CHROMIUM MORDANTS.—Several compounds of chromium are largely used in dyeing and printing. The sesquioxide of chrome dissolved in acids, such as sulphuric, nitric, or acetic, has a con siderable affinity for textile fibres, and also tor certain colouring matters ; but as it gives a pale greenish colour, it is at once excluded in the case of most light and bright colours, to which an admixture of green would be fatal. The aceto-nitrate of chrome is prepared by G. Witz as follows :— Into a large stoneware pan, holding 20 gal., are poured 6 gal. boiling water, 471 lb. nitric acid at 62° Tw., and 6 lb. 9 oz. bichromate potash in very coarse powder. The following mixture is then poured in, by about 17 fl. oz. at a time, stirring with a large glass rod, and allowing the frothing each time to subside before adding a fresh lot lb. white glycerine at 44° Tw., and 90/ lb. acetic acid at 254° Tw. Whilst adding the first half of this mixture, it is needful to proceed slowly, but afterwards it is added more briskly, to keep up the reaction. This process is performed in the open air. When all is dissolved, the liquid is passed into a copper pan fitted with a steam-jacket, heated qniokly to a boil, and kept at that point for two minutes, or until a shallow layer of the liquid takes a fine green. It is then poured back into the stone vessel, and allowed to cool over night. The liquid is next poured off, and the crystals of saltpetre are washed with 4 lb. cold water ; the liquid is decanted off, and mixed with the former, so as to form 2 gal. mordant at about 57 Tw. This mordant may be used for alizarine pieces, for cceruleine shades along with bisulphite of potash, for logwood blacks, and for " Havraneck green," along with prussiate of potash, &c.
Chrome-alum is a double sulphate of chrome and potash. It contains no alumina and no chromic acid, and cannot be prepared, as some dyers erroneously imagine, by mixing bichromate of potash and ordinary alum. It is chiefly obtained as a residual product, e. g. in the conversion of anthraeene into anthraquinone, in the manufacture of artificial alizarine, and is recommended by Dr. Reimann for dyeing logwood blacks, especially in conjunction with iron-alum. (See Iron Mordants.) The chromium-compounds most widely used in dyeing are the combinations of chromic acid with an alkaline base, generally potash. The yellow or neutral salt (potassium chromate, not to be confounded with chrome yellow, a lead chromate) would be very useful, if it could be obtained in an unvarying state. It is generally found contaminated with various proportions of carbonate of potash, and containing more or less moisture. Those who wish to witness its interesting behaviour with many organic colouring matters may avoid this difficulty by using 151 parts of the biehromate and 143 parts of clear soda crystals not effloresced. The yellow chromate is much more widely used on the Continent than in England.
The bichromate of potash (potassium dichromate), and often known as red chromate, bichrome, red chrome, or simply chrome, is used to a very large extent in dyeing blacks upon wool, in conjunction with logwood. Along with fustie, red woods, orchil, &c., with or without logwand, it yields browns, yellows, bottle-greens, dark-greens, olives, purples, and intermediate shades. All these colours are cheap, requiring little time or labour, and are very fairly fast.