CHAPTER VII.

MORDANTS.

HOUGH much has been said, incidentally, in the preceding pages con-

cerning mordants, their systematic consideration remains to be un-dertaken. The subjedt is remarkably complicated from two reasons:—Different dyers and printers produce identical results by means so widelyvarying, that what one finds in his peculiar mode of procedure to be usefulor even necessary, another would at once put aside as worthless. Again, thenomenclature of all the articles used in tindtorial industry is in the highestdegree perplexing. One and the same substance is known under differentnames, some of them originating in the workshop ; others being specimens ofalmost every stage through which chemical theory has passed. As far aspossible we shall endeavour to point out which of these substances areidentical, and which really distindl.

A few general considerations will be necessary before entering upon thepradtical part of the subjedt. That mordants are a class of bodies whichserve to fix colouring bodies upon textile fibres is well known. It will have beenperceived from the foregoing portion of this work that few colouring mattersare able to attach themselves to cellulose—whether in the form of cotton,linen, or jute, &c.—without the interposition of some third body, whichhaving an affinity for both, brings them into a more or less permanent stateof combination. Animal tissues and fibres, wool, alpaca, hair, horn, leather,and silk combine unaided with a much larger number of dyes. Yet even intheir case mordants are often employed to give additional brightness and per-manence to the shades produced, or to modify the effedt in some peculiarmanner. Hence most mordants are at the same time alterants, the shadefixed upon the fibre being not that of the dye-ware in its free state, but thatof a combination formed between some one or more of its constituents andthe mordant.

A mordant to be useful must possess a peculiar combination of qualities.It must have a strong affinity or attradtion at once for the fibre and the colour.But these attradtions must not pass certain easily recognised limits. If toopowerful, the work done will be uneven and spotty; certain parts of the goodseither from first entering the dye-pan, from presenting a slightly abradedsurface,* or from a variety of other causes, will take up an excess of the colour,

* The mechanical condition of the surface of a tissue has a powerfully modifying effedt uponits affinity for mordants and colours. Any portion which has been roughened by fridtion takesa deeper, though often a duller, colour than parts of the same fabric which remain smooth and