Field's Chromatography Part 7

Or iodide of lead, is one of those compounds whose presence on the palette should never have been allowed. Exceedingly brilliant, it is also utterly fugitive, destroyed by exposure or foul gas, and useless in admixture. We may state here that, whatever its colour, no pigment containing iodine can in the slightest degree be relied upon. One of the most unstable of substances, being slowly volatile even at common temperatures, iodine is or ought to be quite inadmissible as a const.i.tuent. Combined with lead, which is in itself objectionable, it forms a yellow possessing every bad quality.

58. _Iron Yellow_,

Or oxalate of protoxide of iron, has very unadvisedly been recommended as a pigment. It is a bright pale yellow, but soon loses the beauty of its tint when submitted to air and light, becoming, by peroxidation, red and buffy. Even in a book the colour changes.

59. _Madder Yellow._

As our (the Editor's) experience of this product is somewhat at variance with that of the author, we subjoin his original statement. "Madder yellow is a preparation from the madder-root. The best is of a bright colour, resembling Indian yellow, but more powerful and transparent, though hardly equal to it in durability of hue; metallic, terrene, and alkaline substances acting on and reddening it as they do gamboge: even alone it has by time a natural tendency to become orange and foxy. We have produced it of various hues and tints, from an opaque and ochrous yellow, to a colour the most brilliant, transparent, and deep. Upon the whole, however, after an experience of many years, we do not consider them eligible pigments."

While agreeing with Mr. Field as to the character given of these yellows, we must confess that we have never been able to obtain, nor have we ever seen, a "most brilliant" madder yellow. Colours bearing that name have come under our notice, but if their hue was pure and vivid, they have always proved to be falsely so called, the madder being conspicuous by its absence. What we have succeeded in producing, and the genuine samples we have met with, have been fawns, buffs, drabs, &c., decidedly "ochrous" yellows, and wanting in stability. It is certain that no true madder yellow, brilliant and pure, ranks as a pigment at the present day. A variety known as Cory's Yellow Madder may be briefly described as Cory's _Brown_ Madder.

60. _Ma.s.sicot_,

Or _Masticot_, is a protoxide of lead, varying from the purest and most tender straw colour to a dull orange yellow, and known as Light, Yellow, and Golden Ma.s.sicot. It has in painting all the properties of white lead, from which it may be prepared by gentle calcination in an open furnace. In tint with that pigment, however, it soon loses its colour and returns to white, probably extracting some carbonic acid therefrom.

If used in an unmixed state, it is permanent in oil under the same conditions as white lead, but should not be employed in water, on account of its changing even to blackness by the action of damp or impure air. It is an admirable dryer, and has much the same effect as litharge in rendering oils siccative.

_Litharge_ is merely fused ma.s.sicot. Old writers speak of litharge of silver and litharge of gold, oxides of lead, pale and reddish yellow respectively. Commercial litharge, especially that which is foreign, contains sometimes a considerable proportion of oxide of copper and iron. The princ.i.p.al impurity, however, is generally silica, left undissolved on treating the litharge with nitric or acetic acid.

Litharge is commonly used in preparing drying oils, which contain a greater or less amount of the oxide in the form of oleate of lead. Oils made siccative by means of litharge are therefore liable to be damaged by foul gas. It is a matter of congratulation that such injury is not lasting, and that the oil, like white lead, recovers its original colour on exposure to air and light. Some drying oil which we exposed on a tile to an atmosphere of sulphuretted hydrogen until it was completely blackened, regained its former yellow hue on being submitted for a day or so to air and light. Hence, although the employment of lead as a siccative is not desirable, its effects are not so deleterious as might be imagined.

61. _Patent Yellow_,

_Turner's Yellow_, _Montpellier Yellow_, _Mineral Yellow_, _Ca.s.sel Yellow_, &c., is a mixture of chloride and oxide of lead, obtainable either as a pale or a deep yellow. It is a hard, ponderous, sparkling substance, of a crystalline texture and bright colour; hardly inferior, when ground, to chrome yellow. Of an excellent body, and working well in oil and water, but soon injured both by the sun's light and impure air.

A variety, mentioned by Merimee, in which bis.m.u.th and antimony are also used, is of greater durability.

62. _Platinum Yellow._

Our own opinion of this costly preparation is that the good qualities of the product do not justify its price. It may be obtained as a bright, rich, deep yellow, of considerable transparency; but the colour is acted upon by foul gas and exposure. Even in a book we have found it a.s.sume a dirty greyish cast, and a specimen which had been kept in a drawer, wrapped up in paper, became perfectly black in a few years. The presence of palladium interferes with the beauty of the original tint, but does not affect its stability.

63. _Thallium Yellow._

The new metal thallium yields in combination with chromic acid two yellow colours, a pale and an orange. They are not absolutely insoluble in water, and the sulphide of thallium being brown, would probably be damaged by impure air. But whatever their properties as pigments may be, their habitudes as such are not yet known. The present scarcity of the metal renders the colours produced from it mere scientific curiosities.

64. _Thwaites' Yellow._

Under this name chromate of cadmium was introduced some few years back.

If well prepared, it is a fine soft powder of a very vivid light yellow colour. The compound is too soluble, however, to be of value, its was.h.i.+ngs even with cold water being continually tinged yellow. Hence it turns green after a time, and becomes otherwise discoloured. Like citron yellow and other chromates apt to a.s.sume a green cast, it should only be employed, if at all, when compounded for foliage tints, &c. This want of durability is to be regretted, for a good sample of cadmium chrome is marked by exceeding beauty, unsurpa.s.sed for clearness and purity by any other yellow.

65. _Turbith Mineral_,

Or _Queen's Yellow_, is a subsulphate of mercury, of a beautiful lemon yellow colour, but so liable to change by the action of light or impure air, that it cannot be used safely, and hardly deserves attention as a pigment.

66. _Uranium Yellow_

Can be produced of a pale or orange tint, differing in brightness and depth of colour according to the mode of preparation. It is fairly eligible as a pigment, and far superior to the many fugitive compounds which have from time to time appeared. Being very expensive, however, and not possessing the good qualities of its compeers lately introduced, uranium yellow has but little chance of being employed now.

67. _Yellow Carmine_

Is a rich transparent colour, somewhat resembling an ochre compounded with Indian yellow. On exposure to light, it behaves much as a mixture of those pigments would do, the rich _yellowness_ entirely disappearing, and the sober-coloured earth being left behind.

From several metals besides those mentioned, yellows more or less vivid and durable may be obtained--from tin, nickel, cerium, molybdenum, &c.; but we do not know that any one of them would be a really desirable addition. To justify its being brought out, a new pigment should own some special advantage, chemical or artistic, by which it may be distinguished from other colours. No purpose would be answered by crowding the palette with mere repet.i.tions, even though they were stable. If, for instance, indium yellow were found exactly similar to that of cadmium, in colour, opacity, permanence, its presence would be quite superfluous. The mistake is often made of offering a fresh compound for a pigment when something as good or better, and cheaper may be, already exists. We remember a patient experimenter, who had produced a pink from cobalt, wondering why his colour should be so generally declined. The product was not wanting in either beauty or stability, but he forgot that the lakes of madder were far more beautiful, at least as durable, and much less expensive. We have said that we do not join in the cry of there being too many pigments, or share the opinion that there is not room for more, but we do enforce the necessity of progress.

Let us have as many good colours as possible, but let the new be superior to the old, and all be distinct from each other.

As far as yellows are concerned, the palette possesses both variety and durability. Opaque or transparent, bright or subdued, deep or pale, it presents a sufficiency of permanent pigments. Most noteworthy are aureolin, the deep and 'pale' cadmiums, lemon yellow, Mars yellow, the modern Naples yellow, the ochres, orient yellow, and raw sienna. Whether used alone or in tint these are, if genuine, perfectly reliable, and comprise the list of those durable colours which may be called pigments of the first cla.s.s.

Among pigments of the second cla.s.s, or the semi-stable, gamboge holds the foremost place, for although not strictly durable in itself, it conduces to the permanence of other colours. Chrome yellows, citron yellow, strontian yellow, and Thwaites' yellow, also belong to this division.

As third cla.s.s pigments, or the fugitive, must be ranked Mutrie yellow and other lemon cadmiums, the true gallstone, Indian yellow, the lakes, orpiment, Gelbin's yellow, ma.s.sicot, patent yellow, and turbith mineral.

It must not be forgotten, however, that these three cla.s.ses are subject to modification. A durable pigment may be so adulterated as to descend to the second or even the third division, while a semi-stable or fugitive colour may be replaced by a permanent or comparatively permanent subst.i.tute, as in the case of strontian yellow and gallstone.

It should likewise be remembered that pigments are apt to vary in stability according to the mode of their preparation; and that, as there are different degrees of permanence, there are different degrees of fugacity.

CHAPTER IX.

ON THE PRIMARY, RED.

Red is the second and intermediate of the primary colours, standing between _yellow_ and _blue_; and is also in like intermediate relation to _white_ and _black_, or light and shade. Hence red is pre-eminent among colours, as well as the most positive of all, forming with yellow the secondary _orange_ and its near relatives, scarlet, &c.; and with blue, the secondary _purple_ and its allies, crimson, &c. It gives some degree of warmth to all colours, especially to those which partake of yellow.

Red is the archeus, or princ.i.p.al colour in the tertiary _russet_; enters subordinately into the two other tertiaries, _citrine_ and _olive_; goes largely into the composition of the various hues and shades of the semi-neutral _marrone_ or chocolate, and its relations, puce, murrey, morelle, mordore, pompadour, &c.; and is more or less present in _browns_, _grays_, and all broken colours. It is likewise the second power in harmonizing and contrasting other colours, as well as in compounding _black_ and all other neutrals, into which it enters in the proportion of five,--to blue, eight,--and yellow, three.

Red is a colour of double power in this respect too; that, in union or connexion with yellow, it becomes hot and _advancing_; but mixed or combined with blue, cool and _retiring_. It is, however, more congenial with yellow than with blue, and thence partakes more of the character of the former in its effects of warmth, the influence of light and distance, and action on the eye, by which the power of vision is diminished on viewing this colour in a strong light. On the other hand, red appears to deepen in colour rapidly in a declining light as night comes on, or in shade. These qualities of red give it great importance, render it difficult of management, and require it to be generally kept subordinate in painting. It is therefore rarely used unbroken, as the ruling or predominating colour, or for toning a picture; on which account it will always seem detached or insulated, unless repeated and subordinated. Hence Nature is sparing with her red, employing it with as much reserve in the decoration of her works as she is profuse in lavis.h.i.+ng green upon them. This latter is of all colours the most soothing to the eye, and the true contrasting or harmonizing equivalent of red, in the proportional quant.i.ty of eleven to five, according to surface or intensity: being, when the red inclines to scarlet or orange, a _blue_-green; and when it tends to crimson or purple, a _yellow_-green.

Red breaks and diffuses with white with peculiar loveliness. It is discordant when standing with orange only, requiring to be joined or accompanied by their proper contrast, to resolve or harmonize the dissonance. In landscapes, &c., abounding with hues allied to green, a red object properly placed as regards light, shade, or distance, conduces wonderfully to the life, beauty, harmony, and connexion of the colouring. Red is, indeed, the chief element of beauty in floral nature, the prime ornament of the green garb of the vegetal kingdom.

Being the most _positive_ of colours, and holding the middle station of the primaries, red contrasts and harmonizes with black and white, which are the _negative_ powers or neutrals of colours, and the extremes of the scale. Moreover, as red is less nearly allied to black or shade than to white or light, this harmony is most remarkable in the union or opposition of white and red, and this contrast most powerful in black and red.

As a primary and simple colour, red cannot be composed by mixture of other colours. So much is it the instrument of beauty in nature and art in flesh, flowers, &c., that good pigments of this genus are most indispensable. On the whole, the palette cannot be considered so well furnished with reds as with yellows. Especially is there wanting a permanent transparent scarlet, a colour for which a prize of 500 has for many years been offered by the Society of Arts.

68. CADMIUM RED.

The deep, pale, and lemon yellows which cadmium at first afforded, were followed by an orange, which has quite recently been succeeded by a red.

This is a most vivid orange-scarlet, the red predominating, of exceeding depth, and intense fire. It is a simple original pigment, containing no base but cadmium, and possessing a large amount of latent colour. It is more orange in hue than vermilion, and has the advantages of flowing and drying well, of greater brilliancy, of retaining that brilliancy when dry, and of considerable transparency. Hence this red is preferably employed where opacity is to be avoided--in sunset clouds for instance.

As day declines or by artificial light, the colour approaches very nearly to a deep pure scarlet; and the best subst.i.tute for a permanent transparent scarlet which has yet been obtained is furnished by admixture of cadmium red with madder carmine, or by using the latter as a glaze. Compounded with white, the red yields a series of fine flesh tints; and it mixes readily and safely with other colours. Without harshness or rankness, neither injured by an impure atmosphere nor exposure to light and air, cadmium red is eligible in every department of art, enamel painting only excepted. In illumination, the red contrasted by viridian will be found most beautiful and effective.

Seeing that previous to its introduction the number of bright reds, not being crimson, nor of a crimson cast, was limited to vermilions, pure scarlet, red chrome, and red lead, of which the first alone were permanent, there was room on the palette for a strictly durable and somewhat transparent pigment like cadmium red, with its many distinctive properties.

COCHINEAL LAKES.

Lake, a term derived from the _lac_ or _lacca_ of India, is the name of a number of transparent red and other coloured pigments of great beauty, prepared for the most part by precipitating coloured tinctures of dyeing drugs upon aluminous bases. Consequently, the lakes form a numerous cla.s.s, both with respect to the variety of their appellations, and the substances whence they are produced. Those under notice are known as _Carmine_, _Crimson Lake_, _Scarlet Lake_, _Purple Lake_, _Chinese Lake_, _Florentine Lake_, _Hamburgh Lake_, _Roman Lake_, _Venetian Lake_, &c., and are obtained from the "coccus cacti," an insect found on a species of cactus, from the juice of which it extracts its nourishment. This coccus is a native of Mexico, where two kinds are recognised, under names which signify wild cochineal and fine cochineal.

The latter may be considered a cultivated product, its food and wants being carefully attended to, while the former is left in a natural state, and is less valuable. Wild cochineal is distinguished by having a woolly downy coat, which is not the case with the fine cochineal. The females, of which there are from one hundred and fifty to two hundred for each male, are marked by the absence of wings, and const.i.tute the commercial article. They are generally killed by immersion in boiling water, which causes them to swell to twice their natural size, and are then dried and packed for market. The insects shrivel in drying, and a.s.sume the form of irregular grains, fluted and concave. The best sorts have a silvery-grey colour, with a purplish reflection, and seem to be dusted with a white powder. This appearance is often given by means of heavy spar, carbonate of lead, Venice talc, &c. A good lens, however, will mostly expose the fraud; or it may be detected by macerating the insect in water, and allowing the loosened pulverised particles to settle.

Cochineal is a very rich colouring substance, yielding about half its weight of real colouring matter, which may be easily extracted by boiling in water. Dr. Warren De La Rue, who examined the living animal, states that on piercing the side of the insect a purplish-red fluid exuded, containing the colouring matter in minute granules. This colouring matter he succeeded in obtaining pure, in the form of a purple-brown friable ma.s.s, pulverizable to a fine red powder, transparent when viewed by the microscope, and soluble both in water and alcohol in all proportions. At temperatures above 136 it decomposed, and by alkalies its colour was turned to purple. These facts account for the care required in drying cochineal lakes, and for their liability to change of hue when in contact with alkaline substances, as in mural decoration.

The lakes of cochineal may be known from those of the dye-woods by their solubility in ammonia, a liquid which purples but does not dissolve the colours produced from the latter.