CHAMBERS'S INFORMATION 'FOR THE PEOPLE. 



4. Formation of insoluble colouring-matter by 

 oxidation of a soluble substance. The most 

 important instance of this method is indigo dye- 

 ing. It has already been stated (see p. 343) that 

 indigo blue (C 8 H 6 NO) is converted into indigo 

 white (C 16 H 1S N S O 8 ), when treated with a reducing 

 agent and an alkali, and that the indigo white 

 thus produced dissolves in the alkaline solution. 

 Further, we have seen that when this solution of 

 indigo white is exposed to the air, indigo blue is 

 reproduced by oxidation. When cloth or yarn is 

 soaked in an alkaline solution of indigo white, 

 and then exposed to the air, the same change 

 takes place, and the insoluble indigo blue is 

 deposited in the fibres. 



The various processes of indigo dyeing differ 

 from one another chiefly in the reducing agent 

 employed. Thus, we have the ' copperas vat,' in 

 which lime and potash are used to render the 

 solution alkaline, and copperas (ferrous sulphate) 

 is the reducing agent. The ferrous sulphate is at 

 once decomposed by the powerful bases, lime and 

 potash, and ferrous hydrate precipitated. This 

 gradually becomes oxidised to ferric hydrate, 

 taking oxygen from the water ; while the hydrogen 

 unites with the indigo blue, and forms indigo 

 white. This vat is usually employed in dyeing 

 cotton. In the vats used for dyeing wool, a pro- 

 cess of reduction resembling that by which indigo 

 white is prepared in the manufacture of indigo, is 

 used. Potash or soda is used as the alkali, and 

 the indigo is reduced by the fermentation of bran, 

 molasses, or a plant called ' woad,' which itself 

 contains a small quantity of indican. 



The production of brown colours by means of 

 tannin is also a case of oxidation, the tannin, 

 especially in the presence of alkalies, being con- 

 verted into a brownish-red substance. Also black 

 peroxide of manganese (MnO^ is produced by 

 oxidation of manganous hydrate (MnO,H 2 O) de- 

 posited in the fibre by the action of an alkali upon 

 a soluble manganous salt. 



Calico-printing is merely a special case of dye- 

 ing in which the action is restricted to parts of 

 the cloth, so as to produce a pattern. 



This may be accomplished in various ways. 

 We shall describe four of these ways, observing 

 that sometimes two of them are used at once. 



1. A mordant may be applied, in the form of 

 the pattern, to the cloth by means of blocks or 

 cylinders, so that the mordant, thickened with 

 starch or gum, is fixed upon those parts of the 

 cloth which are intended to be coloured. The 

 cloth is then passed through the dye-vat, the dye- 

 stuff fixes itself upon the mordanted pattern, and 

 can be washed out of the rest of the cloth. By 

 this method, several different colours can be pro- 

 duced by one operation. Thus, if we print by 

 means of four blocks or cylinders, four patterns, 

 a, with alumina mordant ; b, with weak iron 

 mordant ; c, with strong iron mordant ; d, with 

 a mixture of alumina and iron mordant ; and then 

 dye in a madder vat the pattern a comes out 

 red ; b, violet ; c, black ; and d, chocolate ; while 

 the unprinted parts are white. 



2. The cloth may be printed with what are 

 called resists that is, substances which will pre- 

 vent the dyeing taking effect upon the parts of 

 the cloth covered by them. We thus obtain a 

 white pattern upon a coloured ground, the whole 

 of the cloth except the parts covered by the 



316 



' resist ' being dyed. Resists are most frequently- 

 used in indigo dyeing, and then consist of sub- 

 stances, such as cupric salts, which can oxidise 

 white to blue indigo. When the cloth upon which, 

 such a composition has been printed is dipped in> 

 the indigo vat, the solution of indigo white soaks- 

 into all parts of the cloth except the printed 

 pattern, into which it cannot penetrate, the indigo- 

 white being converted by the resist into insoluble 

 indigo blue. On taking the piece of cloth out of 

 the vat, the pattern is seen to be covered with 

 indigo blue, but not dyed ; the blue is superficial,, 

 and can be washed off. Along with the resist, a. 

 mordant for another colour may be printed ; thus,, 

 alumina mordant mixed with the resist will be ready 

 to receive a red colour from a madder bath, after 

 the ground has been dyed blue in the indigo vat. 



3. A method of producing a white pattern on- 

 a coloured ground by 'discharging' the colour 

 from the pattern has already (p. 339) been de- 

 scribed ; we shall here mention one or two 

 other processes of a similar kind. A piece of 

 cloth uniformly mordanted may have a pattern 

 printed on it in a substance which renders the 

 mordant inoperative. Thus, if an alumina or iron, 

 mordant is used, we can print a pattern upon the 

 cloth with a mixture containing a fixed acid, such 

 as citric acid. This prevents the formation of a 

 basic salt or hydrated oxide, upon which, as 

 already described, the action of the mordant 

 depends the pattern, therefore, is not dyed when 

 the cloth is passed through the dye-vat. 



If the acid mixture is printed before the cloth is 

 mordanted, the same result is produced, and the 

 process becomes one of 'resist' rather than dis- 

 charge. Special discharges are used with some 

 colours. Thus, indigo can be discharged by 

 certain oxidising agents which convert it into 

 isatine (C 8 H 5 NO 2 ), a soluble substance which can 

 be washed away. Prussian blue can be discharged 

 by caustic potash, which converts it into soluble 

 ferrocyanide of potassium, which is washed out 

 with water, and hydrated ferric oxide, which can 

 be removed by means of dilute sulphuric acid. 



4. The colouring-matter itself, mixed with thick- 

 ening material and mordant, may be printed on 

 the cloth, and then, by the action of steam, fixed 

 to the fibre. The thickening material (starch or 

 gum) is then washed away. Colours produced in 

 this way are not so fast as those regularly dyed, 

 but even in this case a certain amount of fixing in 

 the fibre takes place, probably by the mordant 

 and dye-stuff finding their way uncombined into- 

 the fibre, and uniting there. 



In conclusion, we may mention two processes 

 by which patterns are produced upon cloth, which 

 do not fall under any of the above divisions. 

 These are ' China-blue ' printing, and what is 

 called the ' Scottish press ' process. 



In China-blue printing (so called from the 

 resemblance of the colour to that of blue porce- 

 lain or 'china'), finely powdered indigo blue,, 

 mixed with reducing agents, and starch or gum, is 

 printed on the cloth, which is then passed through 

 an alkaline bath. The indigo is thus reduced to 

 indigo white, which soaks into the pattern, but 

 does not spread beyond it, being retained by the 

 gum. By exposure to the air, the indigo white is 

 reoxidised to indigo blue. In the Scottish press 

 process, the cloth is folded up and placed in a 

 powerful press between metal plates with holes 



