Indigo, or Indian Blue, is a pigment manufactured in the East and West Indies from several plants, but principally from the anil or indigofera. It is of various qualities, and has been long known, and of great use in dyeing. Ill painting it is not so bright as Prussian blue, but is extremely powerful and transparent; hence it may be substituted for some of the uses of Prussian blue. It is of great body, and glazes and works well both in water and oil. Its relative permanence as a dye has obtained it a false character of extreme durability in painting, a quality in which it is nevertheless very inferior even to Prussian blue.

It is injured by impure air, and in glazing some specimens are firmer than others, but not durable; id tint with white lead they are all fugitive: when used, however, in considerable body in shadow, it is more permanent, but in all respects inferior to Prussian blue.

2. Intense Blue

Intense Blue is indigo refined by solution and precipitation, in which state it is equal in colour to Antwerp blue. By this process indigo also becomes more durable, and much more powerful, transparent, and deep. It washes and works admirably in water: in other respects it has the common properties of indigo. We have been assured by an eminent architect, * equally able and experienced in the use of colours, that these blues of indigo have the property of pushing or detaching Indian ink from paper. The same is supposed to belong to other blues; but, as this effect is chemical, it can hardly be an attribute of mere colour. It is apt also, when not well freed by washing, from the acid and saline matter employed in its preparation, to penetrate the paper on which it is employed.