DYE MATERIALS .

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fastness with those of alumina or iron. On the contrary, iron mordants arevery difficult to employ on woollen fabrics.

A mordant may be applied to a cloth in three ways with regard to thecolouring matter :—

1. The mordant maybe applied before the- colouring matter; and this isusually the case in calico printing, for dyed goods, and frequently in piecedyeing.

2. The mordant may be applied after the colouring; this process is generallyadopted in piece dyeing, where the pieces are first passed through the extra#of the colouring matter, and then through the metallic mordant.

3. The mordant and colouring matter may be applied simultaneously; thistakes place with many colours in dyeing, and with all steam and spirit coloursin calico printing.

In the first case, where the mordant is applied before the colouring matter,the method is necessary in printing designs upon the fabric to be dyed. It isessential that the mordant should be thoroughly fixed upon the cloth beforepassing the latter into the dye-beck, because a design requires that thereshould be at least two shades of colour, one of which may be the white of thecloth, and it is to be easily conceived that if the mordant were loose or floatingabout in the dye-beck it would attach itself indiscriminately to all parts anddestroy the design. The fixing of the mordant necessitates several processesunknown in piece dyeing.

In the second case, that of applying the colouring matter before the mordant,the method is employed with only self-coloured fabrics, and has arisen frommotives of convenience and economy. It usually happens that the mordantis much cheaper than the colouring matter in regard to the quantities employed,and the colouring matter can be more completely utilised by having themordant in excess.

The method of using the mordant and colouring matter together is one oflimited application, because in the majority of cases insoluble lakes are formed,not well adapted to give good and fast colours. In calico printing it ispossible to use the colouring matter in a concentrated state and in combinationwith acids and other matters, which keep it and the mordant in a state ofsolution until the affinities of the cloth come into play, assisted also by artificialmeans, as steaming, passing through alkaline baths, &c., to cause the formationof an insoluble compound and the adhesion of both mordant and colouringprinciple to the cloth.

In practice, colouring matters are classed according to the process by whichthey are fixed, as—dye colours, steam colours, and colours otherwise applied.This classification is arbitrary and vague. The following classification, althoughit possesses but few chances of general acceptance in practice, has the ad-vantage of being rational:—

A. Mechanical fixation of insoluble colours.

An insoluble colouring-matter is applied in the state of a very finelydivided powder, and is fixed upon the fabric by means of a substancewhich becomes solid and generally at the same time insoluble.Examples are given in the dyeing of tissues with ultramarine, Guignet’sgreen, and lamp-black, fixed by means of albumen, gluten, caseine, orshellac.