COAL-TAR COLORS.—The most recent discovery of importance in dyeing, is the extraction of colored substances of great beauty from coal-tar, and the application of thesd to the coloring of cloth. At the present time, these dyes of coal-tar origin are most extensively employed, and give rise to the fashionable colors named aniline purple, Tyrian purple or mauve, violine, roseine, fuchsine or magenta, solferina, bleu de Paris, aniline green or emeraldine, azuliue, etc. It is only, however, within the last 20 years that these dyes have become practically known, though the preliminary discoveries in connection with their extraction were made in 1826. The condensable product or gas liquor obtained during the destructive distillation of coal in gas-works, consists of aqueous matter holding salts of ammonia in solution, and tar with naphtha. The tar consists of a numerous class of bodies, of which aniline and benzole are two. The aniline is present in minute quantity; and for manufacturing purposes, means are generally resorted to for the conversion of the benzole of gas-tar into aniline. The process fol lowed on the commercial scale is to act upon the benzole by nitric acid, by which it is converted into nitro-benzole, and therefore, by the action of acetate of the protoxide of iron, it becomes aniline.
Aniline Purple.—In the preparation of the dye known as aniline purple, solutions of equal equivalents of sulphate of aniline and bicliromate of potash are mixed together; and when the reaction is complete, a black precipitate is obtained, which is dried, and then digested several times in coal-tar naphtha, to separate all resinous matter. The residue is dissolved by successive quantities of alcohol; to the solution being placed in a retort, the alcohol is distilled off, and the aniline purple is left as a beautiful bronze-colored substance. Aniline purple is slightly soluble in cold water, more so in hot water, and is readily dissolved by the alcohols and aniline itself. It is nearly insoluble in ether and naphtha.
Roseine is most readily prepared on the commercial scale by adding two equivalents of binoxide of lead to a boiling solution of one equivalent of sulphate of aniline, and boiling the whole for a short time. On filtration, a rose-colored solution is obtained, i which is evaporated down to small bulk, when some resin separates, and the roseine is precipitated by soda or potash, and being collected on a filter, can be washed and dried.
This dye is readily soluble in alcohol, and yields a very intense crimson color, which, on being evaporated to dryness, leaves a dark metallic-looking and brittle residue of roseine. It is soluble in water, but not in naphtha.
Violine is procured by the oxidation of aniline, and the process generally followed is to heat a mixture of two equivalents of sulphuric acid, one equivalent of aniline, and some water, to the boiling-point, then add binoxide of lead, boil for some time, and filter hot. A purple liquid is obtained, which is boiled with potash till the aniline present is volatilized, and the coloring matter is precipitated, when the latter is thrown on .a filter, washed with water, and dissolved in a dilute solution of tartaric acid. On filtration, the colored liquid is evaporated to small bulk, refiltered, repre,cipitated by potash and soda, and the precipitate being dissolved in alcohol, yields an alcoholic colored solution, which, on distilling off the alcohol, leaves the violine as a brittle bronze-colored sub -stance. Violine is very slightly soluble in water, is readily dissolved by alcohol, and is insoluble in ether and naphtha.
Fucliaine or magenta is prepared by adding anhydrous bichloride of tin by degrees to aniline. The materials are constantly stirred during the operation, to keep down the intensity of the action, and the result is, that much beat is evolved, the mixture becomes pasty, then liquid and brown; and as the temperature approaches the boiling point, it becomes a dark, almost black liquid, which in very thin layers presents a rich crimson color. This liquid is boiled for some time, much water added, the whole reboiled, so as to volatilize any free aniline, and chloride of sodium (common salt) added till saturation, when the fuchsine or magenta is precipitated as a golden green, semi solid, pitchy substance. Any resinous matter still remaining may be separated by digestion in benzole. This dye may also be obtained by acting upon aniline with nitrate of mercury. Fuchsine or magenta is sparingly soluble in water, dissolves to some extent in alcohol, and is insoluble in ether and naphtha.