INDIGO. This valuable dye and pigment is of vegetable origin, and comes principally from India, being manufactured from the leaves and stems of the indigo plant. There are also West Indian, Egyptian, and Arabian varieties. Indigo is also obtained from woad.
The seeds of the plant are sown in the spring, and the plant is cut as it comes into blossom. Sometimes the indigo is obtained from the fermentation of the fresh leaves and stems, at other times from the dried leaves. An infusion is made which after fermentatici is of a yellowish or greenish colour. It is then decanted and beaten with sticks for some time, which expels the carbonic acid from the liquid, and brings the particles of indigo into contact with the oxygen of the air ; they then separate in grains and fall to the bottom of the vessel, the liquid becoming clear.
Indigo is a dark blue powder, devoid of taste or smell, insoluble in water, cold alcohol, ether, hydrochloric acid, and fat oils, but soluble in strong sulphuric acid, and creasote. The commercial sample con
tains scarmly one half of pure indigo, the remainder being composed of resinous substances, silica, alumina, oxide of iron, carbonate of lime, &c.
When indigo is placed in contact with a substance having a strong affinity for oxygen, it parts with oxygen and takes hydrogen, becoming hydruretted and losing its colour, and forming white indigo, (indigo tine) by exposure to air and heat. This again absorbs oxygen, and acquires its former blue colour. White indigo is soluble in alcohol and ether, and is very unstable. Although, therefore, many substances may deprive indigo of its colour, yet it has always a tendency to recover it by becoming again oxidized.
Indigo dissolved in strong sulphuric acid forms a deep blue liquid, called sulphate of indigo.
Indigo is likely to prove a valuable substance in the photographic process of printing in pigments.