RED DYES-LAC DYE-ALOES-ALOIN. 677 salts are soluble in water, and most of them are also dissolved by alcohol. Cochineal is extensively employed, in combination with oxide of tin, as a scarlet dye for cloth. 6. LAC DYE is very similar to cochineal in its nature, being also furnished by a species of coccus. Of late years this dye-stuff has been extensively substituted for the more costly cochineal, and it furnishes scarlets equally vivid, and even more permanent, but the pinks are not so good. In dyeing scarlets the liquid employed for dissolving the colouring matter is a solution of tin in concentrated hydrochloric acid, 20lb. of acid being required for each pound of tin; the dye-stuff is digested in an equal weight of this liquid, and the cloth, after being cleansed by boiling with fuller's earth, is introduced into a bath containing a mordant composed of a solution of stannous chloride, prepared by gradually dissolving 4lb. of tin in a mixture of 27lb. of hydrochloric acid, and 1lb. of nitric acid (sp. gr. 119). In order to dye 100lb. of cloth, the following process, according to Ure, is adopted :-300 gallons of water are raised to 150° in a tin boiler, and 14lb. of solution of tin mordant and a handful of bran are added; when the liquor boils it is skimmed, and 10lb. of lac dye previously dissolved in the acid, and 233lb. of mordant, are introduced; immediately after which 10lb. of cream of tartar, and 4lb. of ground sumach are suspended in a bag in the bath for five minutes, and then withdrawn; the fire is now extinguished, and 20 gallons of cold water containing 10lb. more of tin mordant are added, after which the cloth is introduced; the liquid is then boiled for an hour, and at the end of that time the cloth is withdrawn and well rinsed. (1510) 7. EXTRACT OF ALOES.-This substance may, by the action of nitric acid, be made to yield various compounds, which admit of being fixed by means of mordants upon silken and woollen fabrics, to which they impart dyes of great durability and beauty. Extract of aloes contains a compound termed aloin, which is soluble both in water and in alcohol; when exposed to the air it absorbs oxygen, and becomes of an intense red colour. Aloin (17H18,? Stenhouse).-In order to extract this substance, powdered Barbadoes aloes, mixed with sand to prevent the particles from agglomerating, is treated with cold water; the dark iquid thus obtained, if evaporated in vacuo to the consistence of syrup, and left for a few days in a cool place, deposits granular crystals, which are more or less deeply coloured: these must be pressed between folds of blotting paper, and recrystallized from water, the temperature of which is not to exceed 149° (65°C.). 678 EXTRACT OF ALOES-BLUE DYES-INDIGO. Aloin may likewise be obtained in hydrated crystals from a warm alcoholic solution, in groups of pale yellow needles. Cape aloes and Socotrine aloes also contain aloin, but it is accompanied by a large proportion of foreign matters, which interfere with its crystallization. Aloin is a neutral substance, with a sweetish, intensely bitter, persistent taste. It constitutes the purgative ingredient in aloes. At a temperature of 212° it is rapidly altered; at 302° (150° C.) it melts, and if heated strongly in the open air it burns with a smoky flame. The alkalies, both caustic and carbonated, dissolve it readily, forming orange-coloured solutions. 2/2 When extract of aloes is treated with nitric acid it yields chrysammic acid (HE,H(NO2)1⁄2Ð1⁄2; Mulder). This compound is obtained by treating extract of aloes for some days with 8 times its weight of nitric acid (sp. gr. 137), distilling off the greater part of the acid, and then adding water; chrysammic acid is precipitated. It is nearly insoluble in water, but soluble in alcohol and in ether, from which it may be obtained in golden yellow plates, and hence its name, from Xpúσeos, golden; on the application of heat it is decomposed with explosion. Its salts are soluble in hot water, and form deep red solutions; most of them may be crystallized, and the crystals exhibit a metallic lustre. Ammonia dissolves the acid with the formation of a beautiful purple-coloured solution, from which, on the addition of a neutral salt, such as potassic nitrate, an amidated compound is deposited in brilliant dark-green needles, which exert a polarizing action upon light when transmitted through them. Schunck finds other acids in the mother liquor from which the chrysammic acid has been separated; one of these is carbazotic acid; two other resinoid acids, the aloetic [(HE ̧H2(N→2),→),, H2O], and aloeretinic, are also formed at the same time: they have an intensely bitter taste, and form red salts. (1511) BLUE DYES.-I. INDIGO.-The most important of the blue colouring matters, and the one which has been most accurately studied, is indigo. Crude indigo contains a definite colouring material, termed indigotin, or indigo blue, from which numerous derivatives belonging to the indigo series have been prepared. Such of them as contain nitrogen have hitherto been procured from natural indigo only, but by the metamorphosis of these azotised compounds, bodies of a less complicated order have been obtained, many of which belong to the salicylic, the quinic, and the phenic groups. The following are the most important of the immediate derivatives of indigo: if indigo blue be described CONDITION OF INDIGO IN THE PLANT. 679 under the term indyl, their relations to it will then be represented by the formulæ of the fourth column: : Indigo blue = Isatin = He, H6 N Indyl hydrate = = Polymeride of indyl Ing Ing, H. (1512) Condition of Indigo in the Plant.-Indigo is extracted from the leaves of a variety of plants, principally the produce of India and America: it is especially obtained from the various kinds of Indigofera. The produce of the Indigofera anil was particularly examined by Chevreul: the Isatis tinctoria, or common woad, and several other plants, also yield indigo in small quantity, and it has been stated to occur occasionally in the milk of cows, and in human urine; so that indigo may be produced under circumstances apparently widely different. The blue colouring matter of indigo is not soluble in water or in alkaline leys, but by treatment with deoxidizing agents in contact with water, it may be made to combine with an additional atom of hydrogen, and produce a white substance termed reduced indigo, which is also insoluble in water, but is soluble in alkaline leys. When an alkaline solution of this reduced indigo is exposed to the air it absorbs oxygen, and is converted into indigo blue, which is precipitated in the insoluble form. In their growing state, the plants which yield indigo give no evidence of its presence, the juice being of a yellow colour. Chevreul supposed that the indigo exists in the plant in the reduced form; but, as Schunck has shown, this view is erroneous, since the juice of the plant is always acid, and reduced indigo requires an alkaline liquid for its solution. Moreover, as soon as such a solution is exposed to the air it becomes blue, and this is not the case with the juice of the Indigofera. Schunck has made the question of the condition in which indigo is contained in these plants the subject of a special inquiry; and although at present his experiments have only been conducted upon the woad, or Isatis tinctoria, which may be readily cultivated in this country, the results which he has obtained possess considerable value and interest [Manchester Memoirs (1855), vol. xii. p. 177]. From these experiments he concludes, that the woad does not contain indigo ready formed either in the blue or in the colourless state; that the indigo-producing substance is soluble in water; and that the formation of the blue colouring matter in watery 680 EXTRACTION OF INDIGO. extracts of the plant is neither caused nor promoted by the action of oxygen or of the alkalies, but that the plant contains a peculiar principle, which he terms indican (Є26H 33 NO18?). This body, by its decomposition, yields indigo. He found that, when heated with sulphuric or hydrochloric acid, it was decomposed: indigo blue, indigo red, and a particular species of sugar, indiglucin (EH), were formed: Indican is a yellow, transparent, amorphous substance; it is very deliquescent, and is also soluble in alcohol and in ether. It has a slightly bitter, nauseous taste; when boiled with caustic alkalies it evolves ammonia. A solution of indican becomes of a bright yellow colour when mixed with alkalies, or with lime or baryta. Acetate of lead occasions a sulphur-yellow precipitate in its alcoholic solution, but none in its aqueous solution till ammonia has been added. Solutions of indican must be evaporated without the application of heat, for if its aqueous solution be heated it undergoes a complete change; the elements of water are assimilated, and a substance is obtained which is not soluble in ether, and is dissolved but in small quantity by alcohol: the aqueous solution of this new body does not form indigo blue when boiled with acids, but it yields black flocculi consisting of two compounds; one of which is a resinous substance, fusible in boiling water, and the other a humus-like compound, which Schunck considers to be identical with the indigo brown of Berzelius. These observations show how important it is to avoid the use of boiling water in the preparation of indigo on the large scale. Solutions of indican which have been mixed with alkalies, and then boiled with acids, yield no indigo blue, but a mixture of this humus-like body and resin. Extraction of Indigo.-If the dried leaves of the Indigofera be digested in cold water for some hours, a green solution is obtained, which when exposed to the air undergoes fermentation, during which indigo is separated in the form of a blue sediment. The mode of preparing indigo, usually adopted in the East Indies, is stated to be the following:-The green plants, cut during the months of June and July, just before flowering time, are placed in wooden or brick troughs of about twenty feet square, and four or five feet deep (6 metres square, and about 1 metres in depth); the plants are then submitted to moderate pressure and covered with water; after the lapse of a few hours INDIGO BLUE, OR INDIGOTIN. 681 a sort of fermentation takes place in the mixture, which is allowed to continue for from twelve to fourteen hours; ammonia and carbonic acid are disengaged, the temperature rises to 100° or 110° (38° to 43° C.), and the liquid in the troughs becomes covered with a blue iridescent film. If the fermentation proceed too far the colouring matter is destroyed: when the process has reached a certain point, of which the manufacturer judges by experience, the liquid acquires a sherry wine colour; it is then drawn off into other troughs, where, on briskly agitating with oars, the pigment is separated as a deep blue granular powder. The supernatant liquor is run off; the sediment, with a portion of the liquid, is heated to the boiling point; the pasty mass is then drained on calico, pressed, cut into cakes, and dried. The indigo of commerce is by no means a homogeneous body. Its most important constituent is indigotin, or indigo blue, but it contains several other substances, and in particular two bodies, known as indigo brown and indigo red. The brown colouring matter is soluble in caustic potash: the indigo red may be extracted by means of boiling alcohol. 8 If a (1513) Indigo Blue, or Indigotin (ЄH ̧NO=131) may be separated from these impurities by the process of sublimation. small quantity of indigo be heated between two watch-glasses so as to protect it from the air, a considerable proportion of it is decomposed, but a portion is sublimed, and becomes condensed in light copper-coloured six-sided crystals upon the surface of the mass operated on. Pure indigo blue is, however, obtained more abundantly by the following plan contrived by Fritzsche:4 ounces (about 113 grms.) of commercial indigo in fine powder, and 4 ounces of grape sugar, are placed in a flask capable of containing 10 pints (5.66 litres) of liquid; 6 ounces of a saturated solution of caustic soda are next added, and the flask is filled up with boiling alcohol; it is then closed so as to prevent the access of air, and the mixture after agitation is set aside. In a few hours it becomes clear; after which the yellowish-red solution is drawn off with a syphon, and if left exposed in open vessels it rapidly absorbs oxygen, becomes brown, and deposits crystals, which after being treated first with alcohol, and then with hot water, and dried, form perfectly pure indigo blue. Good indigo yields nearly half its weight of crystallized indigo blue. Indigotin fuses at about 550° (288° C.), and furnishes purple vapours. Amongst the products of its destructive distillation are cyanide and carbonate of ammonium, aniline, and a peculiar empyreumatic oil. Indigo blue is metameric with benzoyl cyanide. |