Acetate of alumina, always obtained in a liquid form, and standing from 12' to 20' Tw., is extensively used by and, but in a less degree, by dyers, in fixing the colour upon the cloth. For some purposes—as in the printing of pale pinks—the solution is very much diluted, down even to 2° Tw. The mordants owe this property of fixing to their ready decomposition by heat, the acetic mid which they contain being liberated, and the base, with the colouring matter —for which as well as for the fibrous material it has a ,strong affinity—being deposited upon the cloth, As may be inferred from the name (mordre, Fr., to bite), the action of the mordant was formerly supposed to be simply mechanical, corroding and opening the fibre of the cloth, and allow ing the permeation of the colouring principle. It will be readily appreciated that n mordant must be retained by only one portion of the cloth, the rest being left white or occupied by some other mordant or oolour. At the same time, it must be used in the fluid :Ante, a" that the fibre may be thoroughly impregnated ; and it is therefore necessary to guard against a natural inclination of the liquid to spread beyond its proper limits, aided by the capillary attraction of the cloth. For this purpose what aro called " thickenings " are used—gum, starch, flour, overcome the natural inclination of the mordant and the attraction of the fibre, and also allow of the application of a larger amount of mordant than could be made if the latter were a thin liquid. These thicken
ing; which are mixed with all mordants in printing and dyeing, arc only temporary in their use, and have to be removed before the colours are finished.
The acetates of alumina do not act as well as cream of tartar and some other mordants in the treatment of woollen goods, owing to the very strong affinity existing between the base and the fibre of the material. The acetic acid is given off too rapidly, and the mordanting merely super ficial.
Alumina salts as mordants seem to have been known about 140 years, the first patent being taken out by Chappell, in 1742, for a mixture of alum, arsenic, chalk, white argol, and lead acetate. The manufacture is carried on extensively in the Lancashire and Yorkshire cotton and woollen districts; also at Glasgow and at a few places in France. Altogether there are about fifty chemical maiinfacturers engaged in the trade iu this country, but occasionally the calico-printer or dyer manufactures for his own consumption. The turn-out depends very much upon the varying strengths and cpialdies of the liquors required from time to time. In round numbers about 20 per colt. of the total alum manufactured in England is absorbed by the red liquor trade. The plant required is of the simplest and most inexpensive description.
Analysis of the acetates of alumina is but an unreliable test of their real value as mordants, the best guide being actual trial of any given sample upon cloth against mordants of all eady ascer tained quality.