Field's Chromatography Part 4

Is either the anhydrous oxide, the hydrate oxide, or hydrated basic carbonate of zinc. It varies in opacity and colour according to the mode of manufacture, and the purity of the compound, but may always be relied upon as permanent. The whiteness of the best samples rivals that of white lead, and it is not tarnished like the latter by sulphurous vapours. In opacity it never equals white lead, and might perhaps serve advantageously as a glaze over that pigment, either alone or compounded with other colours; as well as act as a medium of interposition between white lead and those colours which are injured by it, such as gamboge, crimson lake, &c. When duly and skilfully prepared the colour and body of this pigment are sufficient to qualify it for a general use upon the palette in oil: in water it has been superseded by Chinese white.

Occasionally, starch, chalk, white clay, and carbonate of baryta, are employed as adulterants; none of which, however, are inimical to stability.

As a pigment, zinc white may be said to be innoxious. As oxide of zinc does not readily form a saponaceous compound with fats or oil like white lead, the paint prepared with it and ordinary linseed oil does not dry or harden so rapidly. For the purpose of causing it to be more siccative, the oil was boiled with a large quant.i.ty of litharge, but by this method the white was liable to tarnish on meeting with foul air.

Instead of litharge, experiments have led to the choice of salts of zinc, such as the chloride or sulphate, a small percentage of which, on being mixed with the oil or oxide, confers upon it the property of rapidly hardening. The same result is attained by employing an oil, dried by boiling with about five per cent of peroxide of manganese. In either case, a paint retaining its white colour permanently is produced.

These agents might, with advantage, be more generally used in the place of litharge for rendering oils siccative. Many pigments which are not naturally affected by sulphurous emanations are apt to suffer if mixed with an oil made drying by means of lead.

16. _Cadmium White._

Provided the metal be freed from iron, which we have commonly found to be more or less present, a white of considerable beauty may be produced; either directly by precipitation as hydrated oxide or carbonate, or indirectly by exposing the brown anhydrous oxide to air and light--the latter mode yielding a product of greater opacity. However prepared, cadmium white is deficient in body, and apt to a.s.sume a yellow tint on meeting with an impure atmosphere.

17. _Pearl White_

Is an insoluble basic nitrate of bis.m.u.th, a pearly white powder of loose texture, turning grey on exposure, and blackened by sulphuretted hydrogen. It is chiefly used as a cosmetic, but is said to injure the skin, rendering it yellow and leather-like; and it has been known to cause a spasmodic trembling of the face, ending in paralysis.

Another preparation under this name, and now obsolete we believe as a pigment, was obtained from mother-of-pearl. It is described as exquisitely white, and of good body in water, but of little force in oil or varnish.

18. _Tin White_

Resembles zinc white in some respects, but has less body and colour, and dries badly. According to its composition, it is liable to turn either black or a dull yellow in contact with sulphurous vapours.

19. _White Chalk_

Is a well-known native carbonate of lime, employed by the artist only as a crayon, or for tracing his designs, for which purpose it is sawed into suitable lengths. White crayons and tracing chalks, to be good, must work and cut free from grit. From this material are prepared whitening and lime, which form the bases of many cheap pigments and colours, used in distemper, paper-staining, &c.

Besides those mentioned, there are other metallic whites varying in beauty and opacity, such as those of mercury, a.r.s.enic, and antimony; but none of them are of any value or reputation in painting, on account of their great disposition to change of colour, whether by light or foul air, both in water and oil.

There are also other terrene whites, under equivocal names, among which are Morat or Modan white, Spanish white, Troys or Troy white, Rouen white, China white, and Satin white; the latter being a sulphate of lime and alumina, which dries with a glossy surface. The common oyster-sh.e.l.l contains a soft white in its thick part, and there is the white of egg-sh.e.l.ls. There is, too, an endless variety of native earths, in addition to those prepared by art. The whole of them, however, are dest.i.tute of body in oil; and several, owing to their alkaline nature, are injurious to many colours in water, as well as to all colours which cannot be employed in fresco.

Among the infinitude of white substances, the artist finds that there are but three white pigments--those of lead, zinc, and baryta. The first possesses the greatest opacity, while the second and third are most durable. The last, however, has so many objectionable qualities, that the number of eligible whites, may almost be said to be two--lead and zinc. Of these, the former is blackened by foul air, and in oil, the latter is wanting in body. In fact, there is but one white pigment which approaches perfection--Chinese white; and this is only a water-colour.

CHAPTER VIII.

ON THE PRIMARY, YELLOW.

Yellow is the first of the primary or simple colours, nearest in relation to, and partaking most of the nature of, the neutral white; it is accordingly a most advancing colour, of great power in reflecting light. Compounded with the primary _red_, it const.i.tutes the secondary _orange_, as well as its relatives, scarlet, and other warm colours.

It is the archeus, or prime colour, of the tertiary _citrine_; characterises in like manner the endless number of semi-neutral colours called _brown_, and enters largely into the complex hues termed buff, bay, tawny, tan, dan, dun, drab, chestnut, roan, sorrel, hazel, auburn, isabela, fawn, feuillemort, &c. Yellow is naturally a.s.sociated with red in transient and prismatic colours, and is the princ.i.p.al power with it in representing the effects of warmth, heat, and fire. Combined with the primary _blue_, yellow furnishes all the variety of the secondary _green_, as well as, subordinately, the tertiaries _russet_ and _olive_.

It also enters in a very subdued degree into cool, semi-neutral, and broken colours, and a.s.sists in minor proportion with blue and red in the composition of _black_.

As a pigment, yellow is a tender delicate colour, easily defiled, when pure, by other colours. In painting it diminishes the power of the eye by its action in a strong light, at the same time becoming less distinct as a colour; while, on the contrary, it a.s.sists vision and becomes more distinct as a colour in a neutral somewhat declining light. These powers of colours upon vision require the particular attention of the colourist. To remedy the ill effect arising from the eyes having dwelt upon a colour, they should be either pa.s.sed gradually to its opposite colour, and refreshed amid compound or neutral tints, or washed in the clear light of day. Hence, in viewing large collections of pictures, their colours will be more duly estimated by sometimes walking to the window, or by taking an occasional glance at a millboard, which may be carried in the hand, painted a cool gray.

In a warm light, yellow becomes totally lost, but is less diminished than all other colours, except white, by distance. The stronger tones of any colour subdue its fainter hues in the same proportion as opposite colours and contrasts exalt them. The contrasting colours of yellow are a purple inclining to blue when the yellow leans to orange, and a purple inclining to red when the yellow tends to green, in the mean proportions of _thirteen_ purple to _three_ yellow, measured in surface or intensity. Being nearest to the neutral white in the natural scale of colours, yellow accords with it in conjunction; while, of all colours, except white, it contrasts black most powerfully. Yellow is discordant when standing alone with orange, unsupported by other colours.

On account of the paucity of fine yellows among the ancients, we find that in many paintings and beautiful illuminated MSS. of old, glowing with vermilion and ultramarine, the place of yellow was supplied by gilding. Now, certainly, no such scarcity exists; of the three primary colours, good yellows being the most numerous. It may be observed of yellow pigments that their colour being primary and therefore simple, they cannot be composed by any mixture of other colours. The same remark of course applies to pigments which are red or blue.

20. AUREOLIN.

In these days a new pigment soon finds its level, standing or falling according to its merits. There are too many colours already on the palette for a fresh comer to have much chance, unless it possess some great distinguishing quality, or can take a place which has never been occupied. Such a void aureolin fills. This "magnificent yellow pigment," says the _Chemical News_, "supplies a desideratum hitherto in vain sought for by artists. It is the nearest approach to a perfect yellow in existence, and more closely resembles the purity of the prismatic spectrum than any other artificial colour. It is transparent, has great brilliancy and richness, both pure and in combination, and is very permanent, being entirely unaffected by exposure to sulphuretted hydrogen and other atmospheric impurities, or to the direct rays of the sun during an entire summer. Aureolin, with ultramarine and madder red, completes the triad of brilliant, permanent, and transparent primitive colours." The above only tallies with the statements of several scientific chemists and artists of note, statements which a prolonged personal experience of the colour enables us to endorse. To our knowledge, aureolin is quite uninjured by the severest tests to which a pigment can be subjected. We have found it bear with impunity, even in its lightest and faintest tints, the foulest gas and the brightest sunshine. Damp has no effect upon it; and in oil, water, or fresco, it is equally eligible. With all other colours aureolin mixes safely and readily, forming combinations of the utmost variety and value. It affords beautiful transparent tints, well defined, and of exceeding purity; the paler washes being at once clear and delicate, and admitting the most subtle gradations of tone. The artistic properties of aureolin, however, will be best described by quoting the following extract from Mr. Aaron Penley's _English School of Painting in Water Colours_:--

"I have fully tested the qualifications of _Aureolin_ for the Landscape Painter, and, without hesitation, p.r.o.nounce it to be the most valuable addition to the 'colour box' since the introduction of Rose Madder. It has supplied a deficiency of a very important character. Hitherto, no Primitive Yellow has been quite satisfactory as to its persistence; so that the Aureolin will not only be regarded by the _artist_ as a great boon in the production of his works, but it must also be considered as a _real_ and _lasting benefit_ to pictorial art in general. The permanence and unaltered purity of its lightest and faintest tints we are a.s.sured may be confidently relied upon, inasmuch as they have been fully established by the most severe tests to which colour can be subjected, by several of our ablest and most talented chemists. It is, therefore, needless to enlarge upon its merits, other than that I, for one, feel grateful for its introduction. Its uses are manifold, and may be considered available for every purpose requiring a Yellow of its character.

As to Gray--perhaps it is not possible to obtain more delicately pure and transparent aerial tints than are to be produced from a combination of Cobalt, Rose Madder and Aureolin; all of which are of a light description and well suited for the representation of soft and thin effects of the atmosphere. These colours are each of them beautiful, and yield a most exquisite range of tones, which, as they mix together most kindly, render them desirable where purity and delicacy are sought. As to Foliage.--In speaking of Aureolin as adapted for the colouring of foliage and herbage, it is impossible to say too much in its praise. It imparts the vividness and freshness of nature to every colour with which it is combined--a quality of the highest order. As a colour for drapery it has no equal, and may be employed with perfect success, either by itself or with any of the other pigments.

"The following table of compound tints will be found extremely useful:--

Aureolin. | Aureolin. | Aureolin. | Aureolin.

Burnt Sienna. | Vand.y.k.e Brown. | Sepia. | Sepia, or Rose Indigo, or | Indigo, or French | | Madder.

French Blue.| Blue. | | Cobalt.

Aureolin. | Aureolin. | Aureolin. | Aureolin.

Indigo. | Oxide of Chromium.| Emerald | Light Red.

| | Green. | Cobalt, or | | | Indigo.

Aureolin. | Aureolin. | Aureolin. | Aureolin.

Burnt Sienna. | Burnt Umber. | Brown Madder.| Rose Madder.

"Aureolin, in combination with Cobalt and Sepia, or Rose Madder, gives most agreeable and delicate tints for distant trees, when under the influence of a soft light, or hazy state of the atmosphere. Having most impartially and diligently tested the qualities of the Aureolin, I can and do most conscientiously recommend its adoption by all who practise water-colour painting."

The foregoing sufficiently proves the value of aureolin in water, and similar flattering notices have been given of the colour in oil. Both in a chemical and artistic sense, therefore, this new primitive yellow merits the highest regard, and justly claims a foremost place among that little band of pigments which are without fear and without reproach.

For mural decoration, aureolin is admirably adapted, but it cannot be used in enamel, the colour being destroyed by great heat.

CADMIUM YELLOWS.

Of these there are three tints, _Deep_, a so-called _Pale_, and _Lemon_.

21. DEEP CADMIUM.

Cadmium Yellow is comparatively a recent introduction, the metal itself not having been discovered till 1818. The cadmium yellows of commerce are (the chromate excepted) all sulphides, and therefore not affected by impure air. Until lately, they were not manufactured in England but imported from abroad, and as a rule were sadly doctored. We have found in them a large proportion of orpiment, chromate of lead, &c., together with quant.i.ties of soluble salt, extracted by boiling water. Owing to careless preparation, there was also present an unnecessary amount of dirt, which interfered as much with the purity of the colour, as sophistication lessened its stability. For these reasons, doubtless, cadmium yellows got to be regarded by some with disfavour and suspicion; and it may fairly be said that they did not attain their present popularity, until they became an article of home produce.

Deep cadmium yellow, if genuine, may without hesitation be declared permanent, both with respect to foul gas, and exposure to light or air.

The variety under notice is of extreme depth, inclining to orange, glowing, l.u.s.trous, and brilliant. It is not very transparent, but wonderfully clear and bright, of great power, and the most richly toned yellow known. For draperies it is particularly adapted, and for gorgeous sunsets is invaluable. It works and washes well, readily throws all other yellows into the shade when used alone, and combines admirably with Chinese white for the light touches of bright clouds or mountains.

By admixture with white, cadmium gives a series of beautiful clear tints. When compounded with white lead, however, the colour has been stated to be destroyed. Theoretically, this might very well happen.

Cadmium yellow is composed of cadmium and sulphur--white lead of lead and carbonic acid. If the former parted with some of its sulphur to the latter, sulphide of lead would result, which is black. Hence, the partly decomposed yellow and white would be mixed with black, and there would be formed a blackish-yellow or a yellowish-black. Again, if the cadmium parted with the whole of its sulphur to the lead, receiving in exchange the carbonic acid of the latter, a mixture of black sulphide of lead with white carbonate of cadmium would be furnished, the result being a grey. Perhaps the following rough diagram may serve to make our meaning plainer:--

Cadmium Yellow {Cadmium. __________ Carbonate of Cadmium (White.) {Sulphur. / / / / / / / / ____/__ White Lead {Lead. / Sulphide of Lead (Black.) {Carbonic Acid.

Such is the theory of the reaction which might take place, but which, as far as our own experience goes, does not. Some deep cadmium yellow which we ourselves prepared was intimately mixed and ground with an equal quant.i.ty by weight of Cremnitz white, and an oil rub of the compound laid upon a tile. Having placed the latter on a shelf in the laboratory, we watched from week to week to see if any approach to blackness occurred, any diminution in the beauty of the tint; but could perceive none. Hence, while admitting the possibility of the colour being damaged or destroyed in the case of an inferior and spurious article, we conclude that an unadulterated cadmium yellow, containing no free sulphur, neither injures, nor is injured by, white lead, and may safely be used therewith. At the same time, the artist should be warned to satisfy himself of the genuineness of his pigment, or otherwise to employ the white of zinc, at least as a medium of intervention.

A good sample of cadmium yellow may rather advantageously than otherwise be compounded with white lead, for we have found that a mixture of equal parts by weight of the two will bear an atmosphere of sulphuretted hydrogen that completely blackens the white alone.