Field's Chromatography Part 12

Beckmann is fully convinced that the _cya.n.u.s_ 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 a.s.suming the tint of the paler but more permanent cobalt. A mixture, be it remembered, necessarily partakes of the qualities of its const.i.tuents, 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 _Indic.u.m_, 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 quant.i.ty.

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 Fact.i.tious 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 was.h.i.+ng 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 ma.s.s 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 ma.s.s while moist, Prussian blue shrinks to a comparatively small compa.s.s 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 circ.u.mstances, 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 a.s.sumes 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 princ.i.p.al 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 was.h.i.+ng 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_, _Fact.i.tious 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 quant.i.ty, and at a less costly rate. In demolis.h.i.+ng 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 _Societe 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 a.s.sumption was shown to be false, by the fact that a blue could be obtained with materials perfectly free from iron. The absolutely necessary const.i.tuents 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 a.n.a.logous 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 quant.i.ty, are often formed in the crucibles, especially on the first ignition. On repeated heating they pa.s.s 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, fact.i.tious 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 gla.s.s 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 was.h.i.+ng 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 p.r.o.nounced 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 fact.i.tious 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 fact.i.tious 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-cla.s.s 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 _Fact.i.tious 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 distinguis.h.i.+ng 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 Vand.y.k.e 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 sh.o.r.es 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 pa.s.sage 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 _cya.n.u.s_ and _coeruleum_ of the ancients; but their cya.n.u.s, 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 a.n.a.lysis; 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 gla.s.s 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 gla.s.s 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 _Musee Mineralogique_ of Paris are two splendid specimens of the stone, in which is seen the transition from the azure to the white. According to the quant.i.ty 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 n.o.bility 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 l.u.s.trous 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 Vand.y.k.e also, five hundred pounds worth of ultramarine, which lay in so small a compa.s.s 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 suns.h.i.+ne--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 ent.i.tled to the whole of this commendation. Frequently it is coa.r.s.e 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 ma.n.u.scripts 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.