OXALIC ACID.
Oxalic acid, a most powerful acid, occurs combined chiefly with potash juices of plants of the genus oxalis and rumex. Artificially it was obtained by the action of nitric acid upon sugar and starch, but has been prepared latterly by treating spent dyestuffs with alkalies. Oxalic acid forms colorless transparent crystals, which are inodorous, intensely sour, and do not grow moist upon exposure. Should they become damp, some nitric or sulphuric acid, used in the preparation, has not been thoroughly removed. It is soluble in its own weight of boiling water, but requires about eight times its weight of water at 65° Fah. Oxalic is one of the largely used acids in feather dyeing, being used in a number of light colors for the purpose of developing the color. In developing blues it is invaluable. Other colors it will totally destroy, violet or safranine, for example; and it is used in place of sulphuric acid for the purpose of extracting color.
INDIGO BLUE.
Indigo is derived from several plants of warm climates. In the plant the color exists as a yellowish liquid; but when extracted and exposed to the action of the air it becomes insoluble, and takes an intensely blue color. The cultivation of the plant is carried on chiefly in India, Java, Egypt and Louisiana. Indigo comes in the market in lumps, which, if of good quality, presents a deep bluish purple color, and exhibits a fine reddish coppery lustre if rubbed with a hard, polished body. If very hard or heavy, or when the color is very dull, blackish, greenish or brownish, the quality is below the standard. It is, however, of very little consequence in ostrich feather dyeing, and its impurities would scarcely at any time be noticeable. It should, however, dilute thoroughly in boiling water, and if there remain a sediment of any proportion, the indigo is impure. Sulphuric acid is generally used to develop the color.
SULPHURIC ACID.
Sulphuric acid, commonly called oil of vitriol, a common, yet very important, acid. Although not used to any great extent in ostrich feather dyeing, it occurs in commerce in various states and degrees of purity. It was at one time prepared by distilling dried copperas at a high temperature. It is now obtained in greater purity from the alkaline bisulphates. It is a clear colorless oily fluid, weighing about eighteen pounds to the gallon. If mixed with cold water, a great increase of temperature takes place. It rapidly destroys organic bodies, depriving them of their oxygen and hydrogen, and leaving the carbon behind, as a blackish mass. If any particle of organic matter falls into a carboy of acid, it is decomposed, and imparts a dark color to the liquid. It takes up water from the air rapidly, if left uncorked, and thus dilutes itself. Its use in feather dyeing is principally to extract colors that are too dark.
COPPERAS.
Copperas is generally prepared from the soft, white variety of iron pyrites, frequently found to a great extent in the coal measures. These, on exposure to air and moisture, decompose the latter, taking up oxygen, and are thus converted into sulphate of iron. Copperas forms pale greenish blue semi-transparent crystals, containing forty-five per cent. of water. If this be expelled, there remains a dull whitish powder. The crystals dissolve readily in one and one-half times their weight of cold water, and less than half their weight of boiling water. The direct uses of copperas have very much diminished in feather dyeing; as for dyeing black in conjunction with logwood it has been almost entirely superseded by bichromate of potash. In drabs and in saddening down light colors it is, to a certain extent, still used. It is used in quantities so small, however, that there is no serious results to be feared, as it must be used in quantity to injure the fibre.