536

DYEING AND CALICO PRINTING.

copperas,” the value of which is very doubtful. If the copperas has beenreally calcined it is reduced to an insoluble powder, which will subside to thebottom of the beck, and will have no more influence on the dyeing processthan an equal weight of sand. If the crystalline water has been merelyexpelled by heat the salt will instantly return to the hydrated state on contactwith water. The solutions of the dry and of the crystalline salt must beabsolutely identical.

Persulphate of Iron (Ferric Sulphate, or Red Sulphate of Iron.)—Thissalt contains iron in the state of peroxide (ferric oxide), instead of protoxideas in copperas. It may be prepared by heating 30 lbs. of hydrated ferric oxideof iron, 70 lbs. of red oxide of iron, and 150 lbs. of oil of vitriol. Or a solu-tion of green vitriol is mixed with half as much sulphuric acid as it alreadycontains, and then heated with nitric, in small quantities, till all the iron isperoxidised.

Persulphate of iron is in the mass a yellowish body, and dissolves in waterwith a pale yellow colour. In the anhydrous state it is a white powder. Per-sulphate of iron is rarely manufactured and used in a state of purity, but itforms a considerable part of many of the mordants sold under the name ofnitrate of iron.

Muriate of Iron (Solution of Protochloride of Iron, or Ferrous Chloride).—This mordant is made by dissolving scrap iron in hydrochloric acid, keepingthe metal in excess. It very readily takes up oxygen, and suffers decom-position if exposed to the air, whence it should be kept in well-stopperedbottles. It generally is sold at about 8o° Tw. If concentrated it depositspale green crystals, which must be carefully kept from exposure to the air,and which are sometimes preferred to the liquid. They are deliquescent, andeasily become peroxidised. Muriate of iron is not extensively used. It assistsin the production of certain catechu drabs and slates, and is sometimesemployed along with salts of manganese.

Permuriate of Iron (Solution of Perchloride, or Ferric Chloride) is madeby dissolving metallic iron in a mixture of nitric and hydrochloric acids. It isvery rarely used.

Black Liquor.(Acetate of Iron, sometimes called also Black Mordant).—The properties, preparation, and uses of this important mordant have beenincidentally described in speaking of madder (p. 290).

Nitrate of Iron.—This name is somewhat promiscuously applied to anentire class of compounds which differ widely in their composition, their pre-paration, and their uses. They may be divided into three classes, known as“blue,” “black,” and “saddening” or “common,” irons. The first-mentioned are used for dyeing blues upon cotton or upon silk (the latter lessfrequently since the introduction of the aniline blues) with the aid of prussiateof potash (ferrocyanide of potassium).

Black irons serve for dyeing or printing blacks upon cotton, and to a lessextent upon animal fibres, along with myrobalans, divi-divi, galls, sumac, log-wood, &c. Common or saddening irons are employed for “saddening”drabs, browns, clarets, &c.

Some nitrates of iron are really what the name implies, compounds ofnitric acid with iron, the latter being, as a rule, entirely in the state of per-oxide. Others are mixtures, in the most variable proportions, of the true