The most commonly used of the rarer dye-woods is sandalwood, obtained from Pterocarpus santalinus. It is garnet red in colour, sinks in water, and produces a fine red meal, which rapidly becomes brown in air owing to oxidation of the colouring matter. According to recent researches, sandalwood contains two colouring matters, one obtained from the other by oxidation. Only one of these, santalin, is known with certainty. Sandalwood contains about 16 per cent. of santalin, which when pure forms a crystalline powder, differing from other wood colouring matters in being almost insoluble in water. Up to the present it has only been used in dyeing to obtain handsome and fast red colours. From it very bright lakes may be obtained, which may be used equally as well as the red wood lakes. The aluminium and tin lakes are especially fine. The extract of sandalwood is obtained by boiling very fine chips with water, then adding alcohol and leaving the mixture. When stannic chloride or alum, or a mixture of the two, is added to the solution of the colouring matter a red precipitate is produced which, after washing and drying, furnishes a bright red lake.
CHAPTER LX.
BLUE LAKES.
Indigo.—The colouring matter of the indigo plant is distinguished by great resistance to the action of light, air, and chemical reagents. It is thus often cited as a particular example of a fast and durable colour. It is very largely used in dyeing, and in certain combinations is important as a painters’ colour.
Indigo comes into commerce in a great variety of qualities, all imported from the tropics, in which alone the plant grows. The principal varieties are Indian, Java, Egyptian, American and African, but in these merchants make numerous subdivisions which are often to be distinguished from one another only by very small differences. Good indigo, whatever its source, has the following characteristics: It appears in almost cubical lumps, but may also form irregular masses. The lumps must show a small specific gravity, a higher might be caused by intentional additions of sand or earth. It is without taste and smell, and the colour of the finest varieties is not particularly bright, but when rubbed with a hard substance, or even by the pressure of the finger nail, it acquires a peculiar coppery lustre, which is the more intense the purer the indigo, and is thus regarded as a common test of quality. Indigo is completely insoluble in almost all solvents; in hot oils a small quantity dissolves, which again separates on cooling. The following are the principal signs by which the quality of indigo is recognised in commerce: It forms a dull-coloured, loose, light mass of earthy nature, a fresh fracture of which has a completely uniform appearance. It must be so porous that it at once adheres to the tongue. Uniformity of colour, light weight and fine fracture are the most important of the practical tests for good indigo. The composition of indigo is well known; we shall glance rapidly over the most important substances which occur in indigo in varying quantity.
The Constituents of Indigo.—Indigo contains the following compounds: Indigo blue, indigo red, indigo brown, indigo gluten, water and salts. The single important constituent for the purposes of the colour maker is indigo blue, or indigotin.
Pure indigo blue can be obtained from indigo only by a lengthy process. The indigo is boiled in turn with dilute sulphuric acid, caustic potash, and strong alcohol. The gluten dissolves in the acid, the indigo brown in the potash solution, and the indigo red in the alcohol. By these operations indigo blue is finally obtained in a state of almost complete purity, it now contains only a small quantity of inorganic substances. The quantity of pure indigo blue obtained in this manner varies, according to the purity of the indigo employed, from 20 to 75 per cent. Indigo blue can also be obtained by direct sublimation from indigo, but the yield is very small, since a large proportion is decomposed by dry distillation.
The most important constituent of indigo, indigo blue, can be obtained in quite pure form by a peculiar chemical process. When finely powdered indigo is brought in contact with a reducing agent in the presence of a strong alkali, the indigo blue is converted into indigo white, which forms a soluble compound with the alkali, which compound is stable only so long as it is protected from the action of the air. In contact with air indigo white is at once oxidised, and is reconverted into indigo blue, which separates from the liquid as a soft deep blue powder; after washing and drying it is quite pure. Whilst sublimed indigo blue forms needles with a splendid coppery lustre, when precipitated from a solution of indigo white by the action of oxygen it is obtained as an amorphous powder, which acquires the coppery lustre only when rubbed.
The process of the reduction of indigo blue by a reducing agent in the presence of a strong base is not only of theoretical interest, but is largely used in practice. Fabrics are generally dyed with indigo by immersing them in a solution of indigo white (known as the indigo vat), and then exposing them to the air. The oxidation of the indigo white then takes place on the fibre, upon which the indigo is deposited in a very finely divided form; the blue colour is not immediately formed, there is an intermediate green stage.
Indigo is used principally in dyeing. Certain of its compounds are used as pigments; the starting point in the preparation of these is a compound of indigo blue with sulphuric acid. When indigo is treated with sulphuric acid, according to the duration of the reaction and the temperature at which it is performed, either the monosulphonic acid or disulphonic acid is formed. The reactions proceed according to the following equations:—
C₁₆H₁₀N₂O₂ + H₂SO₄ = H₂O + C₁₆H₉N₂O₂.HSO₃
C₁₆H₁₀N₂O₂ + 2H₂SO₄ = 2H₂O + C₁₆H₈N₂O₂.(HSO₃)₂