Developer. Colour produced.
Beta-naphthol Bright scarlet. Alpha-naphthol Crimson. Phenylene diamine Brown. Phenol Gold yellow. Resorcine Orange. Naphthylamine ether Blue. Blue developer A N Green.
As regards the dyeing operation, it no way differs from that described for simple direct colours. It should, however, be noted that if good results are required full shades must be
dyed. The cotton must be rinsed in cold water, and be quite cold before it is subjected to the diazotising operation. Diazotising is a simple operation, yet it must be carried out with care if good results are desired. It consists essentially in the use of an acidulated bath of sodium nitrite.
To make the bath for diazotising there is taken (for each 100 lb. of goods) sufficient water to handle them in comfortably, 8 lb. of sodium nitrite and 6 lb. hydrochloric acid. This bath must be quite cold otherwise it does not work well. The goods are handled in this for from fifteen to twenty minutes, when they are ready for the next operation. The bath is not exhausted of nitrite, etc., hence it can be kept standing, and for each succeeding lot of cotton it is strengthened up by adding one-third of the quantities of nitrite and acid originally used. Of course the bath cannot be kept for ever, sooner or later it will get dirty, and then it must be thrown away and a new bath be made up.
The diazo compounds formed on the fibre are not very stable bodies. They decompose on being exposed for any great length of time to the air, while light has a strong action on most, if not all of them; hence it follows that the diazotising process should not be carried out in a room where direct, strong sunlight can enter or fall upon the goods. Then again, after diazotising, the treated goods should not be allowed to lie about exposed to air and light, but the operation of developing should be proceeded with at once, otherwise the diazo body will decompose, and weak and defective colours are liable to be obtained on subsequent development.
For developing, quite a large number of substances are used. Some of these are regular articles of commerce, others are the special productions of certain firms, who advise their use with the dyes that they also manufacture. These latter are sent out under such designations as Developer B, Developer A N, or Fast-blue developer. Those most in use are beta-naphthol for red from Primuline, and for bluish blacks from
Diamine blacks, Diazo blacks, Zambesi blacks, etc.; for dark blues from Diamine blues, Diazo blues, etc.; for greys from Diamine blues, Neutral grey, etc. Alpha-naphthol for dark reds from Primuline, greys from Diamine blues, Neutral grey, etc. Phenylene diamine for blacks from Diamine blacks, Diazo blacks, Zambesi blacks, Triamine blacks, etc.; for dark browns from Diamine browns, Diazo browns, etc.; for light browns from Cotton browns, Diamine cutch, Primuline, etc. Naphthylamine ether for blues from Diamine blacks, etc. Phenol for claret from Diamine cutch, and for gold yellow from Primuline, etc. Resorcine for orange from Primuline, etc. Soda for browns from Diamine cutch, Diazo browns, Zambesi browns, for orange from Diamine orange, and yellow from Primuline.
Beta-naphthol.—This is by far the most important of the developers. It is a white body, insoluble in water, but readily soluble in soda lye, and a solution is easily made by taking 10 lb. beta-naphthol and heating it with 10 lb. caustic soda lye of 70° Tw. and 60 gallons of water. This bath may be used as the developing bath, or it may be diluted with more water. It is not desirable to use any more caustic soda than is necessary to dissolve the beta-naphthol, so that the bath is not too alkaline. To produce full shades it usually takes 1 per cent. of the weight of the cotton of the beta-naphthol, but it is best to use the bath as a continuous one and for the first lot of cotton use 2 per cent. of naphthol, while for each succeeding lot only 1 per cent. more naphthol need be added to the same bath.
This bath is alkaline, while the diazotising bath is acid, unless, therefore, the cotton be well washed when it is taken from the latter bath there is a risk of the alkali of the one being neutralised by the acidity of the other, and the naphthol being thrown out in an insoluble form. This, of course, is easily remedied should it occur.