INDIGO 



909 



which it is reduced to a soft paste in a mill. The indigo-mill is represented in fgs. 

 1210 and 1211. 



a, is a four-sided iron cistern, 2 feet 11 inches long, 19 inches broad, and 18 inches 

 deep, cylindrical or rounded in the bottom, and resting upon gudgeons in a wooden 

 frame. It has an iron lid b, consisting of two leaves, between which the rod c moves 

 to and fro, receiving a vibratory motion from the crank d. By this construction, a 



1210 



1211 



frame e, which is made fast in the cistern by two points e' e', is caused to vibrate, and 

 to impart its swing movement to six iron rollers/,/,/, four inches in diameter, three 

 being on each side of the frame, which triturate the indigo mixed with water into a 

 fine paste. This mill is capable of grinding 1 cwt. of indigo at a time. Whenever 

 the paste is uniformly ground, it is drawn off by the stopcock g, which had been 

 previously filled up by a screwed plug, in order to prevent any of the indigo from 

 lodging in the orifice of the cock, and thereby escaping the action of the rollers. 



Mills of other forms are also used occasionally. One of these consists of a hemi- 

 spherical iron vessel open at the top, in which a stone of corresponding shape is fixed, 

 so as to leave a small space between it and the sides and bottom of the vessel, in which 

 the indigo undergoes the necessary trituration with water, the motion being produced 

 by means of a vertical shaft fixed to the centre of the stone. 



The other ingredients necessary for setting the vat are copperas or protosulphate 

 of iron, newly slaked quicklime, and water. Various proportions of these ingredients 

 are employed, as for instance, 1 part by weight of indigo (dry), 3 parts of copperas, 

 and 4 of lime ; or 1 of indigo, 2 J of copperas, and 3 of lime ; or 8 of indigo, 14 of 

 copperas, and 20 of lime ; or 1 of indigo, f of copperas, and 1 of lime. The sulphate 

 of iron should be as free as possible from the red oxide of iron, as well as from sulphate 

 of copper, which would re-oxidise the reduced indigo-blue. The vat having been 

 filled with water to near the top, the materials are introduced, and the whole after 

 being well stirred several times is left to stand for about twelve hours. The chemical 

 action which takes place is very simple. The protoxide of iron which is set at liberty 

 by the lime reduces the indigo-blue, and the indigo-white is then dissolved by the 

 excess of lime, forming a solution, which, on being examined in a glass, appears per- 

 fectly transparent and of a pure yellow colour, and becomes covered wherever it 

 comes into contact with the air, with a copper-coloured pellicle of regenerated indigo- 

 blue. The sediment at the bottom of the vat consists of sulphate of lime, peroxide of 

 iron, and the insoluble impurities of the indigo, such as indigo-brown in combination 

 with lime, as well as sand, clay, &c. If an excess of lime is present, a little reduced 

 indigo-blue will also be found in the sediment in combination with lime. 



The copperas vat is employed in dyeing cotton, linen, and silk. For cotton goods 

 no other kind of vat is used at the present day. The dyeing process itself is very 

 simple. The vat having been allowed to settle, the goods are plunged into the clear 

 liquor, and after being gently moved about in it for some time are taken out, allowed 

 to drain, and exposed to the action of the atmosphere. Whilst in the liquid the fabric 

 attracts a portion of the reduced indigo-blue. On now removing it from the liquid it 

 appears green, but soon becomes blue on exposure to the air in consequence of the 

 oxidation of the reduced indigo-blue. On again plunging it into the vat, the de- 

 oxidising action of the latter does not again remove the indigo-blue which has been 

 deposited within and around the vegetable or animal fibre, but on the contrary, a 

 fresh portion of reduced indigo-blue is attracted, which on removal from the liquid is 

 again oxidised like the first, and the colour thus becomes a shade darker. By repeat- 

 ing this process several times, the requisite depth of colour is attained. This effect 

 cannot in any case be produced by one immersion in the vat, however strong it may 

 be. The beauty of the colour is increased by finally passing the goods through 



