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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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