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Online Encyclopedia
Originally appearing in Volume V08, Page 748 of the 1911 Encyclopedia Britannica.
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DEVELOPED CoLouRs.—This group. includes certain azo colours which are developed or produced upon the fibre itself (usually cotton) by the successive application of their constituent elements. It may be conveniently divided into the following sub-groups:—Insoluble Azo Colours, Developed Direct Colours, Benzo Nitrol Colours. (a) The Insoluble Azo Colours are produced as insoluble coloured precipitates by adding a solution of a diazo compound to an alkaline solution of a phenol, or to an acid solution of an amido compound. The necessary diazo compound is prepared by allowing a solution containing nitrous acid to act upon a solution of a primary aromatic amine. It is usually desirable to keep the solutions cool with ice, owing to the very unstable nature of the diazo compounds produced. The colour obtained varies according to the particular diazo compound, as well as the amine or phenol employed, '3- naphthol being the most useful among the latter. The same coloured precipitates are produced upon the cotton fibre if the material is first impregnated with an alkaline solution of the phenol, then dried and passed into a cold solution of the diazo solution. The most important of these colours is para-nitraniline red, which is dyed in enormous quantities on cotton pieces. The pieces are first " prepared " by running them on a padding machine through a solution made up of 30 grins. $ - naphthol, 20 grms. caustic soda, 5o grms. Turkey red oil, and 5 grms. tartar emetic in loon grms. (1 litre) water. They are then dried on the drying-machine, and are passed, after being allowed to cool, into the diazo solution, which is prepared as follows: 15 grms. para-nitraniline are dissolved in 53 C.C. hydrochloric acid (34° Tw.) and a sufficiency of water. To the cold solution a solution of toe gyms: sodium nitrite is added while stirring. The whole is then made up to 1200 c.c., and just before use 6o grms. sodium acetate are added. The colour is developed almost immediately, but it is well to allow the cotton to remain in contact with the solution for a few minutes. The dyed cotton is squeezed, washed, soaped slightly, and finally rinsed in water and dried. A brilliant red is then obtained which is fast to soap but not to light. If the para-nitraniline used in the fore-going process is replaced by meta - nitraniline, a yellowish-orange colour is obtained ; with a-naphthylamine, a claret-red ; with amidoazo-toluene, a brownish red; with benzidine, a dark chocolate; with dianisidine, a dark blue; and so on. The dyed colours are fast to washing and are much used in practice, particularly the paranitraniline red, which serves as a substitute for Turkey-red, although it is not so fast to light as the latter. (b) Developed Direct Colours.—The primuline colours were the first representatives of this class and are derived from the yellow dyestuff known as primuline, which dyes cotton in the same manner as the direct colours. The primuline yellow thus obtained is fugitive to light and of little practical value, but since the colouring matter is an amido base it can be diazotized in the fibre and then developed in solutions of phenols or amines, whereby azo dyes of various hues may be obtained, according to the developer employed; thus, j9-naphthol develops a bright red colour, resorcin develops an orange, phenol a yellow, naphthylamine a brown, &c. The dyeing of the primuline yellow is effected by boiling the cotton for one hour in a solution of primuline (5 %) and common salt (lo to 20 %). The diazotizing operation consists in passing the dyed and rinsed cotton for 5 to to minutes into a cold solution of nitrous acid—i.e. a solution of oz. sodium nitrite per gallon of water, slightly acidified with sulphuric acid. The diazotized material should not be exposed to light, but at once washed in cold water and passed into the developer. The developing process consists in working the diazotized material for 5 to io minutes in a cold solution of the necessary phenol, and finally washing with water. The only developer of any practical importance is a solution of #- naphthol in caustic soda, which produces primuline red. The primuline colours are best adapted for cotton dyeing, and the colours obtained are fast to washing and to moderate soaping, but they are not very fast to light. If cotton is dyed with other direct colours containing free amido groups, the colour can be diazotized on the dyed fibre exactly in the same manner as in the case of prim uline-dyed cotton, and then developed by passing into the solution of an amine or phenol, or by treating it with a warm solution of sodium carbonate. In this manner a new azo dye is produced upon the fibre, which differs from the original one not only in colour, but also by being faster to washing and other influences. A treatment with copper sulphate solution after development is frequently beneficial in rendering the colour faster to light. Some Direct Colours, indeed, are of little value, owing, for example, to their sensibility to acids, until they have been diazotized and developed, the usual developers being $-naphthol, resorcinol, phenol and phenylene-diamine. The following Direct Colours, after being applied to cotton, may be submitted to the above treatment, the colours produced being chiefly blue, brown and black: Blue.—Diazurine, diazo blue, diamine blue, diaminogene. Red.—Rosanthiene. Brown.—Diazo brown, diamine cutch, diamine brown, cotton brown. Grey and Black.—Benzo blue, diazo blue black, diazo black, diamine black, diazo brilliant black. (c) Benzo Nitrol Colours.—These are certain Direct Colours, dyed on cotton in the ordinary manner, which are then developed by passing into a diazo solution—e.g. diazotized para-nitraniline, &c. The dyed colour here plays the part of a phenol or amine, and reacts with the diazo compound to produce a new colour. The process is similar to the production of the Insoluble Azo Colours, the )3- naphthol which is there applied to the fibre being here replaced by a Direct Colour. The colour of the latter is rendered much deeper by the process, and also faster to washing and to the action of acids. The dyestuffs recommended for application in the manner described are: Benzo nitrol brown, toluylene brown, direct fast brown, Pluto black, direct blue black. " Topping " Direct Colours.—The direct colours possess the remarkable property of precipitating the basic colours from aqueous solution. Use is frequently made of this property for " topping ' cotton dyed with direct colours either with a view to obtain compound shades or to brighten the colour. Thus by dyeing cotton first yellow in chrysamine and then dyeing it again in a cold bath of methylene blue a brilliant shade of green results. If, on the other hand, a direct blue is topped with' methylene blue, its brilliancy may be enhanced.
End of Article: DEVELOPED
CHARLES DEVENS (1820-1891)

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