MORDANTS. These are substances capable of combining with other substanees (the commer cial dyestuffs) to form insoluble colored com Thus. aluminum salts make an in soluble red compound with alizarin, arid are therefore used a= mordants in calico-printing. Ferrie salts make, with alizarin. an insoluble dark purple compound, and are likewise em ployed mordants in ealieo-printing. Soluble mordant salts must themselves he transformed into insoluble compound, before they are allowed to form the dyes, in order to prevent the mor &tilting material from being dissolved out of the fabric when the latter is introduced into the dye-bath. Therefore, after the fabric has laconic impregnated with the soluble mordant salt, it is treated with ammonia, lime, or hot steamówith a view to transforming the soluble salt into an insoluble hydroxide, or an insoluble basic salt; or else the fabric is treated with sodium phos phate or sodium arsenate with a view to trait, forming the soluble salt an insoluble phos phate or arsenate. in the case of animal fibres, the same end is often attained by simply im mersing the fabric in a boiling dilute solution of the soluble salt, the insoluble mordant being then deposited in the fibre directly. The fabric containing the insoluble mordant is ready to be treated with the dyeing substance, the resulting color of the fabric being evidently that of the compound formed by the latter with the mor dant. Another method of dyeing with the aid of
mordants, extensively employed in ealieo.print iig% consists in mixing the dye directly with the soluble form of the mordant and with starch. dextrin. gum, or some other thickening sub stance, printing the mixture on the fabric, and then subjecting the latter to a process of steam ing. The result is of course the same as in case the other methods are employed: for the steaming process has the effect of transforming the soluble into an insoluble form of the mor dant. the latter then combining with the dye ing substance. Mordants may be either basic or acid, the former combining with 'acid dyes,' the latter with 'basic dyes! The mordants men tioned above are basic. and their compounds with dyes are termed 'lakes.' Among the acid mor dants may be mentioned tannin, or tannic acid. After the fibre has been impregnated with this mordant in its ordinary, soluble form. it is passed through a weak solution of chloride of tin or of tartar emetic. These produce, respect ively. the tannate of tin or the tannate of anti mony (tartar emetic is a compound of anti mony), and these insoluble tannates further combine into insoluble colored compounds with dyes.