This method is now being superseded by the Freiberger process, a simpler and more pro ductive way of discharging, the reagents used being either chromic acid, sodium chlorate or, in some cases, bromate and, still more recently, nitrates. This latter method passes the cotton fabric imprinted with nitrate discharging color through hot and concentrated sulphuric acid and, by accurate control of the strength, tem perature and time exposed, not only preserves the fabric in undiminished strength but obtains the most accurate and permanent discharge effects.
Bronze Style.— Manganese brown or bronze is decolorized by reducing agents; hence white discharge patterns on a bronze ground are obtained by printing plain manganese-brown dyed calico with a mixture of stannous chloride and oxalic acid, and then steaming. Colored discharge patterns are obtained if coloring mat ters are added to the printing mixture which are not affected by reducing-agents, or which even require stannous chloride as a mordant to de velop the color as Prussian blue, chrome yel low, Persian-berry yellow, Brazil-wood pink, safranine, acridine orange, etc.
Aniline Black Style.—Aniline black being a product of the oxidation of aniline, patterns in this color on a white ground are obtained by printing a thickened solution of aniline hydro chloride containing the oxidizing agent, sodium chlorate and a salt of copper or vanadium. When the printed fabric is slightly steamed or exposed to a moist, warm atmosphere, the im pression, which is at first devoid of color, grad ually becomes dark green, and this by a final treatment•with an alkaline solution, soap, etc., changes at once to a rich black. The color is extremely fast to light, alkalis, acids, etc., and it is largely employed by the printer, both alone and in conjunction with dyed or steam colors. The development of the black during the age ing or oxidizing process occurs only in the pres ence of a mineral acid, hence resist whites are obtained by first printing the design on the white calico with thickened solutions of sub stances of an alkaline or reducing character or salts of organic acids, as acetate of soda, and then printing or padding over all with the ani line black mixture, ageing', steaming, Where the design is printed the alkalinity entirely pre vents the development of the black. Pigment colors thickened with albumen, also certain benzidine colors, containing an admixture of chalk, acetate of soda, etc., are largely em ployed in this manner. These resist colors may also be printed immediately after the ap plication of the aniline black mixture, before the development of the color by ageing.
Azo Color Style.— The so-called insoluble azo colors result from the interaction of an azo compound and a phenol. Two methods of printing based upon this principle are employed.
One method is to print the design with a thick ened solution of 13-naphthol on the white calico, and then pass the fabric through a very cold solution of the azo compound (developing bath), when the design at once appears in a color corresponding to the azo compound em ployed. Another method is to print the design with a thickened solution of the azo compound upon calico which has been previously impreg nated with a solution of sodium-naphthol and dried; in this case the color of the design is de veloped in the moment of impression. The necessary azo compounds are obtained by the action of nitrous acid, on salts of amido sub stances for example, paranitraniline, naphthy lamine nitrotoluidine, dianisidine, etc., each of which yields a distinct color, bright red, claret red, orange, blue, etc. The naphthol-prepared cloth and also the azo compounds are somewhat unstable, so that this style is not successfully printed without considerable care. The in soluble azo colors, also the direct or benzidine colors, are capable of furnishing discharge pat terns, since, in common with the azo colors gen erally, they are readily decomposed and .destroyed by reducing-agents. It suffices to print calico dyed with these colors, as benzopur purine, chrysophenine, benzoazurine, Mikado brown, etc., with a mixture containing stannous acetate, zinc powder, or other similar reducing agent, and then steam the printed fabric, to ob tain white discharge patterns. If there be added to the printing mixture such mordants and coloring matters as are not affected by reduc ing-agents, for example, safranine, auramine, etc., a variety of colored discharges are ob tained, exactly as in the bronze style. Many of the benzidine colors may also be printed direct on white calico to furnish color designs, but such prints are not particularly fast to washing.
A newer method of printing on cloth by a lithographic process has recently been invented. The principle involved is printing lithographically upon the fabric with lithographic, or oil, colors and dispensing with the practice of fixing the colors after printing, which is essential when aniline or allied colors are used. The actual process is continuous, as in ordinary calico printing, but instead of ordinary copper rollers being used for impress ing the pattern the effects are obtained by the use of continuous metal plates, or tubes, upon which the designs are transferred directly in the manner employed in ordinary lithographic printing on paper. It is said to cheapen pro duction greatly, for as soon as an order is com pleted of one design, the rollers may be cleaned and fresh patterns immediately transferred.
Bibliography.— Blackwood, William, 'Cali co Engraving' (London 1913) ; Stiitz, Ernest, in the 'Journal of the Franklin Institute> (Janu ary 1914).