DYEING 169 



Such substances, used to unite tho colour with the stuff) have been termed mordants-, 

 it appeared that they had a mutual attraction for the stuff and colour, and combining 

 with the stuff first, they afterwards took up the colour ; but this is only so in some 

 instances. A few examples will illustrate the action of these mordants. If a piece 

 of cotton stuff is put into a decoction of logwood, it will get stained of a depth accor- 

 ding to the colour of the solution, but this stain or colour may be washed from tho 

 cotton by putting it into pure water, tho colour being soluble. If another piece of 

 cotton stuff be put into a solution of protosulphate of iron, and then washed from this, 

 a portion of the iron will have undergone oxidation, and left the acid, and become 

 fixed upon the fibre and insoluble in water. Whether this oxidation is the result of 

 an influence of the stuff, or the effect of the oxygen of the air and water in which the 

 goods are exposed, it does not now matter ; this fixed oxide, however, constituted an 

 example of a mordant, by its combining with the stuff. If this stuff is now put into 

 a decoction of logwood, the colouring matter of the logwood will combine with the 

 oxide of iron fixed upon the fibre, and form an insoluble colour, which after washing 

 will not remove from the stuff. If, instead of washing the stuff from the sulphate-of- 

 iron solution in water, it be passed through an alkaline lye of soda or potash, the 

 acid holding the iron in solution is taken hold of by the alkali, and removed. The 

 oxide of iron is thus left upon the stuff in a much larger quantity than in the 

 former case, and as firmly fixed, although not by any attraction between it and the 

 fibre, but simply by being left within it. And this stuff being now put into the logwood 

 liquor, will form a dye of a depth according to the quantity of iron thus fixed upon 

 the stuff, and equally permanent with that which had been fixed on the stuff by the 

 oxidation in working. 



Such then are the methods of fixing within the stuff insoluble colours from soluble 

 compounds, and from these remarks the necessity of having the dye in solution will 

 also be evident. 



Suppose again that the sulphate of iron be mixed with the logwood decoction, there 

 will be produced the same colour or dye as an insoluble precipitate : if the cotton 

 stuff is put into this, no colour worthy of the name of a dye will be obtained, as the 

 cotton will not imbibe within its fibre this precipitate. Place woollen stuff in the same 

 liquid, and there is formed a very good dye, the woollen fibre having imbibed a great 

 portion of the solid precipitate, probably owing to woollen fibres being much larger than 

 those of cotton. Thus, with cotton and other stuff that will not imbibe freely solid pre- 

 cipitates, the mordant must be fixed within the fibre previously to applying the colouring 

 substances, such as the vegetable decoctions. It will also be seen that the dye whicli 

 is the product of combination between the mordant and colour is not that of the natural 

 colour of the drug, but the colour of the compound. Hence the great variety of tints 

 capable of being produced from one dye-drug, by varying either the kind or intensity 

 of the mordant. So that in the above instances, it is not the colour of the hematoxylin 

 fixed on the stuff, but its compound with iron, or tin, or alumina, as the case may be, 

 all of which give different tints. 



It is this principle of rendering bases insoluble while within the fibre, by chemi- 

 cal means, that has brought to the use of the dyer a great number of mineral dyes 

 which in themselves, whether separate or combined, have no attraction whatever 

 for the fibre ; such as solutions of sulphate of copper, and yellow prussiate of potash, 

 nitrate of lead, and bichromate of potash, &c. Suppose the stuff to be dyed a yellow 

 by the two last-named salts were first put into the solution of lead and then washed 

 previously to being put into the bichromate solution ; the greater portion of the lead 

 would be dissolved from the stuff, and a very weak colour would be obtained. If the 

 stuff from the lead solution was put directly into the bichromate solution, a very good 

 dye would be the result ; but the portion of the solution remaining upon the surface of 

 the stuff will combine with the chrome and form a precipitate which the fibre cannot 

 imbibe, but it will form an external crust or pigment upon the surface, which blocks up 

 the pores, and exhausts to no purpose the dye, causing great waste : hence the stuff 

 from the solution of lead is put into water containing a little soda or lime, and the 

 lead is thus reduced to an insoluble oxide within the fibre. The goods may now be 

 washed from any loose oxide adhering, and then passed through the bichromate 

 solution, when the chromic acid combines with the oxide of lead, forming a permanent 

 yellow dye. Thus it will be seen that whether the combination of the colour with 

 the stuff be chemical or mechanical, the production of the dye which is fixed upon the 

 fibre is certainly a chemical question, and the dyer should be familiar with the nature 

 and principles of these reactions. 



There are a few instances where the dye produced does not come within the sphere 

 of these principles, there being no mordants required, nor any combination of the 

 colour formed within the stuff, but the dye-drug in its natural hue is fixed within the 

 fibre. Such colours have been termed substantive, to distinguish them from those pro- 



