Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/761

of soda or alkaline pink (used as a mordant when it has to act as a resist for another colour such as aniline black), and

acetates of chromium, copper, tin, and other metals.

Dye Colours.—The principal dye colour is madder or some of its derivatives, including artificial alizarin, the dyeing principle of madder obtained synthetically from anthracene. Madder is the root of a plant, Riibia tinctoria, a native of Central Asia, but introduced and extensively culti vated in south Europe, especially at Avignon in France. For the purposes of the calico-printer, madder-root is pre pared by simply grinding, or in the form of flowers of madder (jleurs deyarance), of garancin, of garanceux, or of alizarin. Fleurs de garance is powdered madder deprived of its soluble constituents and reclried, whereby the tinctorial strength of the preparation is increased nearly one-half. Garaucin is prepared by boiling powdered madder in sulphuric acid ; garanceux is spent madder similarly prepared ; and alizarin, the chief tinctorial principle of madder, is obtainable from garancin by the action of superheated steam. Among the chemical principles of tinctorial value yielded by madder there is, besides alizarin, an allied substance named purpurin. Alizarin of precisely similar composition and behaviour is now artificially made from anthracene, one of the products of coal-tar, and purpurin also is obtainable by the oxidation of artificial alizarin. By chemical agency the essential ingredients of madder arc thus now produced in a cheaper, more convenient, and more effective form than it was formerly possible to extract them from the cultivated root. Madder extract, garancin, and alizarin dye heavier and more brilliant colours than madder, and they require less soaping or other treatment to clean the whites after dyeing. Madder extract and artificial alizarin are also used as steam colours.

We may now briefly follow the stages in printing a madder style, taking for example a calico printed in four colours (the technical name for whatever is printed by the machine, whether mordant or dye), with a padding or blotch of weak iron liquor. In this case the mordants or colours are—

Black from strong solution of iron liquor.

Purple from weak solution of iron liquor.

Red from solution of red liquor.

White resist of solution of citric acid (lime juice).

Purple pad or cover of weak iron liquor, which falling on the acid resist forms a soluble ferric citrate.

Drying.—The cloth after receiving these impressions passes into a drying apparatus, generally a closed chamber, highly heated by radiation from steam-chests of cast-iron. Through such a chamber the cloth passes up and down over numerous rollers, traversing a long distance before it emerges dry and ready for the next process. Another means of drying, employed in some, of the best establish ments, such as Thornliebank, is by passing the cloth round a long series of revolving steam cans or cylinders, the metallic surface of which is covered with felt. Recently a most effective system of drying has been introduced, which consists of forcing a strong current of heated air through an enclosed chamber by the action of a fan, connected with which is an apparatus filled with pipes, through which the air passes, while surrounding the pipes is a steam space. By this plan any temperature may be obtained, and the current of air adjusted by the speed of the fan.

Ageing.—From the drying apparatus the goods pass to the ageing room, a lofty and spacious chamber (fig. 4), where they are exposed to the combined influence of heat and steam. The pieces pass, as shown by the arrows, up and down over rollers from end to end of the room, travelling over a long space, for twenty minutes or thereby. The atmosphere is rendered moist by jets of steam blowing from pipes a, a, and hot by radiation from the same and other steam pipes. A difference of four degrees is main tained between the dry and wet thermometers ; the readings average 80 and 76 Fahr. The cloth takes up about 5 per cent, of its weight of moisture in its passage, and as it issues at the further end of the apartment, itis piled up in loose bundles, and so left for two or three days in a warm moist atmosphere. The object of the operation of ageing is to precipitate the mordants in the fibre of the cloth, they in the meantime being partly decomposed with the disengagement of abundant fumes of acetic acid. The practical development of the modern process of agein"- is due to the scientific ingenuity of the late Mr Walter Cram of Thornliebank, the method previously practised having been tedious and uncertain, depending upon variable states of the weather.

FIG. 4. Vertical Section of Ageing Eoom.

Dunging.—It is next necessary to remove any superfluous uncombined mordant which may be on the cloth, and to take away the thickening agent with which the mordant was printed. These objects are accomplished by passing the goods through hot water, in which it was formerly the practice to dissolve cow s dung, hence the name ; but now some of the numerous dung substitutes are chiefly used, the principal of which are the silicate and the arseniate of soda. The first operation in dunging is to pass the pieces through the " fly dunging " apparatus, a cast-iron trough with rollers top and bottom, by which the cloth is made to pass, in the open state, through the hot solution. This operation fixes the mordant in the fibre and prevents it from spreading to unmordanted parts of the cloth in the subsequent washing and dyeing operations to which it is subjected. Immediately after the fly-dunging the goods are washed and submitted to a second dunging, this time in a different kind of apparatus, through which they are passed in a coil or loose rope form. They are then thoroughly washed at a machine to remove the last traces of thickening matter and all uncombined ingredients.

FIG. 5. Sections of Dye Vat.

Dyeing.—At this point the goods are ready for dyeing,

the most important process in the whole series of opera tions. The dye-beck or vat, one form of which is shown in longitudinal and transverse section in fig. 5, consists of an iron cistern or trough A, into which the dyeing solution

is introduced. Running along the whole length of the