinchonin'e, caffeine, and alizarine. - In spite of the numerous investigations that have been made of madder, chemists are still in doubt as to the nature of many of its constituents. Some attribute its coloring powers to the presence of at least two substances, alizarine and purpurine; while others say that only one of these produces the true madder colors.
Alizarine was discovered and obtained from madder, as a crystalline sublimate, by Robiquet and Colin, in 1831; but little importance was attached to this discovery until Schunck in 1848 showed that all the finest madder colors contain only alizarine combined with bases and fatty acids. The second coloring matter, termed purpurine, was discovered by Persoz. It contributes to the fall and fiery red colors in ordinary madder dyeing, but dyes a bad purple, alizarine being essential to the latter. Purpurine disappears during the purifying processes of soaping, etc, being far less stable than alizarine. It is distinguished from alizarine by its solubility in boiling alum liquor. These two coloring principles may likewise be easily distinguished by the spectrum, alizarine producing a set of dark absorption bands, quite different from those of purpurine, which again vary according to the nature of the solvent. Alizarine can be obtained in yellow needle-shaped crystals by simple sublimation from the dried madder; but this coloring matter is, singularly enough, not contained ready formed in the fresh madder root, but is the product of a peculiar decomposition.
A crystalline glucoside, termed rubianic acid (Schunck), is contained in the root, and it is this which splits up simply into alizarine and glucose. This acid crystallizes in line yellow needles, and gives a definite and crystalline potash salt, from which it was shown to contain 26 atoms of carbon in the molecule. Hence, as no other product but glucose is formed, it follows that alizarine must contain C26 - C12 = C14. The formation of alizarine in extracts of madder root is effected by fermentation peculiar to the plant, and called erythrozine. It is sui generis, since no other ferment produces the same effect. When mixed with a solution of rubian or rubianio acid, at the ordinary temperature, the latter is rapidly decomposed as with acids. This is what takes place in making fleur de garance. Dyers raise the temperature of their madder baths gradually up to the boiling point, because the application of a high temperature destroys the ferment. "When the temperature is gradually raised, the ferment acts upon the glucoside, and produces alizarine. According to Schunck, the origin of purpurine, and its relation to alizarine, are still (1870) involved in obscurity.
The formula assigned to ali-zarine by some chemists is C10116O3 +2H2O; while Grabe and Liebermann prefer C14118O4 +H2O. - Artificial Alizarine. In studying the transformations of alizarine under the action of chemical reagents, Messrs. Grabe and Liebermann in 1869 were led to connect it with anthracene, one of the coal-tar series of bodies (see Anthracene), and to devise a mode of forming it artificially. This is justly regarded as one of the most important as well as beneficent discoveries of the age: important as affording a new source for a valuable dye, and beneficent as restoring to agriculture large tracts of land now devoted to the culture of the madder root. The method employed by Grabe and Liebermann in the artificial production of alizarine is as follows: One part of anthracene is heated with four parts of sulphuric-acid, of sp. gr. 1.845, for three or four hours, to a temperature of 212° C, and then for about an hour at 300° C. The mixture is allowed to cool, and to it is added water equal to three times the weight of the anthracene employed, and manganese equal to four times that weight.
The whole is boiled for three hours, and milk of lime added, which gives rise to a deposit consisting of the excess of lime and manganese used, and protoxide of manganese, while there remains in solution a double sulphate of anthraquinone and lime. The solution is now acted upon by carbonate of soda in excess; carbonate of lime separates, and the salt of soda thus produced is evaporated to dryness. The solid mass is then mixed with two or three parts of caustic soda and a little water, and heated under pressure in suitable vessels, at a high temperature, whereby the anthraquinone is further oxidized and converted into alizarine. The alkaline mass, on cooling, is dissolved in water and sulphuric or acetic acid added in slight excess, when an orange-yellow flocculent substance precipitates, which, when perfectly washed and dried, is artificial alizarine. The artificial product appears to be entirely identical with the coloring matter obtained from the madder root. Both the natural and the artificial products crystallize in needles and dissolve in caustic alkalies, forming violet solutions of the same tint and possessing precisely the same tinctorial value. Applied to mordanted fabrics, they produce exactly the same colors, and they resist equally well the action of light.