This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
Prominent among natural dyestuffs is the coloring matter obtained from logwood and known as "hæmatein." The color-forming substance (or chromogen), hæmatoxylin, exists in the logwood partly free and partly as a glucoside. When pure, hæmatoxylin forms nearly colorless crystals, but on oxidation, especially in the presence of an alkali, it is converted into the coloring matter hæmatein, which forms colored lakes with metallic bases, yielding violets, blues, and blacks with various mordants. Logwood comes into commerce in the form of logs, chips, and extracts. The chips are moistened with water and exposed in heaps so as to induce fermentation, alkalies and oxidizing agents being added to promote the "curing" or oxidation. When complete and the chips have assumed a deep reddish-brown color, the decoction is made which is employed in dyeing. The extract offers convenience in transportation, storage, and use. It is now usually made from logwood chips that have not been cured. The chips are treated in an extractor, pressure often being used. The extract is sometimes adulterated with chestnut, hemlock, and quercitron extracts, and with glucose or molasses.
Fustic is the heart-wood of certain species of trees indigenous to the West Indies and tropical South America. It is sold as chips and extract, yields a coloring principle which forms lemon-yellow lakes with alumina and is chiefly used in dyeing wool. Young fustic is the heart-wood of a sumac native to the shores of the Mediterranean, which yields an orange-colored lake with alumina and tin salts.
Cutch, or catechu, is obtained from the wood and pods of the Acacia catechu, and from the betel nut, both native in India. Cutch appears in commerce in dark-brown lumps, which form a dark-brown solution with water. It contains catechu-tannic acid, as tannin and catechin, and is extensively used in weighting black silks, as a mordant for certain basic coal-tar dyes, as a brown dye on cotton, and for calico printing.
Indigo, which is obtained from the glucoside indican existing in the indigo plant and in woad, is one of the oldest dyestuffs. It is obtained from the plant by a process of fermentation and oxidation. Indigo appears in commerce in dark-blue cubical cakes, varying very much in composition as they often contain indigo red and indigo brown, besides moisture, mineral matters, and glutinous substances. Consequently the color varies. Powdered indigo dissolves in concentrated fuming sulphuric acid, forming monosulphonic and disulphonic acids. On neutralizing these solutions with sodium carbonate and precipitating the indigo carmine with common salt there is obtained the indigo extract, soluble indigo, and indigo carmine of commerce. True indigo carmine is the sodium salt of the disulphonic acid, and when sold dry it is called "indigotine."
One of the most important of the recent achievements of chemistry is the synthetic production of indigo on a commercial scale.
Artificial dyestuffs assumed preponderating importance with the discovery of the lilac color mauve by Perkin in 1856, and fuchsine or magenta by Verguin in 1895, for with each succeeding year other colors have been discovered, until at the present time there are several thousand artificial organic dyes or colors on the market. Since the first of these were prepared from aniline or its derivatives the colors were known as "aniline dyes," but as a large number are now prepared from other constituents of coal tar than aniline they are better called "coal-tar dyestuffs." There are many schemes of classification. Benedikt-Knecht divides them into I, aniline or amine dyes; II, phenol dyes; III, azo dyes; IV, quinoline and acridine derivatives; V, anthracene dyes; and VI, artificial indigo.
Of the anthracene dyes, the alizarine is the most important, since this is the coloring principle of the madder. The synthesis of alizarine from anthracene was effected by Gräbe and Liebermann in 1868. This discovery produced a complete revolution in calico printing, turkey-red dyeing, and in the manufacture of madder preparations. Madder finds to-day only a very limited application in the dyeing of wool.
In textile dyeing and printing, substances called mordants are largely used, either to fix or to develop the color on the fiber. Substances of mineral origin, such as salts of aluminum, chromium, iron, copper, antimony, and tin, principally, and many others to a less extent and of organic origin, like acetic, oxalic, citric, tartaric, and lactic acid, sulphonated oils, and tannins are employed as mordants.
Iron liquor, known as black liquor or pyrolignite of iron, is made by dissolving scrap iron in pyroligneous acid. It is used as a mordant in dyeing silks and cotton and in calico printing.
Red liquor is a solution of aluminum acetate in acetic acid, and is produced by acting on calcium or lead acetate solutions with aluminum sulphate or the double alums, the supernatant liquid forming the red liquor. The red liquor of the trade is often the sulpho-acetate of alumina resulting when the quantity of calcium or lead acetate is insufficient to completely decompose the aluminum salt. Ordinarily the solutions have a dark-brown color and a strong pyroligneous odor. It is called red liquor because it was first used in dyeing reds.
It is employed as a mordant by the cotton dyer and largely by the printer.