ON THE PRIMARY, BLUE.
The third and last of the primary or simple colours is _blue_, which
bears the same relation to shade as yellow to light. Hence it is the
most retiring and diffusive of all colours, except purple and black; and
all colours have the power of throwing it back in painting, to a greater
or less extent, in proportion to the intimacy of their relations to
light--first white, then yellow, orange, red, &c.
Blue alone possesses entirely the quality technically called 'coldness'
in colouring, and it communicates this property variously to all other
colours with which it happens to be compounded. Most powerful in a
strong light, it seems to become neutral and pale in a declining light,
owing to its ruling affinity with black or shade, and its power of
absorbing light. Consequently, the eye of the artist is liable to be
deceived when painting with blue in too low a light, or toward the close
of day, to the endangering of the warmth and harmony of his picture.
Entering into combination with yellow in the composition of all
_greens_, and with red in all _purples_, blue characterizes the tertiary
_olive_, and is also the prime colour or archeus of the neutral _black_,
&c., as well as of the semi-neutral _gray_, &c.: it therefore is changed
in hue less than any other colour by mixture with black, as it is
likewise by distance. Blue is present subordinately in all tertiary and
broken colours, and being nearest in the scale to black, breaks and
contrasts powerfully and agreeably with white, as in pale blues, skies,
&c. Being less active than the other primaries in reflecting light, it
is sooner lost as a local colour by assimilation with distance. There is
an ancient doctrine that the azure of the sky is a compound of light and
darkness, and some have argued hence that blue is not a primary colour,
but a mixture of black and white; but pure or _neutral_ black and white
compound in infinite shades, all of which are neutral also, or _grey_.
It is true that a mixture of black and white is of a _cool_ hue, because
black is not a primary colour, but a compound of the three primary
colours in which blue predominates, a predominance which is rendered
more sensible when black is diluted with white. As to the colour of the
sky, in which light and shade are combined, that is likewise neutral,
and never blue except by contrast; thus, the more the light of the sun
partakes of a golden or orange hue, and the more parched and burnt the
earth is, the bluer appears the sky, as in Italy and all hot countries.
In England, where the sun is cooler, and a perpetual verdure reigns,
infusing blue latently into the landscape, the sky is warmer and nearer
to neutrality, partaking of a diversity of greys, which beautifully
melodize with blue as their key, and harmonize with the light and
landscape. Therefore the colour of the sky is always a contrast to the
direct and reflected light of the scene: if this light were of a rose
colour, the neutral of the sky would be converted into green, or if
purple, the sky would become yellow. Similarly would it be in all cases,
according to the laws of chromatic equivalence and contrast, as may be
often seen in the openings of coloured clouds at the rising and setting
of the sun.
In art, blue is apt to be discordant in juxtaposition with green, and
less so with purple, both which are cool colours; consequently blue
requires its contrast, _orange_, in equal proportion whether of surface
or intensity, to compensate or resolve its dissonances and correct its
coldness. In nature, however, blue is not discordant with either green
or purple, nor are any two colours (as we have said before) ever found
so. On the palette of nature each _colour_ is an example of _colouring_:
no colour is too absolute or defined, no perfectly pure blue appears
beside a perfectly pure green. A blue flower nestled in its green leaves
does not offend the nicest eye, but the blue and green are not blue and
green alone. There is, perhaps, but a single gleam of pure colour in
each: the rest is composed of such varied hues and tints and shades, so
broken and blended and beautifully harmonized, that no jarring discord
is possible. Hue melts into hue, tint into tint, shade into shade; and
thus does the simplest weed teach a lesson in colouring the proudest
painter may stoop to learn.
We have spoken of blue, which is termed a cold colour, as retiring; and
of yellow and red, which are called warm colours, as advancing. By this
we must not be understood to mean that blue, as blue, expresses
distance; or that yellow and red, as yellow and red, express nearness.
Colours are advancing or retiring in their _quality_--as depth,
delicacy, &c., not in their hue. A blue object set side-by-side a yellow
one will not look an inch farther off, but a red or orange cloud, in the
upper sky, will always seem to be beyond a blue cloud close to us, as it
is in reality. We grant that in certain objects, blue is a _sign_ of
distance, but that is not because blue, as a mere colour, is retiring;
but because the mist in the air is blue, and therefore any warm colour
which has not strength of light enough to pierce the mist is lost or
subdued in its blue. Blue in itself, however, is no more, on this
account, retiring, than brown is retiring, because when stones are seen
through brown water, the deeper they lie, the browner they appear.
Neither blue nor yellow nor red possesses, as such, the smallest power
of expressing either nearness or distance; they merely express
themselves under the peculiar circumstances which render them at the
moment, or in that place, signs of nearness or distance. Thus, purple in
a violet is a sign of nearness, because the closer it is looked at the
more purple is seen; but purple in a mountain is a sign of distance,
because a mountain close at hand is not purple, but green or grey. It
may, indeed, be generally assumed that a tender or pale colour will more
or less denote distance, and a powerful or dark colour nearness; but
even this is not always so. Heathery hills will usually give a pale and
tender purple near, and an intense or dark purple far away: the rose
colour of sunset on snow is pale on the snow at one's feet, but deep and
full on the snow in the distance; and the green of a Swiss lake is pale
in the clear waves on the beach, but intense as an emerald in the
sunstreak, six miles from shore. And in any case, when the foreground is
in strong light, with much water about it or white surface, casting
intense reflections, all its colours may be perfectly delicate, pale,
and faint; while the distance, when it is in shadow, may relieve the
whole foreground with deepest shades of purple, blue green, or
ultramarine blue.
There is one law, however, about distance, which has some claims to be
considered constant, namely, that dulness and heaviness of colour are
more or less indicative of nearness. All distant colour is pure colour:
it may not be bright, but it is clear and lovely, not opaque nor soiled;
for the air and light coming between us and any earthy or imperfect
colour, purify or harmonize it; hence a bad colourist is peculiarly
incapable of expressing distance. It is not of course meant that bad
colours are to be used in the foreground by way of making it come
forward; but only that a failure in colour there will not put it out of
its place. A failure in colour in the distance will at once do away with
its remoteness; a dull-coloured foreground will still be a foreground,
though coloured badly; but an ill-painted distance will not be merely a
dull distance, it will be no distance at all.
This seeming digression is not out of place, as it will enable the
artist better to understand that it is in their quality, not in their
hue, that colours are advancing or retiring; and that he must rely on
the depth, delicacy, &c., of his pigments, and not simply on their
colours, to produce effects of distance.
Of all colours, except black, blue contrasts white most powerfully. In
all harmonious combinations of colours, whether of mixture or
neighbourhood, blue is the natural, prime, or predominating power.
Accordingly, blue is universally agreeable to the eye in due relation to
the composition, and may more frequently be repeated therein, pure or
unbroken, than either of the other primaries; whence the employment of
ultramarine by some masters throughout the colouring of a picture.
Blue pigments, like blue flowers, are more rare than those of the other
primary colours. In permanent blues the palette is very deficient, the
list being exhausted when the native and artificial ultramarines and the
cobalts have been mentioned. That there is room for new blues, durable
and distinct, cannot therefore be denied. A good addition has been made
of late years in the German _Coëlin_, known here as Cerulian Blue and
Coeruleum. What is chiefly wanted, however, is a colour combining the
wonderful depth, richness, and transparency of Prussian blue with the
strict stability of ultramarine. A permanent Prussian blue would be the
most valued gift the palette could receive.
COBALT BLUES
comprise _Cerulian Blue_ or _Coeruleum_, _Cobalt Blue_, _Smalt_,
_Royal Blue_, _Dumont's Blue_, _Saxon Blue_, _Thénard's Blue_,
_Leithner's Blue_, _Hungary Blue_, _Dutch Ultramarine_, _Zaffre or
Enamel Blue_, _Vienna Blue_, _Paris Blue_, _Azure_, &c., and are
obtained by the action of heat on mixtures of earthy or metallic bases
with cobalt. They are divisible into three classes--the stannic cerulian
blue, the aluminous cobalt blues, and the siliceous smalts. Of these,
the first possesses the least depth; the second hold a middle position;
while the third are marked by exceeding richness. Although not to be
ranked with ultramarine, the stannic and aluminous blues may be
described as durable, or at least as durable rather than semi-stable.
There are, as we have before observed, different degrees of permanence,
and the blues in question are not readily affected. With regard to
smalts, they are, as artist's pigments, inferior in stability to other
blues of cobalt.
123. CERULIAN BLUE,
or _Coeruleum_. Under the name Coëlin there has of late years been
imported from Germany the cobalt blue with a tin base to which reference
has just been made. This comparatively new pigment--which likewise
contains or is mixed with gypsum, silica, and sometimes magnesia--has
the distinctive property of appearing a pure blue by artificial light,
tending neither to green on the one hand nor to purple on the other.
This advantage, added to its permanence, has conferred a popularity upon
coeruleum which its mere colour would scarcely have gained for it. A
light and pleasing blue, with a greenish-grey cast by day, it possesses
little depth or richness, and is far excelled in beauty by a good
aluminous cobalt. A certain chalkiness, moreover, somewhat detracts from
its transparency, and militates against its use in water. It is in oil,
and as a night colour, that coeruleum becomes of service, as our
present system of lighting picture galleries by gas affects the purity
of blues generally. If those galleries were illuminated by means of the
electric light, we have it on the authority of Chevreul that all colours
and shades would show as well as by day: the same purpose would be
answered by the magnesium light. Some artificial lights are the ruin of
colours; in the soda flame (alcohol and salt) for instance, yellow
chromate of lead appears white, while red ochre and aniline blue appear
black.
Like other blues of cobalt, coeruleum assumes a greenish obscurity in
time, but like them it resists for a lengthened period both the action
of light and impure air, although chemically it is more open to the
influence of the latter, owing to its tin base. In admixture it may
safely be employed, as well as in fresco or enamel. For stage skies,
&c., in high-art scenery, the blue is admirably adapted. Now that there
are so many scene-painters who are artists--and so many artists who are
scene-painters--in bringing Nature to the foot-lights the effect of gas
on colours is of importance.
124. COBALT BLUE,
to which the various appellations have been given of _Thénard's Blue_,
_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.
Thénard, 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.
125. 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
with cobalt blue, the name _Azure_ has sometimes been given to it.
Varying exceedingly in quality and colour, the rougher kinds have been
employed by the laundress, and in the making of porcelain, pottery,
stained glass, encaustic tiles, &c.; as well as to cover the yellow
tinge of paper. For this last purpose, however, smalt is not perfectly
adapted, the colour being difficult to lay on uniformly, and the paper
when written on blunting the nibs of pens. Hence it has been superseded
to a great extent by artificial ultramarine, the presence of which may
be detected by the yellow spot which a drop of acid leaves on the paper.
A coarse gritty texture is peculiar to smalt, whether it be the _Powder
Blue_ of the washtub and _Blue Sand_ of the pottery, or the _Dumont's_
and _Royal Blue_ of the artist and high-class manufacturer. But the
strict stability which is a feature in smalt when used for painting on
glass and enamel does not follow it to the studio: both in water and oil
its beauty soon decays, as is often the case with other vitrified
pigments; nor is it in other respects eligible, being, notwithstanding
its richness and depth, very inferior to the cobalts preceding. It may
seem a paradox that the same colour should be at once so durable and so
fugitive, but we may briefly explain it by saying _when vitreous
pigments are reduced to that extreme state of division which the palette
requires, they lose the properties they possess in a less finely divided
state_. The best smalt in lumps appears black, yields a blue powder on
grinding, becomes paler on further grinding, and may be almost
decolourised by continued and excessive grinding. Smalt, it has been
stated, is merely a blue glass; and when a piece of blue glass, or a
blue crystal of sulphate of copper, is reduced to the fineness of flour,
the blue is lost. In vitrified and crystallised compounds, colour
depends on cohesion: sufficiently separate the particles, and the colour
more or less disappears. Not only, moreover, does grinding effect an
optical change in vitreous pigments, but it imposes further alteration.
That colour which was safe when locked up in a mass, crushed to minute
atoms is no longer so: imbedded in glass or enamel it will endure for
ages, but ground to impalpable powder becomes as liable to influence as
though it had never been subjected to heat at all. To sum up, vitreous
pigments are durable in a coarse or compact form, but are not more
stable than others when reduced to extreme division. As far as regards
artists' colours, therefore, vitrification does _not_ impart
permanence.
The grittiness to which we have referred is one of the defects of smalt,
which cannot, consistently with preserving its colour be entirely freed
from that drawback--an objection which pertains to vitreous pigments in
general. Hence it does not wash well, and in mural decoration is
sometimes applied to work by strewing the dry powdered colour upon a
flat ground of white or blue oil paint immediately after the latter is
laid on, whilst it yet remains wet. Of little body, it is a vivid and
gorgeous blue; bright, deep, and transparent, bordering on the violet
hue. It is chiefly employed in illumination and flower painting. The
inferior kinds of smalt are occasionally adulterated with chalk.
126. CYANINE.
Beckmann is fully convinced that the _cyanus_ of Theophrastus and the
_coeruleum_ of Pliny were a blue copper earth. However that may be, in
these days both names signify cobalt compounds, coeruleum being a
stannate of cobalt, and cyanine a mixture of cobalt and Prussian blue.
Unlike the former, cyanine, being composed of two old colours, can lay
no claim to originality. In the fourth chapter it was observed, "it is
quite possible for the artist to multiply his pigments unnecessarily.
Colours are sometimes brought out under new names which have no claim to
be regarded as new colours, being, indeed, mere mixtures. Compound
pigments like these may most frequently be dispensed with, in favour of
hues and tints composed extemporaneously of original colours upon the
palette." Whether these remarks are applicable to cyanine or not is a
question for artists to decide: in our opinion, with so many semi-stable
original pigments, the introduction of semi-stable compounds is to be
deprecated. Cyanine is a rich, deep, transparent blue, but its richness
and depth, as well as to a great extent its transparency, depend upon
Prussian blue, which is not strictly stable. Hence the peculiar
properties of cyanine remain unchanged only so long as the Prussian blue
itself, the pigment losing its colour by degrees on exposure to air and
light, and gradually assuming the tint of the paler but more permanent
cobalt. A mixture, be it remembered, necessarily partakes of the
qualities of its constituents, and if one of these be fugitive, the
compound cannot preserve its original hue.
Within the last few years, a compound similar to cyanine has appeared,
under the name of _Leitch's Blue_.
127. INDIGO,
or _Indian Blue_, was known to the ancients under the name of _Indicum_,
whence its present appellation. In modern Europe, it first came into
extensive use in Italy; but about the middle of the sixteenth century,
the Dutch began to import and employ it in considerable quantity.
Present in the woad plant, which is a native of Great Britain, indigo is
chiefly derived from a genus of leguminous plants called _Indigofera_,
found in India, Africa, and America. The colouring matter of these is
wholly in the cellular tissue of the leaves, as a secretion or juice;
not, however, in the blue state in which one is accustomed to see
indigo, but as a colourless substance, which continues white only so
long as the tissue of the leaf remains perfect: when this is by any
means destroyed, oxygen is absorbed from the atmosphere, and the
principle becomes blue. The best indigo is so light as to swim upon
water, but the commercial article seldom contains more than 50 per cent.
of blue colouring matter or true indigo, the remainder consisting of
either accidental or intentional impurities.
In painting, indigo is not nearly so bright as Prussian blue, but it is
extremely powerful and transparent, and may be described as a Prussian
blue in mourning. Of great body, it glazes and works well both in water
and oil. Its relative permanence as a dye has obtained it a false
character of extreme durability as a pigment, a quality in which it is
nevertheless very inferior even to Prussian blue. By impure air it is
injured, and in glazing some specimens are firmer than others, but not
durable; while in tint with white lead they are all fugitive. Employed
in considerable body in shadow, it is more permanent, but in all
respects Prussian blue is superior.
Despite this want of stability, indigo is a favourite colour with many
artists, who sacrifice by its use future permanence to present effect.
It is so serviceable a pigment for so many purposes, especially in
admixture, that its sin of fugacity is overlooked. Hence we find indigo
constantly mentioned in works on painting, their authors forgetting or
not caring to remember that wholesome axiom, a fugitive colour is not
rendered durable by being compounded. Artistically, it is adapted for
moonlights, and when mixed with a little lamp black, is well suited for
night clouds, distant cliffs, &c. With a little raw umber and madder it
is used for water in night effects. With the addition of a little madder
it forms a good gray; and with madder and burnt Sienna is useful for
dark rocks, this combination, with raw Sienna, being also eligible for
boats. For these and other mixed tints, however, Prussian blue saddened
by black with a suspicion of green in it, is equally fitted, and is more
permanent. Indeed, it would be perhaps justifiable to introduce such a
compound, under the name say, of Factitious Indigo.
Indigo in dust, or in small bits, is often adulterated with sand,
pulverized slate, and other earthy substances. That indigo is best which
is lightest, brightest, most copper-coloured, most fine-grained, and
inodorous.
128. INTENSE BLUE
is indigo refined by solution and precipitation. By this process, indigo
becomes more durable, and, being separated from impurities, is rendered
much more powerful, transparent, and deep. It washes and works admirably
in water; in other respects it possesses the common properties of
indigo. It is apt, however, to penetrate the paper on which it is
employed, if not well freed by washing from the acid and saline matter
used in its preparation. This is not always easily effected, and we
cannot help thinking that in the manufacture of intense blue a dry
method would be preferable. Indigo may, by cautious management, be
volatilized, and therefore be most thoroughly purified without the aid
of acids and alkalies. The best mode of subliming this substance is to
mix one part of indigo with two parts of plaster of Paris, make the
whole into a paste with water, spread it upon an iron plate, and, when
quite dry, heat it by a spirit lamp. The volatilization of the indigo
is aided by the vapour of water disengaged from the gypsum, and the
surface of the mass becomes covered with beautiful crystals of pure
indigo, which may be readily removed by a thin spatula. At a higher
temperature, charring and decomposition take place.
129. PRUSSIAN BLUE,
otherwise called _Berlin Blue_, _Paris Blue_, _Prussiate of Iron_,
_Ferrocyanide of Iron_, &c., was accidentally discovered in 1710 by
Diesbach, a colour-maker at Berlin. It is a compound of iron and
cyanogen, of varying composition, formed by adding yellow prussiate of
potash to a persalt of iron, or by oxidizing the precipitate obtained
from the prussiate and a protosalt. The finest blue is furnished by
sesquinitrate of iron, but the salt almost exclusively employed is the
protosulphate, the freedom of which from copper is essential to the
colour of the blue. As is the case with other pigments, Prussian blue
differs considerably in colour, in depth, and in permanence, according
to the purity of the materials, the mode of manufacture, and the absence
of adulterants. Like smalt, it is known in the washtub as well as in the
studio; and in the cheaper varieties, alumina, starch, chalk, oxide of
iron, &c., are often largely present. A good unsophisticated sample in
the dry state is intense blue, almost black, hard and brittle, much
resembling in appearance the best indigo, and having a similar
copper-red fracture. It does not effervesce with acids, as when
adulterated with chalk; nor become pasty with boiling water, as when
sophisticated with starch. Further, it feels light in the hand, adheres
to the tongue, is inodorous, tasteless, not poisonous, and is insoluble
in water. Forming a bulky mass while moist, Prussian blue shrinks to a
comparatively small compass when well washed and dried by gentle heat;
and, when once dried, being difficult to reduce again to the state of
extreme division which it possessed while wet, it is frequently sold and
used in paste for common purposes. We have said that a good sample of
Prussian blue is insoluble in water, and for artistic use it should
certainly be so, as otherwise it has a tendency to stain the fabric on
which it is employed, a defect formerly very prevalent. All Prussian
blues, however, are not insoluble, and these are not only liable to the
drawback named, but are less to be depended on for permanence. Improper
proportions, for instance, of sesquichloride of iron and
potash-ferrocyanide will yield a blue which, when washed even with cold
water, continually imparts to it a yellow or green colour, through the
partial solution of the prussiate. All commercial Prussian blue, and
indeed that which is prepared by careful chemical processes, give up the
ferrocyanide to boiling water, thereby colouring it greenish yellow; but
a sample which parts with its prussiate to _cold_ water is quite
unfitted for the palette, for which the most perfect specimen is none
too stable.
In spite of the learned researches of Professor Williamson, whose name
is as closely connected with the pigment as are the names of Schunck and
De La Rue with madder and cochineal, Prussian blue is not yet entirely
understood. Complex and uncertain in composition, uncertain too in its
habitudes, our best course perhaps will be not to attempt a complete
survey, but to state briefly those facts which bear on the artist's
craft.
Prussian blue is a colour of vast body and wonderful transparency, with
a soft velvety richness, and of such intense depth as to appear black in
its deepest washes. Notwithstanding it lasts a long time under
favourable circumstances, its tints fade by the action of strong light;
becoming white, according to Chevreul, in the direct rays of the sun,
but regaining its blue colour in the dark; hence that subdued light
which is favourable to all colours is particularly so to this blue. Its
colour has the singular property of fluctuating, or of coming and going,
under certain conditions; and which it owes to the action and reaction
by which it acquires or relinquishes oxygen alternately. It also becomes
greenish sometimes by a development of the oxide of iron; and is
purpled, darkened, or otherwise discoloured by damp or impure air. Time
has a neutralizing tendency upon its colour, which forms tints of much
beauty with white lead, though they are not equal either in purity,
brilliancy, or permanence to those of cobalt and ultramarine. When
carefully heated, Prussian blue gives off water and assumes a pale green
hue; its colour, therefore, depending on the presence of water, must not
be exposed to a high temperature. And as it is likewise injured or
destroyed by alkalis, which decompose it into oxide of iron and a
soluble prussiate, the blue should be avoided in fresco, on account of
the lime; neither should it be employed with pigments of an alkaline
nature, nor with hard water containing bicarbonate of lime in solution,
but with clean rain or distilled water, either of which is preferable
for colours generally.
Prussian blue dries and glazes well in oil, but its great and principal
use is in painting deep blues, in which its body helps to secure its
permanence, and its transparency gives force to its depth. It is also
valuable in compounding deep purples with lake, and is a powerful
neutralizer and component of black, to the intensity of which it adds
considerably. Prussian blue borders slightly on green, a quality which
militates against its use in skies and distances. In spite, however, of
its want of, or deficiency in, durability, the old water-colour painters
so employed it, neutralized by the addition of a little crimson lake. It
is serviceable in mixed tints of greens, affording with light red a
sea-green neutral. Dissolved in oxalic acid, the blue is available as an
ink, or for tinting maps.
Besides the preceding, there is a _Basic Prussian Blue_, formed by
simply submitting to the air the bluish-white precipitate which falls on
adding yellow prussiate of potash to green vitriol. This compound
dissolves entirely by continued washing with water, yielding a beautiful
deep blue solution, from which the colour may be thrown down in a solid
form by the addition of any salt. Probably it was this basic
preparation, so cheaply and easily made, that conferred upon Prussian
blue the character of staining paper. In name, there is also another
variety of this pigment, known as _Native Prussian Blue_; which is
really a native phosphate of iron, occurring as a blue earthy powder, or
as a white powder that becomes blue by exposure.
130. ANTWERP BLUE,
_Haerlem Blue_, _Berlin Blue_, _Mineral Blue_, is a lighter and
somewhat brighter Prussian blue, with less depth and less permanence. It
is a species of lake, having a considerable proportion of aluminous
base, to which its paler tint is due. As the stability of Prussian blue
rests in a great measure on the marvellous amount of latent colour the
pigment contains, when its particles of colour are set farther apart by
the intervention of the alumina, the permanence of its hue is
endangered. It was remarked, with respect to vitrified pigments, that
colour depends on cohesion. More or less, this holds good as regards all
pigments; but not only, as was also observed, does colour rest on
cohesion, in many instances durability depends likewise. It is only when
a colour is stable in itself that its particles will bear separating:
native ultramarine, for example, may be weakened almost to white, and
will still preserve its hue. If, however, a colour be naturally
fugitive, and rely chiefly on its extreme depth for what permanence it
possesses, that colour cannot with impunity be paled: witness the
cochineal lakes, which the deeper they are, the more durable they are
found; and so it is with Prussian blue. Antwerp blue is distinguished
from the latter by its more earthy fracture.
131. TURNBULL'S BLUE,
Or _Ferricyanide of Iron_, is formed by adding the red prussiate of
potash to a protosalt of iron. This blue is lighter and more delicate
than ordinary Prussian blue, and is believed to resist the action of
alkalies longer. It is a question whether the common Prussian blue
obtained by oxidizing the precipitate yielded by green vitriol and the
yellow prussiate is not in reality this variety. However that may be,
there is, as far as permanence goes, little or no difference between the
two kinds.
ULTRAMARINES.
ARTIFICIAL ULTRAMARINES
comprise the varieties known as _French Ultramarine_, _French Blue_,
_Brilliant Ultramarine_, _Factitious Ultramarine_, _Guimet's
Ultramarine_, _New Blue_, _Permanent Blue_, _Gmelin's German
Ultramarine_, _Bleu de Garance_, _Outremer de Guimet_, &c. The
unrivalled qualities of native ultramarine prepared from the lapis
lazuli rendered it most desirable to obtain an artificial compound
which, while possessing similar properties, could be produced in
quantity, and at a less costly rate. In demolishing some furnaces
employed in making soda, by means of decomposing sulphate of soda, some
earth had been found impregnated with a light blue, which was proved to
have so close a resemblance to ultramarine as to foster hopes of
success. As a stimulus, there was offered a prize of six thousand francs
or £500 for the production of artificial ultramarine by the _Société
d'Encouragement_ of Paris, which was won in 1828 by M. Guimet. It is
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 believed 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
_blackened_, the cobalt blue _greened_, the genuine ultramarine remained
a _pure azure_, like a spot of light, while their ground, the Prussian
blue sky, seemed by contrast with the ultramarine of a _grey_ or _slate
colour_.
Other things being equal, those artificial ultramarines are most durable
which possess the most colour; and all are, perhaps, most permanent in
water. If used in that vehicle, care should be taken to employ a gum
free from acid; also, whether in water or oil, not to compound the blue
with a pigment which may possibly contain acid, such as constant white.
Acid, as we have said, is the great test for ultramarine; whence if a
sample be sophisticated with cobalt, its blue colour will not be
entirely destroyed. With high-class artistic pigments, however,
adulteration is the exception and not the rule. It is as a powder-blue
for the washtub that ultramarine gets disguised, when it is ground up
with soda-ash, chalk, gypsum, &c., and sold sometimes under its own
name, but more frequently as superfine Saxon smalts.
132. BRILLIANT ULTRAMARINE,
lately called _Factitious Ultramarine_, is a specially fine preparation
of M. Guimet, presenting the nearest approach to the natural product of
any artificial ultramarine, both in transparency, purity of hue, and
chemical characteristics. Equalling in depth and power the ordinary
French ultramarine, it possesses greater clearness, beauty, and
brightness; and has, in a subdued degree, that quality of light in it,
and of the tint of air, which forms so distinguishing a feature in the
native blue.
133. FRENCH ULTRAMARINE,
or _French Blue_, is a rich deep colour, but less transparent and vivid
than the preceding variety, which is preferable in unmixed tints. For
compound hues, French blue is sufficiently well adapted, and is
extremely useful. With aureolin and burnt Sienna, or Vandyke brown, it
affords valuable autumn greens; and with lamp black, or lamp black and
light red, good stormy clouds. A sombre gray for distant mountains is
furnished by French blue and madder brown, with a very little gamboge;
and a deep purple for sunsets, by the blue and purple madder, or Indian
red and rose madder. With cadmium and orient yellows, sepia, viridian,
and many other colours, this ultramarine is of service.
134. NEW BLUE
Is confined to water-colour painting, and is an artificial ultramarine,
holding a middle position between French blue and permanent blue, being
less deep than the one and less pale than the other. It may be said to
hover in tint between a rich ultramarine and cobalt.
135. PERMANENT BLUE
Is a pale ultramarine, with a cobalt hue; and, in spite of its name,
less permanence than belongs to the richer and deeper sorts. What
Antwerp blue is to Prussian blue, this is to French blue--that is, as
regards colour. With respect to durability, however, permanent and
Antwerp blues cannot be compared; the former being a weakened variety of
a stable, and the latter a weakened variety of a semi-fugitive, pigment.
Hence permanent blue justifies its name, although that name would be
more suited to the brilliant, or French, ultramarine.
136. GENUINE ULTRAMARINE,
_Native Ultramarine_, _Natural Ultramarine_, _Real Ultramarine_, _True
Ultramarine_, _Ultramarine_, _Pure _Ultramarine_, _Azure_, _Outremer_,
_Lazuline_, _Lazulite Blue_, and _Lazurstein_. This most costly, most
permanent, and most celebrated of all pigments, is obtained by isolating
the blue colouring matter of the _lapis lazuli_, a stone chiefly brought
from China, Thibet, and the shores of Lake Baikal. About the antiquity
of the stone, and its colour, much has been written, and many
conflicting statements have been made; but there is little doubt that
our lapis lazuli was the sapphire of the ancients; and that the first
certain mention of ultramarine occurs in a passage of Arethas, who lived
in the eleventh century, and who, in his exposition of a verse in the
book of Revelation, says, the sapphire is that stone of which
_lazurium_, as we are told, is made. It has been common to confound
ultramarine with the _cyanus_ and _coeruleum_ of the ancients; but
their cyanus, or Armenian blue, was a kind of mineral or mountain blue,
tinged with copper; and their coeruleum, although it may sometimes
have been real ultramarine, was properly and in general a copper ochre.
That ultramarine was known to the ancients there seems every
probability, for it is certain they were acquainted with the stone; and
modern travellers describe the brilliant blue painting still remaining
in the ruins of temples of Upper Egypt as having all the appearance of
ultramarine. Whether it is so or not, however, could only be proved by
analysis; for, be it recollected, although the colour had preserved its
hue during so many centuries, it had been completely buried, and
therefore most perfectly secluded from light and air. Mr. Layard, in his
'Nineveh,' referring to some painted plaster, remarks that "The colours,
particularly the blues and reds, were as brilliant and vivid when the
earth was removed from them as they could originally have been; but, on
exposure to the air, they faded rapidly." In all likelihood, these were
of organic, or semi-organic, origin, prepared in some such manner as
that mentioned by Pliny, who speaks of an earth which, when boiled with
plants, acquired their blue colour, and was in some measure inflammable.
As a pigment, cobalt was unknown to the ancients; but to these vegetable
and copper blues of theirs, a third blue may perhaps be added.
Experiments made upon blue tiles, found in a Roman tesselated
foot-pavement at Montbeillard, showed that the colour was due to iron.
M. Gmelin has proved that a blue tint can be imparted to glass and
enamel by means of iron; and it is probable that the ancients were first
induced by the blue slag of their smelting-houses to study the colouring
of glass with iron; that in this art they acquired a dexterity not
possessed at present, and that they employed their iron-smalt as a
pigment, as we do our smalt of cobalt. To sum up, there are grounds for
believing that the ancients were acquainted with copper blues,
vegetable blues, and iron blues; and that, consequently, the blue
described by travellers as having all the appearance of ultramarine may,
or may not, be that pigment.
Lapis lazuli, or lazulite, is usually disseminated in a rock, which
contains, among other substances, a fine white lazulite. In the _Musée
Minéralogique_ 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.
* * * * *
137. _Blue Carmine._
In a former edition of this work there appeared the following:--"Blue
carmine is a blue oxide of molybdenum, of which little is known as a
substance or as a pigment. It is said to be of a beautiful blue colour,
and durable in a strong light, but is subject to be changed in hue by
other substances, and blackened by foul air: we may conjecture,
therefore, that it is not of much value in painting." In his estimate of
this colour the author was certainly right. It is formed when a
solution of bichloride of molybdenum is poured into a saturated, or
nearly saturated, solution of molybdate of ammonia. A blue precipitate
falls, which is a molybdate of molybdic oxide, hydrated, and abundantly
soluble in water. When dried, it furnishes a dark blue powder,
resembling powdered indigo, having a bitter, rough, metallic taste, and
reddening litmus strongly. The solubility of this hydrated oxide is
alone fatal to its employment as a pigment. It may, indeed, be rendered
comparatively insoluble in water by ignition; but the anhydrous oxide so
obtained is nearly black, and as a colour worthless.
A more eligible preparation is the molybdate of baryta, produced by
mixing solutions of molybdate of potash and acetate of baryta. A white,
flocculent precipitate results, which rapidly condenses to a crystalline
powder, and turns blue on ignition. It is, however, a costly compound,
of little merit, and not likely to come into use. It is insoluble in
water.
138. _Blue Ochre_,
which has been improperly called Native Prussian Blue, is a native
hydrated phosphate of iron of rare occurrence, found with iron pyrites
in Cornwall, and also in North America. What Indian red is to the colour
red, and Oxford ochre to yellow, this pigment is to the colour blue,
being sober and subdued rather than brilliant. It has the body of other
ochres, more transparency, and is of considerable depth. Both in water
and oil it works well, dries readily, and does not suffer in tint with
white lead, nor change when exposed to the action of strong light, damp,
or impure air. As far as its powers extend, therefore, it is an eligible
pigment, though not generally employed nor easily procured; it may,
however, be artificially prepared. Answering to similar acid tests as
ultramarine, it is distinguished therefrom by assuming an olive-brown
hue on exposure to a red heat.
139. _Cobalt Prussian Blue._
Gmelin states that yellow prussiate of potash yields with a solution of
oxalate of sesquioxide of cobalt a blue resembling Prussian blue--that,
in fact, there can be obtained a Prussian blue with a base of cobalt
instead of iron. In the moist state, the similarity is sufficiently
great, but when washed and dried, the product is, with us, a dingy slate
colour. Possibly, if such a blue could be produced, it might exceed in
permanence the ferro- and ferri-cyanides of iron. Of course the compound
would be much more expensive.
_Copper Blues_
are now seldom or never employed as artists' pigments. The following
are the principal varieties:--
140. _Bice_,
Blue Bice, Iris, Terre Bleu, was prepared, when true, from the Armenian
stone, which is a calcareous kind of stone coloured with copper. It was
of a light bright hue, but is completely superseded by pale ultramarine.
The Persian lazur appears to have been a similar pigment, being a sort
of copper ore, which, when the stone was pounded and sifted, furnished a
fine paint, very bright and pleasant. It could not, however, stand the
effects of the atmosphere like the Tartarian lazur or lapis lazuli, in
the course of time becoming of a dark and dismal colour.
Ground smalts, blue verditer, and other pigments, have passed under the
name of bice.
141. _Blue Ashes_, or _Mountain Blue_,
are both hydrated carbonates of copper, the first being artificially
prepared, and the second found native in Cumberland. Neither is durable,
especially in oil; and, as pigments, both are precisely of the character
of verditer. By treating the natural malachite green with an alkali, it
may be converted into blue.
142. _Blue Verditer_,
or Verditer, is an oxide of copper, formed by precipitating nitrate of
copper with lime. It is of a beautiful light blue colour, little
affected by light, but greened and ultimately blackened by time, damp,
and impure air--changes which ensue even more rapidly in oil than in
water. It is mostly confined to distemper painting and paper-staining.
143. _Egyptian Blue_,
called by Vitruvius, Coeruleum, is frequently found on the walls of
the temples in Egypt, as well as on the cases enclosing mummies. Count
Chaptal, who analysed some of it discovered in 1809 in a shop at
Pompeii, found that it was blue ashes, not prepared in the moist manner,
but by calcination. He considers it a kind of frit, of a semi-vitreous
nature; and this would appear to be the case from Sir H. Davy obtaining
a similar colour by exposing to a strong heat, for two hours, a mixture
of fifteen parts of carbonate of soda, twenty of powdered flints, and
three of copper. The colour is very brilliant when first made, and
retains its hue well in distemper and decorative painting; but it has
the common defect of copper blues of turning green in oil, when ground
impalpably for artistic use. One remarkable effect of this copper
smalt--for it is nothing else--is, that by lamp-light it shows somewhat
greenish, but shines by day with all the brightness of azure. Mérimée
believes that Paul Veronese employed this sort of blue in many of his
pictures where the skies have become green.
144. _Saunders Blue_,
a corrupt name from _Cendres Bleues_, the original denomination probably
of ultramarine ashes, is of two kinds, the natural and artificial. The
first is a blue mineral found near copper mines, while the last is
simply a verditer.
145. _Schweinfurt Blue_,
or Reboulleau's Blue, is prepared by fusing together equal weights of
ordinary arseniate of protoxide of copper and arseniate of potash, and
adding one-fifth its weight of nitre to the fused mass. The result is,
so to speak, a sort of blue Scheele's green, into which latter colour it
soon passes when rubbed with oil.
146. _Cotton Seed Blue._
Cotton seed oil is bleached by treatment with either carbonate of soda
or caustic lime. In both cases, a considerable residue is left after
drawing off the bleached oil. This residue is treated with sulphuric
acid, and distilled at a high temperature, when there is left a compact
mass of a deep greenish-blue colour. On further treatment of this mass
with strong sulphuric acid, the green tint disappears, and a very
intense pure blue colour is produced. The blue mass is a mixture of the
coloured substance with some sulphuric acid, sulphate of soda, and fats.
The two former may be removed by washing with water; the latter by
treatment with naptha. Alcohol now dissolves the blue colour, and water
precipitates it from the solution chemically pure.
This blue has not been introduced as a pigment; and of its permanence,
and other attributes, we know nothing.
147. _Gold Blue._
Gold purple, under the name of Purple of Cassius, was once very well
known: a like compound of tin and gold may be made to yield a blue.
Resembling indigo, the colour is not remarkably brilliant, and, unless
several precautions are carefully observed, is rather violet than blue.
When obtained, the colour must be quickly washed by decantation, or it
changes first to violet and then to purple. Its costliness, lack of
brightness, and tendency to redden, are against its employment on the
palette. In enamelling it would doubtless preserve its colour, and in
exceptional cases might be useful.
148. _Iodine Blue._
It is curious that iodine, which gives a yellow with lead, should also
afford a blue with the same metal. When a solution of iodine in aqueous
soda (carbonate of soda is not so good) is added to nitrate or acetate
of lead-oxide, a transient violet-red precipitate falls, which
decomposes spontaneously under water, yielding iodine and a beautiful
blue powder. The colour, however, is exceedingly fugitive, even the
carbonic acid of the air separating iodine from it and forming a lead
salt. Bearing in mind the scarlet iodide of mercury, iodine is capable
of furnishing the three primary colours, distinguished alike by their
brilliancy and fugacity.
149. _Iridium Blue._
The rare metal iridium affords a blue which is a mixture of the oxide
and the sesquioxide. But being slightly soluble in water and
decolourised by sulphuretted hydrogen, it would not, other
considerations apart, be an acquisition.
150. _Manganese Blue._
An aqueous solution of permanganate of potash yields with baryta-water a
violet mixture, which afterwards becomes colourless, and deposits a blue
precipitate. This retains its colour after washing and drying, but
cannot be recommended as a pigment, being liable to suffer in contact
with organic substances, which deoxidize and decolourize the manganates
and permanganates.
151. _Platinum Blue._
With mercurous nitrate, the platinocyanide of potassium forms a thick
smalt blue, and the platinidcyanide a dark blue precipitate. The
compound is a mixture of platino- or platinidcyanide of mercury and
mercurous nitrate. Upon the presence of the latter the colour seems to
depend, for on washing with cold water containing nitric acid, the
nitrate is not removed nor the blue affected; but boiling water extracts
the nitrate and leaves a white residue. A blue containing mercurous
nitrate must necessarily be injured by impure air, and be otherwise
objectionable.
152. _Tungsten Blue_
is an oxide formed by the action of various deoxidizing agents on
tungstic acid. It remains unaltered in the air at ordinary temperatures,
is opaque, and of a blackish indigo-blue colour. As a pigment, there is
little to recommend it.
153. _Wood-Tar Blue._
The colours obtained from coal-tar have become household words, and it
is not impossible that those from wood-tar may be some day equally
familiar. At present wood-tar is comparatively unexplored, but the fact
that picamar furnishes a blue is at least as suggestive and hopeful as
that transient purple colouration by which aniline was once chiefly
distinguished. As aniline is a product of coal-tar, so picamar is a
product of wood-tar; and as the former gives a purple with
hypochlorites, so the latter yields a blue with baryta-water. Both are
distinguished by coloured tests, but there is this advantage in the
picamar blue--it is comparatively permanent.
Picamar blue is produced when a few drops of baryta-water are added to
an alcoholic solution of impure picamar, or even to wood-tar oil
deprived of its acid. The liquor instantly assumes a bright blue tint,
which in a few minutes passes into an indigo colour. From [Greek: pitta]
pitch, and [Greek: kallos] ornament, the blue is named _Pittacal_.
The mode of separating pittacal has not been clearly described. Dumas
states, that when precipitated in a flocculent state from its solutions,
or obtained by evaporation, it closely resembles indigo, and, like it,
acquires a coppery hue when rubbed. It is inodorous, tasteless, and not
volatile; and is abundantly soluble in acetic acid, forming a red
liquid, which, when saturated by an alkali, becomes of a bright blue. It
is represented as a more delicate test of acid and alkalis than litmus.
With acetate of lead, protochloride of tin, ammonio-sulphate of copper,
and acetate of alumina, it yields a fine blue colour with a tint of
violet, said not to be affected by air or light, and therefore
recommended for dyeing.
Like indigo, pittacal is believed to contain nitrogen, but its ultimate
composition has not been accurately determined. Dumas considers it
identical with a blue product obtained in 1827 from coal-tar by MM.
Barthe and Laurent. If this be the case, its greater stability over
coal-tar blues and colours generally admits of doubt. That, however, has
yet to be ascertained. Our object in noticing this blue has been
two-fold: first, to direct attention to wood-tar as a possible source of
colour; and secondly, to point to pittacal as a possible substitute for
indigo, possessing greater durability.
154. _Zinc-Cobalt Blue._
Cobalt, as furnishing a blue colour, is usually associated with alumina,
silica, or tin; and, as furnishing a green colour, with zinc. But there
is obtainable a compound of zinc and cobalt which gives a blue not only
free from green, but inclining rather to red. It is made by adding to a
solution of ordinary phosphate of soda in excess a solution first of
sulphate of zinc and then of sulphate of cobalt, and washing and
igniting the precipitate. The result is a vitreous blue with a purple
cast, of little body, and exceedingly difficult to grind. Altogether, it
is not unlike smalt, over which it has no advantages as an artistic
pigment either in colour or permanence. For tinting porcelain, however,
it is admirably adapted, imparting thereto a very pure dark blue of
extraordinary beauty. This blue is distinguished from smalt by
dissolving in acetic acid.
* * * * *
Compared with the wide range of yellows, or even with reds, the artist
finds the number of his blues limited. The perfect native and excellent
artificial ultramarines, the good blues of cobalt, the fair Prussian
blue, and the doubtful indigo, are the four varieties he has for years
been in the habit of using, and is still mainly dependent on. Our
division, therefore, into permanent, semi-stable, and fugitive, is
easily effected.
In the front rank, pre-eminent among blues as among pigments generally,
stands genuine ultramarine. Behind it, are the artificial ultramarines;
and behind them again, cobalt and cerulian blue. To a greater or less
extent, all these are durable.
Among the semi-stable, must be classed cyanine or Leitch's blue, smalt,
and Prussian blue.
To the fugitive, belong indigo and the somewhat more permanent intense
blue, Antwerp blue, and the copper blues.
In this list of blues, which grace or disgrace the palette of the
present day, there is one colour which, although not permanent, is
almost indispensable. As yet, the chemist cannot in all cases lay down
the law as to what pigments may or may not be employed. The painter who
unnecessarily uses fugitive colours must have little love for his
craft, and a poor opinion of the value of his work; but, even with the
best intentions and the utmost self-esteem, the artist cannot always
confine himself to strictly stable pigments. He has no right to use
orpiment instead of cadmium yellow, or red lead instead of vermilion, or
copper blue instead of cobalt: he has no business to employ indigo when
Prussian blue saddened by black will answer his purpose; but--what
pigment can he substitute for Prussian blue itself? None. In its
wondrous depth, richness, and transparency, it stands alone: there is no
yellow to compare with it, no red to equal it, no blue to rival it. In
force and power it is a colour among colours, and transparent beyond
them all. The great importance of transparent pigments is to unite with
solid or opaque colours of their own hues, giving tone and atmosphere
generally, together with beauty and life; to convert primary into
secondary, and secondary into tertiary colours, with brilliancy; to
deepen and enrich dark colours and shadows, and to impart force and tone
to black itself. For such effects, no pigment can vie with Prussian
blue. What purples it produces, what greens it gives, what a matchless
range of grays; what velvety glow it confers, how it softens the
harshness of colours, and how it subdues their glare. No; until the
advent of a perfect palette, the artist can scarcely part with his
Prussian blue; nor can the chemist, who has nothing better to offer,
hold him to blame. It is for Art to copy Nature with the best materials
she possesses: it is for Science to learn the secrets of Nature, and
turn them to the benefit of Art.