ORGANIC MoRDA.Nms.—Next come a number of bodies having little in common, save the property of fixing colours upon tissues; but which do not, like the bulk of the substances above mentioned (silica and sulphur being the only exceptions), form "lakes," if brought into contact with solutions of colouring matters. Among these bodies, the first to deserve mention is argol, otherwise known as tartar, or cream of tartar (potassium bitartrate). This substance, which consists of tartaric acid in combination with potash, is deposited from the juice of the grape in wine-tuna, and is sold in various grades of purity as red argol, white argol, grey tartar, and crystal tartar. It is very extensively used in woollen-dyeing, along with alum, salts of tin, chrome, &c., and would be employed more extensively were it less expensive. The question whether tartar is a true mordant, or an alterant—serving to modify rather than to fix colours—has been debated with some warmth, and is scarcely decided.
A variety of substitutes for tartar and argol under such names as pro-titrtar, pro-argol, tartar spirits, &c., some of them containing a proportion of tartaric acid, and others being mainly mixtures of bisulphate of soda, common salt, &c., have been used more or less ; but their success is by no means unequivocal.
A very important part as mordants is played by the astringents. (See Tannin.) The animal mordants are chiefly albumen, caseine, and gelatine. If applied to linen, cotton, or other vegetable fibre, they give it the property of taking up colours in the same manner as is done by the animal fibres, silk, wool, &c. The cotton, &c., is then said to be animalized, and can be dyed with magenta, picric acid, &c., without the aid of any ordinary mordant. In other cases, as in the so-called pigment style of calico-printing, the animal mordant in a liquid state is ground up with the colour to be applied, printed upon the fibre, and then rendered insoluble by some appropriate means.
Among these animal mordants, the principal place is due to albumen, the finest quality of which is white of egg, an article necessarily limited in quantity, and very costly. Albumen from blood, if
well prepared, can be used for all hut the very lightest and brightest colours. It is said that blood albumen, perfectly colourless, and equal to the finest egg-albumen, has recently been produced in Germany. The albumen from the roe of fishes, and from certain molluscous animals, cannot be readily made available, on account of the difficulty of removing accompanying substances. Albumen in its natural state is soluble iu water, but is rendered insoluble if heated to 71° (160° F.), a property which thus supplies an easy method of fastening it permanently upon the fibre. It is also coagulated and fixed by tannin, and by sugar of lead.
Caseine agrees very closely with albumen in its chemical composition, but differs from it in several of its properties. In dilute solutions, it is not rendered insoluble by the action of heat. For use, caseine is dissolved in an alkali, generally ammonia, in which case it may be permanently fixed upon the fibre by means of evaporation. It is to be regretted that, in an English patent for the use of caseine as a mordant, it received the utterly needless and unscientific name of " lactarine," to which the trade still cling.
Gelatine or glue in its various forms is also used as a mordant, though it is not well adapted for the pigment style of printing. It is generally, when required, fixed upon the fibre by a subsequent treatment with some astringent, such as decoction of nut-galls.
Another class of organic mordants consists of oily or fatty bodies. Oil has for centuries been found necessary in fixing the colouring-matter of madder (natural alizarine) upon vegetable fibre in the brightest and most permanent condition.
For such purposes, not all oils are suitable, but merely those kinds which are emulsive, i. e. which, if shaken up with a solution of pearl-ash or soda-ash, form a white, milky fluid, from which the oil does not separate for some time.