360 BLACK DYE 



precision, it is better that the sulphate of iron should predominate, because the as- 

 tringent, -when in excess, counteracts the precipitation of the black colouring par- 

 ticles, and has the property of even dissolving them. 



This action of the astringent is such that, if a pattern of black cloth be boiled -with 

 gall-nuts, it is reducible to grey. An observation of Lewis may thence be explained. 

 If cloth be turned several times through the colouring bath, after it has taken a 

 good black colour, instead of obtaining more body, it is weakened, and becomes 

 brownish. Too considerable a quantity of the ingredients produces the same effect ; 

 to which the sulphuric acid, sot at liberty by the precipitation of the oxide of iron, 

 contributes. 



It is merely the highly oxidised sulphate which is decomposed by the astringent ; 

 whence it appears that the sulphate will produce a different effect according to its 

 state of oxidation, and call for other proportions. Some advise, therefore, to follow 

 the method of Proust, employing it in the oxidised state ; but in this case it is only 

 partially decomposed, and another part is brought, by the action of the astringent, 

 into the lower degree of oxidation. 



The particles precipitated by the mixture of an astringent and sulphate of iron have 

 not at first a deep colour ; but they pass to a black by contact of air while they are 

 moist. 



Black dye is only a very condensed colour, and it assumes more intensity from 

 the mixture of different colours likewise deep. It is for this reason advantageous 

 to unite several astringents, each combination of which produces a different shade. 

 But blue appears the colour most conducive to this effect, and it corrects the 

 tendency to dun, which is remarked in the black produced on stuffs by the other 

 astringents. 



On this property is founded the practice of giving a blue ground to black cloths, 

 which acquire more beauty and solidity the deeper the blue. Another advantage of 

 this practice is to diminish the quantity of sulphuric acid -which is necessarily disen- 

 gaged by the precipitation of the black particles, and which would not only counteract 

 their fixation, but would further weaken the stuff, and give it harshness. For common 

 stuffs, a portion of the effect of the blue ground is produced by the rooting. 



The mixture of logwood with astringents contributes to the beauty of the black in 

 a twofold way. It produces molecules of a hue different from what the astringents 

 do, and particularly blue molecules, with the acetate of copper, commonly employed 

 in the black dyes ; which appears to be more useful the more acetate the verdigris 

 made use of contains. 



The boil of -weld by which the dye of black cloth is frequently finished, may also 

 contribute to its beauty, by the shade peculiar to its combination. It has, moreover, 

 the advantage of giving softness to the stuffs. 



The processes that are employed for -wool yield, according to the observation of 

 Lewis, only a rusty black to silk ; and cotton is hardly dyed by the processes proper 

 for wool and silk. Let us endeavour to ascertain the conditions which these three 

 varieties of dyeing demand. 



Wool has a great tendency to combine with colouring substances ; but its physical 

 nature requires its combinations to be made in general at a high temperature. The 

 combination of the black molecules may therefore be directly effected in a bath, in 

 proportion as they form ; and, if the operation be prolonged by subdividing it, it is 

 only with the view of changing the necessary oxidation of the sulphate and 

 augmenting that of the colouring particles themselves. 



Silk has not the same disposition to unite with the black particles. It seems to be 

 assisted by the agency of the tannin, with which it is previously impregnated, espe- 

 cially after it has been scoured. A very deep black may be obtained upon 100 Ibs. 

 of silk, by -working it for two hours in a solution of 20 Ibs. of copperas and 3 pints 

 of nitrate of iron. Wash from this thoroughly, and then wash for two hours more in 

 a decoction of 100 Ibs. of logwood and 20 Ibs. of fustic. Lift up. and add to the bath 

 a solution of 3 Ibs. of copperas, and work half an hour longer, and wash. A beau- 

 tiful rich blue-black is produced by dyeing the silk a deep royal blue, then working 

 for an hour in a solution of copperas (2 ounces to the pound of silk), washing from 

 this, and working in a bath of logwood, using half a pound to each pound of silk, 

 and adding, after an hour's working, a few ounces of copperas ; working half an 

 hour longer, and finishing. 



Cotton has no affinity for the black dye, and has always to be impregnated or com- 

 bined with astringent substances, in order to produce the dye. A good deep black 

 will be imparted to 100 Ibs. of cotton by steeping it in a decoction of 30 Ibs. of 

 sumach, at a boiling heat, and allowing it to stand till perfectly cool ; then passing it 

 through lime-water, and, immediately after this, working for an hour in a solution of 

 20 Ibs. of copperas. After this, expose for an hour to the air ; then pass through lime- 



