910 INDIGO 



diluted sulphuric or muriatic acid, which removes the adhering lime and oxide of 

 iron. After being used for some time, the vat should be refreshed or fed with copperas 

 and lime, upon which occasion the sediment must first be stirred up, and then allowed 

 to settle again, so as to leave the liquor clear. The indigo-blue, however, is in course 

 of time gradually removed, and by degrees the vat becomes capable of dying only pale 

 shades of blue. When the colour produced by it is only very faint, it is no longer 

 worth while using it, and the contents are then thrown away. In dying cotton with 

 indigo, it seems to be essential that the reduced indigo-blue should be in combination 

 with lime. If potash or soda be used in its stead it is impossible to obtain dark 

 shades of blue. 



When cotton piece goods are to be dyed of a uniform blue, they are not submitted 

 to any preparatory process of bleaching or washing, Indeed, the size contained in 

 unbleached goods seems rather to facilitate than to impede the dyeing process. In 

 dying these goods a peculiar roller apparatus is employed. When certain portions 

 of the fabric are to retain their white colour a different plan is adopted. The pieces 

 having been bleached, those portions which are to remain white are printed with so- 

 called resists. These resists consist essentially of some salt of copper, mixed with an 

 appropriate thickening material. The copper salt acts by oxidising the reduced 

 indigo-blue at the surface, and thus rendering it insoluble before it can enter the 

 interior of the vegetable fibre, since it is only when deposited within the fibre itself 

 that the colouring matter becomes durably fixed. The pieces are now stretched upon 

 square dipping frames, made of wood or of iron, furnished with sharp hooks or points 

 of attachment. These frames are supended by cords over a pulley, and thus 

 immersed and lifted out alternately at proper intervals. In dyeing, a set of 10 vats is 

 Tised, the first vat containing 5 or 6 Ibs. of indigo, and the quality increasing gradually 

 up to 80 Ibs. in the last vat. The pieces are dipped for 7 2 minutes in the first vat, 

 then taken out and exposed to the air for the same length of time, then dipped in the 

 second vat, and so on to the last. After passing through the last vat, a small bit of 

 the calico is dried, in order to see whether the colour is sufficiently dark. If it is not, 

 the whole series must be dipped once more in the same vat in which the last dipping 

 was performed. When the bottom of the vat is raked up so as to have more lime in 

 suspension, the vat becomes what the dyer calls hard, that is to say, the oxide of 

 copper of the resist is precipitated in a compact state, and consequently acts with 

 more efficiency. But when the vat has been at rest for some time, and there is little 

 lime in suspension, then it is called soft. When it is in this state, the oxide of copper 

 is thrown down in a bulky form, and when the pieces are afterwards agitated in the 

 liquor, in order to detach the oxide of iron which always floats about in the vat, and 

 attaches itself to the fabric, and which, if left adhering, would cause light stains, 

 technically called grounding ; then the oxide of copper is also detached, and the iadigo 

 penetrates to those parts which are to remain white. When cotton yarn is dyt-d in 

 the copperas vat, the latter is generally heated by means of steam-pipes passing 

 through the liquor, the object being to give to the colour the peculiar gloss or lustiv, 

 which is required in this class of goods. No preparatory process is required, except 

 simply steeping in hot water. In dyeing, wooden pins are put through the hanks, 

 their ends resting on supports passing over the top of the vat, and the yarn is then 

 slowly turned over, one half being in the liquor, the other half over the pins. It is 

 then taken out, wrung, exposed to the air, and again dipped, this operation being 

 repeated until the requisite shade is obtained. 



The methods employed for producing the colours called China blue and pencil blue 

 on calico have been described under Calico-Printing. 



The urine vat is prepared by digestion of the ground indigo into warmed stale urine, 

 which first deoxidises the indigo-blue, and then dissolves it by means of its ammonia. 

 Madder and alum are likewise added, the latter being of use to moderate the fermen- 

 tation. This vat was employed more commonly formerly than at present, for the 

 purpose of dyeing woollen and linen goods. 



Wood Vat. In former times, woad was the only material known to the dyers of 

 Europe for producing the blue colour of indigo. For this purpose it was previously 

 .submitted to a peculiar process of fermentation, and the product was named pastel in 

 France. For most purposes indigo has taken the place of woad in the dye-house, 

 and for cotton goods it is now used alone. In the dyeing of woollen goods. however, 

 the use of woad has been retained to the present day, for the purpose rather of 

 exciting fermentation and thus reducing the indigo which is employed at the same 

 time, than of imparting any colour to the material to be dj-cd. Indeed, ti 

 used by woollen dyers in this country contains no trace of colour! up: matter. \ ariou- 

 suhsUnces, such as rhubarb-leaves, turnip-tops, weld, and other vegetable matters, 

 have accordingly been tried, but without success, since the fermentation is nu.ro 

 steadily maintained by means of woad than by any other material. Pastel, which 



