D'encouragement Of Paris Which Was Won In 1828 By M Guimet It Is

: ON THE PRIMARY, BLUE.

fitting that the discoverer of a colour should excel in its manufacture,

and to this day Guimet's ultramarine is the finest made. As an instance

of how the researches of different men may, almost simultaneously, lead

to the same results, it is curious that very shortly after the problem

was also solved by Gmelin.



The cause of the blue colour of ultramarine was long a matter of

controversy, but was believe generally to be due to iron. When,

however, the discovery of artificial ultramarine was made, this

assumption was shown to be false, by the fact that a blue could be

obtained with materials perfectly free from iron. The absolutely

necessary constituents of ultramarine are silica, alumina, sulphur, and

soda; and there is little doubt that the colouring matter consists of

hyposulphite of soda and sulphide of sodium: it is certain that the blue

colour is dependant on the soda, inasmuch as potash yields an analogous

compound which is purely white. A number of substances, such as iron,

lime, magnesia, and potash, may be present as impurities, and were, in

part at least, purposely added to the earlier manufactures; but they are

found to be superfluous. Nevertheless, as regards iron, it is probable

that a very small portion, such as is usually contained in the

ingredients, greatly facilitates the production of the blue, and may

even be essential in some cases.



The colour of ultramarine is brought out by successive heatings. Green

portions, more or less in quantity, are often formed in the crucibles,

especially on the first ignition. On repeated heating they pass into a

blue tint. Artificial ultramarines are said to be seldom entirely freed

from all traces of the green modification, and are therefore less

beautiful than the natural varieties, having a shade of green or grey.

This defect, however, is certainly not discernible in Guimet's products,

which sometimes incline so much to purple as to require neutralizing

with a little Prussian blue. Depth for depth, the artificial are darker

and less azure than the natural varieties, but the superiority of the

latter consists not so much in their greater purity of hue, although

this is considerable, as in their far greater transparency. The finest

French ultramarine is never so transparent as the native; it is

brilliant, it is powerful, it is permanent, it is nearly--but only

nearly--transparent. Possessing in a subdued degree the characteristics

and qualities of the genuine, it works, washes, and dries well; and is

useful either in figures, draperies, or landscape. Rivalling in depth,

although not equalling in colour, the pure azure of native ultramarine,

it answers to the same acid tests, but is sometimes distinguished

therefrom by the effervescence which ensues on the addition of an acid.

Not a bubble escapes in such case from the natural blue; unless, indeed,

as occasionally happens, it retain a portion of alkali, with which it

may have been combined in the preparation, but from which it should have

been freed. Darkened as a rule by fire, factitious ultramarine becomes

dingy blue, and at last white, when strongly ignited for a long time;

and is, like the true variety, decolourised by ignition in an atmosphere

of hydrogen gas. At a high temperature, this effect is even produced by

silica, whence the unfitness of ultramarine for painting on glass or

porcelain; and simply by a prolonged red heat the blue is rendered

white. Being unaffected by alkalis, it is eligible in mural decoration,

and is particularly adapted to siliceous painting, on account of the

silica and alumina which it contains, two substances with which a

soluble silicate readily unites. If artificial ultramarine be mixed with

a soluble silicate, for example silicate of potash, and be laid on a

properly prepared ground, it will become so firmly fixed, says Mr.

Barff, that no amount of washing nor the slow action of moisture will

remove it, or affect its brilliancy. Judging from the behaviour of

ultramarine, therefore, if the colours employed in siliceous painting

contain silica and alumina, they should adhere as firmly to the surface

on which they are placed; and this is really the case. It is possible to

produce a mixed solution of aluminate and silicate of potash which will

remain liquid for twenty-four hours. If, while in the liquid state,

colours are saturated with this solution and allowed to dry, their

particles will be very intimately mixed with silica and alumina

chemically combined with potash. According to the author quoted, the

admixture of silica and alumina does not interfere with the brilliancy

or depth of the colours, and the method may be used for all those which

are not injured by potash, and are in themselves adapted to the art.



With respect to permanence, the finer varieties of artificial

ultramarines may, undoubtedly, be pronounced stable; but, like all other

colours, these blues are apt to vary in quality, and inferior kinds are

liable to lose their purity in a measure, and become grayer. Moreover,

they are made by different processes, and the mode adopted for the

manufacture of a pigment not only tells upon the colour, but may

influence to some extent its durability. From the following experiment

of an ingenious artist and friend of the author, it is evident that the

production of artificial ultramarine was not carried in its early days

to that state of perfection at which it has now arrived. He took a

picture, the sky of which had been recently painted in the ordinary

manner with Prussian blue and white; and having painted over the clear

part of the sky uniform portions with tints formed of the best

factitious ultramarine, cobalt blue, and genuine ultramarine, so as to

match the ground of the sky, and to disappear to the eye thereon by

blending with the ground, when viewed at a moderate distance, he set the

picture aside for some months. Upon examination, it appeared that the

colour of these various blue pigments had taken different ways, and

departed from the hue of the ground: the factitious ultramarine had