The most important of these is indigo. Being insoluble in water, it is prepared for use by sulphate of iron, and alkalies, or lime, by fermentation and alkalies, and by solution in sulphuric acid.

1. Triturate 1 lb, of indigo with water and a little caustic potash; then add 3 lbs. of lime, and afterwards 2 1/2 lbs. of sulphate of iron in solution, stirring them well together. The solution contains refined indigo, which is soluble in lime and alkalies. The cotton, linen, etc, to be dyed is repeatedly dipped in the solution, and afterwards rinsed in water soured with hydrochloric acid.

2. To 45 or 50 gallons of water, heated to 122° P., add 12 oz., of indigo, 8 oz. of madder, 8 oz. of bran, and 24 oz. of potash. In 36 hours introduce 12 oz. more of potash, and the same in 12 hours after. In 72 hours, add a little lime to check the fermentation. Wool, silk, linen and cotton may be dyed in this bath. Another form of this dye is - Indigo 2 parts, common soda 5 parts, lime 2 parts, clarified honey 1 part, water as much as may be sufficient. Keep it warm in an earthen jar till the indigo is dissolved.

4. A solution of indigo in sulphuric acid (see Chemic Blue) is used as a dye, but a purer tone of colour is obtained by the following method: The sulphate of indigo, mixed with water, is heated in a copper kettle; wool is immersed in it, and the whole is allowed to cool for 24 hours. The wool is then taken out, washed till the water comes off colourless and free from acid; it is then boiled in water containing about 2 per cent. of pearlash, or other alkaline carbonate, for a quarter of an hour. The quantity of pearlash should be equal to one third the weight of the indigo.

Logwood, with verdigris, or sulphate of copper, gives a blue dye, bordering on violet: with alum and tartar, a violet.

Prussian Blue is sometimes used in dyeing, after being triturated and digested for 24 hours with its weight of hydrochloric acid. A blue is also given by immersing silk, etc, in a solution of peracetate of iron, then in a solution of prussiate of potash, and afterwards rinsing it in acidulated water. Boiling water is sufficient to discolour articles thus dyed.