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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
inodorous.

TTITLE INTENSE BLUE

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.

TTITLE PRUSSIAN BLUE,





Next: Otherwise Called Berlin Blue Paris Blue Prussiate Of Iron

Previous: Or Indian Blue Was Known To The Ancients Under The Name Of Indicum



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