COPPER MORDANTS.—Compounds of copper, though used in dyeing and printing to some extent* serve rather as oxidizing agents or alterauts, than for fixing colours upon the fibre. The principal are :— Ammoniuret of copper is formed by adding liquid ammonia in excess to a solution of sulphate of copper, till the precipitate is redissolved, and a beautiful violet-blue liquid is produced. When diluted, it is sometimes used for giving a pale-green upon vegetable tissues by padding, dyeing, and rinsing.
Chloride (muriate) of copper is most used on the Continent in printing. In England, nitrate or sulphate of copper is used in its place, along with sal ammoniac. It may be easily prepared, if desired, by mixing solutions of sulphate of copper and chloride of calcium, drawing off the clear liquid for use.
Nitrate of copper is generally obtained as a residual product in cleaning articles of copper and its alloys. Tho nitrate is sold as a deep-blue liquid at about 90° Tw., and is often very impure, containing zinc, iron, &c., not intentionally, but from carelessness. It is chiefly used as an oxidizing agent in printing, e. g. in cateehu browns.
Sulphate of copper, known also as blue vitriol, blue-stone, and Roman vitriol, is generally pre pared directly from copper ore. It is used by printers as a resist, and by dyers as a mordant, generally in conjunction with eopperas, alum, argol, &c. Sulphate of copper sometimes contains more than 60 per cent. of sulphate of iron. Such mixtures are frequently known by the names of Salzburg vitriol, Cyprus vitriol, admont vitriol, and eagle vitriol. They are to be condemned. If the dyer requires the joint action of blue-stone and copperas, he had better have both these substances in a pure state, and mix them himself in known proportions.
Verdigris (acetate of copper) is now comparatively little used. It is employed in eatechu colours, in resists for indigo-styles, and as an oxidizer in some steam colours. In dyeing blacks an silks, and logwood blues on wools, it is also used. An old Act of Parliament (George III., 20)
imposes a penalty of 20/. for every piece of woollen cloth dyed a logwood blue in this manner. This curious statute is said to be still unrepealed.
IRON MORDANTS.—These are both numerous and important.
Copperas (green vitriol), scientifically known as ferrous sulphate, may be made by dissolving scrap iron in dilute sulphuric acid. Practically it is obtained by exposing the soft whitish iron pyrites of the Coal-measures, often known as "brass-lumps " or " coal-brasses," to the action of air and moisture. The water being decomposed, the hydrogen escapes, and the sulphur and iron are both oxidized. The eolution thus obtained is concentrated, when it deposits copperas in pale greenish blue semi-transparent crystals, containing 45 per cent, of water, which, on exposure to a gentle beat, escapes, leaving a white powder. Copperas should be hard, clear, and dry. If soft, and of a whitish or greyish-green colour, sulphate of alumina is probably present—the most objectionable impurity—which is often met with if the pyrites have not been duly selected, or if the crystalliza tion has been carried on at too high a point of concentration. To detect the presence of alumina, a little of the sample is dissolved in water, heated with -some pure nitric acid to peroxidize the iron, and the solution, after being heated till all the free nitric acid has been expelled, is mixed with a considerable excess of pure caustic soda. The mixture is heated in a clean iron vessel, diluted with water, filtered, and a solution of sal ammoniac is added to the clear liquid. If the copperas contains alumina, a white precipitate will be formed.
Old copperas, especially if it has been exposed to moist air, turns brown—a feature preferred by some consumers, though only valuable as a sign that the crystals do not contain any sulphuric acid. This brown appearance is therefore imitated by scattering lime-dust over the heaps, or watering them with stale urine.