"Calcined copperas" is an article for which some dyers have a peculiar liking. It is prepared by heating ordinary raw copperas. If merely the water of crystallization is given off, the copperas remains soluble, and is, weight for weight, stronger than in its original state. If actually calcined, it becomes to a greater or less extent insoluble, and is therefore, pro tanto, wasted.
Copperas is now very extensively used in the manufacture of so-called nitrate of iron, and some times of acetate of iron. As a mordant, it is less employed than formerly.
Persulphate of iron, otherwise known as red sulphate (ferric sulphate), is not produced in the form of crystals. It may be made by dissolving copperas in water, applying heat, and gradually adding nitric acid in small proportions, till a little of the liquid no longer gives a blue precipitate with potassium ferricyanide (red prussiate of potash). Or native hydrated ferric oxide is boiled in oil of vitriol. Persulphate of iron is little used under its own name, but much of the nitrate of iron, commonly so called, is little else than persulphate.
Iron-alum is much used abroad, and is recommended by Dr. Reimann in conjunction with chromg alum when dyeing logwood blacks. It may be obtained as follows :-78 parts of the red oxide of iron are dissolved in 147 parts of oil of vitriol with the aid of heat. The solution is then diluted with water, and mixed with 87 parts sulphate of potash. The solution is next allowed to crystallize. These crystals, being absolutely unvarying in composition, are free from many of the objections made to the nitrates and the persulphate.
Muriate of iron (solution of ferrous chloride) is prepared by dissolving waste iron in hydro chloric acid of the common commercial strength (32°-34° Tw.) till the solution reaches 80°-85° Tw. It requires to be kept from contact with the air. It is sometimes used in producing catechu drabs and slates.
Permuriate of iron (solution of ferric chloride) is made by dissolving scrap iron in a mixture of nitric and hydrochloric acids. It is used still less than the muriate.
Nitrate of iron is a name given to a whole group of mordants, differing widely in their nature, manufacture, and applications. Some of these compounds contain no acid except nitric, others are made with a mixture of nitric and sulphuric, and in others, again, acetic acid is present in addition. In general, the iron is entirely in the ferric or most highly oxidized state ; whilst in others, there is more or less of the proto or ferrous salt. Some nitrates of iron are made from copperas, others from scrap-iron, and others of a mixture of both. In sp. gr., they range from 40° to 120° Tw. However they may differ in other respects, it is important that they contain no hydrochloric acid, nor any chloride. If such is present, the iron will be less readily deposited upon the fibre, and—a greater evil —in dyeing mixed goods, it will work upon the woollen or worsted in preference to the cotton. Another very important point is that the acid and the base should be accurately proportioned, or, as it is technically termed, " balanced." If there is an excess of acid, the fibre will not be able
to take up a sufficient quantity of the base, whence the shades produced will be meagre. The tissue itself, and in case of printing, the rollers and doctors, will be corroded. If, on the other hand, the proportion of iron is relatively too great, much of the colour will be deposited not on the fibres, but at the bottom of the dye-beck, and the shades obtained will be dull, irregular, and deficient in fastness. This balance between the acid and the base varies, however, according to the purpose for which the nitrate of iron is prepared.
A " black-iron," i. e. for dyeing and printing black upon cotton yarns, pieces, or the cotton warps of mixed goods, should be very thoroughly neutralized, or, as it is termed, " killed." Still, even here, ' if the oxide of iron is deposited too rapidly and irregularly to combine with the organic colouring matter, the goods will not only appear cloudy or streaky, but may even display buff patches. For this purpose, a nitrate of iron—or rather a nitro-sulphate—made from copperas is preferable to one from the metal. It is not necessary that the whole of the iron should be peroxidized. A mordant containing a mixture of proto- and per-salt (ferrous and ferric nitro-persulphate) yields fuller and richer blacks ; the former of these compounds, if used alone, giving a bluish, and the latter a brownish cast. The following process will yield a good black-iron. Take a large, strong cask, holding about 140 gal.; remove one end, and put into it 5 cwt. of dry, clean copperas, free from alumina and from both fine powder and very large lumps. Upon it, pour 120 lb. of nitric acid at 64° Tw. (the so-called double aquafortie of commerce); and stir the whole up together with a long pole, so that every part of the copperas may come into contact with the acid. In the stirring, there is a certain art. A violent revolving movement must be avoided ; the copperas should be turned over from the very bottom, working as if trying to find some lost article with the end of the pole. As the fumes given off are absolutely suffocating, the cask should be placed in a shed with open sides, so that the workmen may stand on the windward aide. In case of inhaling these fumes, relief Is obtained by drinking very strong vinegar, or acetic acid diluted with water. On no account should whisky or rum be used. In the evening, the cask is stirred up again. This process is repeated on the second day at morning and evening, and again on the morning of the third day. At night the reaction will be over, and if the operation has been well conducted, the copperas will have been entirely dissolved, or a very small quantity may be left in the cask. Water is then poured in, until the whole, after being thoroughly stirred up, marks about 75°-80° Tw. The liquid is allowed to settle, and is run off into carboys, which should stand in a dark place, exposed neither to heat nor cold.