Scientific American 1.7.1868
Until within a recent period, most colors used in calico printing, paper staining, dyeing, etc., were chiefly obtained from vegetable sources. Mineral dyes, however, have been much in favor, and have the advantage, where they can be used, of being lasting and easily applied. Some of these thus used, and also as pigments, have been described in our previous pages; we shall, themfore, here chiefly direct attention to colors of a vegetable origin.
The natural colors of leaves and flowers aro due to a peculiar principle which is subject to the action of heat, light, and chemical action, but which is lost on the death of the plant. It is there a vital principle only. Colors employed for dyeing, etc., are extracted from the plant after its vitality has ceased, and are resident in the leaves, stems, routs, and flowers. Red dyes are obtained from madder, or rubia tinctorum, safflower, or carthamus, Brasil wood, logwood, sapan wool, the cochineal insect, etc.; blue colors are afforded by indigo, archil, litmus, etc.; and yellow dyes are produced from fustic turmeric, saffron, etc.
But all theme colors per se are fugitive, and require a mordant to fix them in the fabrics to which they are applied. The action of a mordant is readily illustrated by that of iron and an infusion of logwood used in dyeing black. If a piece of cotton were simply dipped in the infusion of logwood, it would only acquire a dirty red-brown color; but if it be first soaked in a solution of sulphate of iron, the oxide of the metal attaches itself to the fibers of the material, and, on being introduced into the logwood infusion, a black and permanent color is produced. The mordants usually employed are salts of iron, alumina, and tin, others being used only to a limited extent.
Madder is chiefly employed in dyeing red. It is the root of a plant, and is imported into this country from the continent in a state of powder, having a dark red color. From it a peculiar principle, called garancine is produced by means of sulphuric acid. Thin preparation is superceding the use of the raw madder, because it is more economical, cleanly and effective. Other principles may be extracted from madder, such as purpurine, alizarine, xanthine, etc., of which the alizarine is the most important, because it is really the coloring principle of the rest, and is the chief constituent of the garancine of commerce. The celebrated Turkey-red dye, which with stands the action of most chemical substances, is obtained by means of madder.
Cochineal is properly an animal dye, but its coloring powers are due to the cactus, on which it feeds. With alumina, a decoction of the insect affords a rich red color, used in dyeing and in producing "carmine." We have succeeded in producing some rich red precipitates from a cold infusion of the cactus flower and solutions of carbonate of soda and citric acid, employed in the manner we are about to describe in connection with safflower.
Safflower is a kind of saffron, and affords two coloring principles — a yellow and a red — the former being abundant and useless, while the latter is obtained only in small quantities, and is very valuable as a dye for silks, producing reds of the purest color and of every shade, from pink to deep poppy.
The safflower must be washed in cold water, until all the yellow color is removed. The residue of solid matter is then to be steeped in a solution of carbonate of soda, also cold; and, after some time, cotton wool is to be introduced so as to absorb the color. It will appear of a muddy red tint ; but on the addition of a solution of citric acid, or lemon juice, a magnificent red color is afforded. By a repetition of this process the cotton may be filled with color, which can afterwards be removed by the same means as those just employed. The color is thus obtained isolated. A large quantity of the color is manufactured on the banks.of the Lea, near London ; and also near Paris. The winter months are chosen for the purpose, as the heat of summer spoils the tint. From the small quantity of color produced from the raw material, it is very expensive, and its use is chiefly confined to dyeing silks, and making "carmine saucers."
Decoctions made by boiling the chips of Japan, Brazil, and logwood, afford a red color, with tin and alum as a mordant ; and a black with salts of iron. These substances are chiefly used for dyeing wool and cotton yarn. Each of them contains some tannic acid, as do sumac, gall nuts, oak bark, walnut peels, etc.; and such with iron, afford black dyes of various depths.
Yellow colors are produced by the action of alumina, as a mordant on infusions of turmeric, etc; but these colors are generally fugitive. Intermediate tints of brown, maroon, etc., are produced by successively dyeing the stuffs a yellow and red color, until the desired tint are arrived at.
Blue vegetable colors are obtained from some lichens, amongst which the rocella tinctoria is that most commonly used. All the lichens, however, afford colors of some kind and even those of a yellow and red tint. Archil, used in dyeing silk, is obtained from the rocella by means of ammonia, or more economically, urine, which has so far undergone decomposition as to afford ammonia. From these plants peculiar coloring principles, such orcine, etc., may be obtained, which are analogous to garancine, alazarine, etc.
Indigo has, until the discovery of the production of aniline from coal-tar, been the chief source of the permanent blue of the dyer. The color from indigo is not obtained, as in most other cases, by infusion or decoction in water, but by means of the strongest sulphuric acid. The raw material, as imported is in blue colored cakes; these are powdered, and to them the strongest sulphuric acid is added. By this, the real principle of indigo is dissolved out. The principle of dyeing with this substance is that of first deoxidizing it, which is done by means of the proto-sulphate of iron. This renders the indigo in a state fitted for absorption by the fabric, which, after being dipped into the solution, becomes of a deep blue color on exposure to the oxygen of the air, and the color then becomes permanent. In the process, the alkaline earth, lime, is used with the proto-salt of iron. And this introduces us to the production of aniline from a vegetable, as we have shown its production already from coal tar.
If indigo be acted upon by a hot solution of potassa and then distilled, analine is produced as a nearly colorless liquor. It is highly volatite, soluble in water, and on being oxidized by chromic acid, affords a rich purple tint. It matters not what it is produced from, for it is equally obtainable from indigo, nitro-benzole, and coal tar. And thin is a matter of not only deep philosophical interest, but, in the means to which it is applied, has become a most important article of commerce.
Into the varied treatment which this substance undergoes, we of course cannot enter; and the have made our remarks more extended on it than we should have done solely because it affords an instance of a most remarkable practical application of purely scientific research, and which we are glad to say has been the means of enriching those to whose perseverance we have been indebted to its manufacture in quantities.