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Vienna Blue Paris Blue Azure Cobalt-ultramarine &c Is The








name now exclusively confined to that preparation of cobalt which has a
base of alumina. It may, therefore, be not improperly called a blue
lake, the colour of which is brought up by fire, in the manner of enamel
blues. The discovery of this important pigment was made in 1802 by M.
Thenard, who obtained it by calcining a well-combined mixture of alumina
and crystals of cobalt. There may be employed with the aluminous base,
either the arseniate, the borate, or the phosphate of cobalt; but the
latter in preference, as it produces the purest colour. The arseniate
has always a violet tinge, more visible by gas-light than by day; while,
on account of the arsenic, the blue is more apt to be greened by impure
air, by reason of the formation of yellow sulphide of arsenic. The
purity of the colour, however, does not altogether depend on the
compound of cobalt used; in a great measure--as with other pigments--it
rests on the purity of the materials. To obtain a perfect blue, neither
inclining to purple nor green, the cobalt and alumina should be freed
from iron, and the former, as much as possible, from nickel also. With
the absence of these and proper skill, a true and brilliant blue may be
produced, almost rivalling the finest ultramarine. Apart, too, from its
increased beauty, a cobalt blue containing no iron or nickel is of
greater permanence than the ordinary products, being less liable to that
greenness and obscurity which time confers.

Though not possessing the body, transparency, and depth of ultramarine,
nor its natural and modest hue, commercial cobalt blue works better in
water than that pigment in general does; and is hence an acquisition to
those who have not the management of the latter. Resisting the action of
strong light and acids, its beauty declines by time, while impure air
greens and ultimately blackens it. Nevertheless, these changes are not
readily effected, especially in well made samples full of colour, and
sometimes the green tone is mechanically imparted. What wheat is to a
loaf, colour is to a pigment--it has to be ground and made up for use;
in the one vehicle to be mixed with gums, in the other with oils. It
often happens that colours have an antipathy to the latter, and refuse
to compound kindly therewith. Occasionally this repugnance manifests
itself in a few days, occasionally not for months. We know of a green
which flatly declines to have anything to do with oils, sinking and
separating therefrom in the course of a week, and leaving the clear oil
on the top. Repeatedly have colours to be coaxed to behave themselves as
pigments, coaxed not to 'run,' to work well, to dry well, &c.; and in
the humouring of their likes and dislikes the skill and patience of the
artist-colourman are sometimes severely taxed. Given a colour, it might
puzzle most chemists to convert it into a pigment; luckily Commerce
lends her aid. Lasting success, it is true, does not always follow, and
oils will rise to the surface now and then, giving green hues to blues,
orange hues to reds, and buff hues to yellows. Hence changes of colour
have been imputed before now to chemical alteration, when in reality the
results have been physical, caused by the subsidence of the pigments,
and the floating of the vehicles employed.

Cobalt blue dries well in oil, does not injure or suffer injury from
pigments in general, and may be used with a proper flux in enamel, as
well as in fresco. It affords clear bright tints in skies and distances,
but is apt to cause opacity if brought too near the foreground, and to
assume a violet tinge by artificial light. With madder brown it yields a
range of fine pearly neutrals; and with light red, in any proportion,
gives beautiful cloud tints. In combination with aureolin and sepia, or
rose madder, cobalt furnishes most agreeable and delicate tints for
distant trees, when under the influence of a soft light, or hazy state
of the atmosphere. In water-colour painting, cobalt is tolerably firm on
paper, and consequently answers better for some purposes than French
blue. In middle distances, if the cobalt possess a tendency to
chalkiness, the addition of a little indigo is a good corrective,
especially where the blue tone is required to be sombre and dark: it
should, however, be observed that the change is but temporary, indigo
being a fugitive pigment. In marine painting in water-colours, cobalt is
most useful for the remotest parts of seas and headlands. When dry, it
can be changed by going over it with a slight wash of vermilion or light
red, whereby a prismatic character is realized. Any strength of tone can
be obtained by repeating the washes, and should the colour be too
powerful, it may be reduced by pouncing it with a soft wet sponge; or if
too cold and blue, by a thin wash of burnt Sienna, merely the water
stained.

The blues of cobalt, on whatever base they may be prepared, are
distinguished from native and artificial ultramarines by not being
decolorised by acids.

TTITLE SMALT,

Invented about the year 1540, in Saxony, is a vitreous compound of
cobalt and silica, in fact a blue glass. Since the fifteenth century,
cobalt has been used in different parts of Europe to tinge glass; and
so intense is the colouring power of its oxide, that pure white glass is
rendered sensibly blue by the addition of one thousandth part, while one
twenty-thousandth part communicates a perceptible azure tint. In common





Next: With Cobalt Blue The Name Azure Has Sometimes Been Given To It

Previous: To Which The Various Appellations Have Been Given Of Thenard's Blue



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