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





Very Slight Solubility In Water Would Be A Fatal Objection; And Violet De Mars Purple Ochre Or Mineral Purple Is A Dark Ochre facebooktwittergoogle_plusredditpinterestlinkedinmail

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