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 5050 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 πιττα pitch, and καλλος 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.
