Chiefly Derived From A Genus Of Leguminous Plants Called Indigofera

found in India, Africa, and America. The colouring matter of these is

wholly in the cellular tissue of the leaves, as a secretion or juice;

not, however, in the blue state in which one is accustomed to see

indigo, but as a colourless substance, which continues white only so

long as the tissue of the leaf remains perfect: when this is by any

means destroyed, oxygen is absorbed from the atmosphere, and the

principle becomes blue. The best indigo is so light as to swim upon

water, but the commercial article seldom contains more than 50 per cent.

of blue colouring matter or true indigo, the remainder consisting of

either accidental or intentional impurities.

In painting, indigo is not nearly so bright as Prussian blue, but it is

extremely powerful and transparent, and may be described as a Prussian

blue in mourning. Of great body, it glazes and works well both in water

and oil. Its relative permanence as a dye has obtained it a false

character of extreme durability as a pigment, a quality in which it is

nevertheless very inferior even to Prussian blue. By impure air it is

injured, and in glazing some specimens are firmer than others, but not

durable; while in tint with white lead they are all fugitive. Employed

in considerable body in shadow, it is more permanent, but in all

respects Prussian blue is superior.

Despite this want of stability, indigo is a favourite colour with many

artists, who sacrifice by its use future permanence to present effect.

It is so serviceable a pigment for so many purposes, especially in

admixture, that its sin of fugacity is overlooked. Hence we find indigo

constantly mentioned in works on painting, their authors forgetting or

not caring to remember that wholesome axiom, a fugitive colour is not

rendered durable by being compounded. Artistically, it is adapted for

moonlights, and when mixed with a little lamp black, is well suited for

night clouds, distant cliffs, &c. With a little raw umber and madder it

is used for water in night effects. With the addition of a little madder

it forms a good gray; and with madder and burnt Sienna is useful for

dark rocks, this combination, with raw Sienna, being also eligible for

boats. For these and other mixed tints, however, Prussian blue saddened

by black with a suspicion of green in it, is equally fitted, and is more

permanent. Indeed, it would be perhaps justifiable to introduce such a

compound, under the name say, of Factitious Indigo.

Indigo in dust, or in small bits, is often adulterated with sand,

pulverized slate, and other earthy substances. That indigo is best which

is lightest, brightest, most copper-coloured, most fine-grained, and



is indigo refined by solution and precipitation. By this process, indigo

becomes more durable, and, being separated from impurities, is rendered

much more powerful, transparent, and deep. It washes and works admirably

in water; in other respects it possesses the common properties of

indigo. It is apt, however, to penetrate the paper on which it is

employed, if not well freed by washing from the acid and saline matter

used in its preparation. This is not always easily effected, and we

cannot help thinking that in the manufacture of intense blue a dry

method would be preferable. Indigo may, by cautious management, be

volatilized, and therefore be most thoroughly purified without the aid

of acids and alkalies. The best mode of subliming this substance is to

mix one part of indigo with two parts of plaster of Paris, make the

whole into a paste with water, spread it upon an iron plate, and, when

quite dry, heat it by a spirit lamp. The volatilization of the indigo

is aided by the vapour of water disengaged from the gypsum, and the

surface of the mass becomes covered with beautiful crystals of pure

indigo, which may be readily removed by a thin spatula. At a higher

temperature, charring and decomposition take place.


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