Mineralogique Of Paris Are Two Splendid Specimens Of The Stone In


which is seen the transition from the azure to the white. According to

the quantity and quality of blue present, the lapis varies from an

almost uniform tint of the deepest indigo-blue to grayish-white, dotted

and streaked at intervals with pale blue. The exceeding beauty of good

samples has caused the lazulite to be much sought after, both as a gem

for adorning the person, and for inlaid works in ornamental decoration.

In China the stone is highly esteemed, being worn by mandarins as badges

of nobility conferred only by the Emperor; and in the apartments of a

summer palace near St. Petersburg, the walls are covered with amber,

interspersed with plates of this costly lapis. Besides the colouring

principle of the lazulite, there are always more or less mica and iron

pyrites, the latter a lustrous yellow bisulphide of iron, which has

often been mistaken for pellets of gold. Having chosen portions of the

stone most free from these impurities, it is simply requisite to reduce

them to an impalpable powder to obtain a blue pigment; and probably this

was the original mode of preparing it before the discovery of the modern

process. This curious method, which is mechanical rather than chemical,

depends for its success on the character and proportions of the

materials employed, as well as on the nicety of working. When well

carried out, it perfectly isolates the blue from all extraneous matter,

yielding the colour at first deep and rich, then lighter and paler, and

lastly of that gray tint which is known by the name of Ultramarine Ash.

The refuse, containing little or no blue, furnishes the useful pigment,

Mineral Gray.

The immense price of ultramarine--or, as it was at first called, azurrum

ultramarinum, blue beyond-the-sea--was almost a prohibition to its use

in former times. It is related that Charles I. presented to Mrs.

Walpole, and possibly to Vandyke also, five hundred pounds worth of

ultramarine, which lay in so small a compass as only to cover his hand.

Even in these days, despite the introduction of artificial ultramarines,

the native product continues costly, commanding in proportion to its

intensity and brightness, from two to eight guineas an ounce. To say,

however, that the merits of the blue at least equal its expense, is to

give the genuine ultramarine no more than its due. It has, indeed, not

earned its reputation upon slight pretensions, being, when of fine

quality, and skilfully prepared, of the most exquisitely beautiful blue,

ranging from the utmost depth of shadow to the highest brilliancy of

light and colour,--transparent in all its shades, and pure in all its

tints. A true medial blue, when perfect, partaking neither of purple on

the one hand, nor of green on the other, it sustains no injury either by

damp and impure air, or by the intensest action of light, and is so

eminently durable, that it remains unchanged in the oldest paintings.

Drying well, working well in oil and fresco, ultramarine may be safely

compounded with pigments generally, excepting only an acid sulphate of

baryta or constant white. The blue has so much of the property of light

in it, and of the tint of air--is so purely a sky-colour, and hence so

singularly adapted to the direct and reflex light of the sky, and to

become the antagonist of sunshine--that it is indispensable to the

painter. Moreover, it is so pure, so true, so unchangeable in its tints

and glazings, as to be no less essential in imitating the marvellous

colouring of nature in flesh and flowers. To this may be added that it

enters so admirably into purples, blacks, greens, grays, and broken

hues, that it has justly obtained the character of clearing or carrying

light and air into all colours, both in mixture and glazing, as well as

gained a sort of claim to universality throughout a picture.

Nevertheless, ultramarine is not always entitled to the whole of this

commendation. Frequently it is coarse in texture, in which case it is

apparently more deep and valuable; yet such blue cannot be used with

effect, nor ground fine without injuring its colour. Again, it is apt to

be separated in an impure state from the lapis lazuli, which is an

exceedingly varying and compound mineral, abounding with earthy and

metallic parts in different states of oxidation and composition: hence

ultramarine sometimes contains iron as a red oxide, when it has a purple

cast; and sometimes the same metal as a yellow oxide, when it is of a

green tone; while often it retains a portion of black sulphuret of iron,

which imparts a dark and dusky hue. Occasionally, it is true, artists

have preferred ultramarine for each of these tones; still are they

imperfections which may account for various effects and defects of this

pigment in painting. Growing deeper by age has been attributed to

ultramarine; but it is only such specimens as would acquire depth in the

fire that could be subject to the change; and it has been reasonably

supposed that in pictures wherein other colours have faded, it may have

taken this appearance by contrast. Ultramarine, prepared from calcined

lapis, is not liable to so deepen; but this advantage may be purchased

at some sacrifice of the vivid, warm, and pure azure colour of the blue

produced from unburnt stone. We have frequently found ultramarine to be

darkened, dimmed, and somewhat purpled by ignition; and the same results

ensue, in many instances, when the lazulite is calcined. In burning the

stone, the sulphur of the pyrites is in a great measure expelled, and

during its expulsion has probably a deteriorating influence on the

beauty of the colour: our belief in this being so is strengthened by the

fact that certain samples of ultramarine, ignited with sulphur, were not

improved thereby. Similar effects are likewise caused by a careless or

improper mode of treatment, for the finest lapis may yield dingy blues,

containing particles of mica, metal, &c., and possessing a dull green,

black, or purple hue. Of course the perfection of the pigment is

dependant to a large extent upon the quality of the stone itself.

Though unexceptionable as an oil-colour, both in solid painting and

glazing, it does not work so well as some other blues in water; nor is

it, unless carefully prepared, so well adapted for mixed tints, on

account of a gritty quality, of which no grinding will entirely divest

it, and which causes it to separate from other pigments. When extremely

fine in texture, however, or when a considerable portion of gum, which

renders it transparent, can be employed to give connexion or adhesion

while flowing, it becomes no less valuable in water than in oil; but

when its vivid azure is to be preserved, as in illuminated manuscripts

and missals, little gum must be used. The fine greens, purples, and

grays of the old masters, are often unquestionably compounds of

ultramarine; and formerly it was the only blue known in fresco. Pure

ultramarine varies in shade from light to dark, and in hue from pale

warm azure to the deepest cold blue.

Native ultramarine consists of silica, alumina, sulphur, and soda; its

colouring matter seeming to be due to hyposulphite of soda and sulphide

of sodium. In these respects, as well as in that of being decolourised

by acids, the natural product resembles the artificial. As a precious

material, the former has been subject to adulteration; and it has been

dyed, damped, and oiled to enrich its appearance; attempts of fraud,

however, which may be easily detected. In the preceding edition of this

work the author adds--"and the genuine may be as easily distinguished

from the spurious by dropping a few particles of the pigment into

lemon-juice, or any other acid, which almost instantly destroys the

colour of the true ultramarine totally, and without effervescence." With

this statement, so far as it pretends to be a test for the two kinds, we

are not inclined to agree. Genuine ultramarine is always decolourised by

acids; but it depends on the mode and nicety of its preparation whether

it is decolourised without effervescence: that this is the case the

author himself admits in his article on artificial ultramarine.

Moreover, the "violent effervescence" which he describes as ensuing on

the latter being dropped into an acid, does not of necessity take place:

in M. Guimet's finest variety, the brilliant ultramarine, acid produces

little or no effervescence. Seeing, therefore, that both sorts are

decolourised by acids, and that both may or may not effervesce

therewith, the acid test must be considered fallacious. Experiments made

with different samples of each, showed that native ultramarines offered

greater resistance to acid than the artificial, taking longer to

decolourise; and that the residues of the first were in general of a

purer white than those of the last. It was also found that the brilliant

ultramarine, above referred to, was less readily decolourised than other

French or German kinds.

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