Field's Chromatography Part 6

As its name denotes, was likewise a preparation of that metal, of a deeper colour than Naples yellow, but similar in its properties. It was princ.i.p.ally used in enamel and porcelain painting, and differed greatly in tint. One variety, brighter than the rest, is stated not to have been affected by foul air, and therefore could not have had a lead basis.

OCHRES,

Known as _Yellow Ochre_, _Brown Ochre_, _Roman Ochre_, _Transparent Gold Ochre_, _Oxford Ochre_, _Stone Ochre_, _Di Palito_, &c., are native earths, consisting chiefly of silica and alumina in combination with iron, which latter forms the princ.i.p.al colouring matter. They are among the most ancient of pigments, and their permanency is proved by the state of the old pictures. In a box of colours found at Pompeii, and a.n.a.lyzed by Count Chaptal, he discovered yellow ochre purified by washing, which had preserved its original freshness. They may all be produced artificially in endless variety as they exist in nature, and are all converted by burning into reds or reddish-browns. Several ochres are found in the natural state of so very fine a quality, that they require no other preparation than that of being washed. Their colours may be imitated to a certain extent by means of iron alone, uncombined with silica and alumina; but such ferruginous preparations are not equally durable, and as their chemical action is stronger, they are more likely to affect those pigments which are damaged by iron. It often happens in colours that one component of weak stability, or powerful for evil, is strengthened and held in check by another; thus in the case of the ochres, the silica and alumina by keeping a tight hand on the iron, both ensure its safety, and prevent it injuring others.

41. YELLOW OCHRE,

Called also _Mineral Yellow_, is found in most countries, and abundantly in our own. It differs much both in const.i.tution and colour, ranging from a tolerably bright though not vivid yellow to a brown-yellow, and is generally of a warm cast. Its natural variety is much increased by artificial dressing and compounding. The best yellow ochres possess no great force, but as far as they go are valuable pigments, particularly in fresco and distemper, being neither subject to change by ordinary light, nor sensibly affected by impure air, or the action of lime. By time, however, and the direct rays of the sun, they are somewhat darkened. Like other ochres, they may be safely used in admixture with pigments which are themselves permanent. With carmine and the cochineal lakes, or intense blue, the ochres are best not employed.

The impunity with which yellow ochre bears foul gas is one of its many recommendations. No immediate effect whatever is produced by sulphuretted hydrogen, and only a slight dirty brown tint is imparted by its prolonged action. This discolouration a short exposure to air and light quickly removes. By keeping the ochre sufficiently long in contact with sulphide of ammonium a jet black is obtainable, but a rub of it in a moist unwashed state completely regains its yellow hue in a day or so.

Hence, yellow ochre compounded with pigments which suffer from an impure atmosphere doubtless acts as a preservative agent.

Yellow ochre is usually employed in the distance and middle ground of a landscape. It possesses a slight degree of turbidness, and is esteemed for this property, which is considered to give it a retiring quality. By admixture with Antwerp blue or indigo it affords a fine range of quiet greens, also a very serviceable yellowish drab with Vand.y.k.e brown. The ochre is valuable in warm skies, the sails of ships and boats, sandstone rocks and cliffs, buildings, hay, sheep, &c. It does not compound kindly with any of the cold colours, and should therefore be used as a wash over others that are dry, when required to qualify their tints.

42. ROMAN OCHRE

Is rather deeper and more powerful than the preceding, as well as more transparent and cool in tint. In other respects it is similar, and forms with Antwerp blue and indigo a like excellent range of greens. We may observe, however, that as indigo is not a permanent pigment, the colours it yields by admixture cannot be durable as far as the blue is concerned. Roman ochre and brown madder are admirably adapted for red sails, and autumnal effects of foliage.

43. BROWN OCHRE,

Likewise known as _Spruce Ochre_ and _Ocre de Rue_, or, more correctly _Ru_, is a dense, deep-toned brownish yellow, fine in sandy foregrounds.

With Indian yellow it gives a dark autumnal tint of great richness, but stable only as respects the ochre. When mixed with other colours, it furnishes a series of rich yet sober tones of extensive use. It covers well, without being too opaque; and compounded with black and a little brown-red is good for backgrounds, &c.

44. TRANSPARENT GOLD OCHRE

Resembles in a great degree Roman ochre, but is clearer in its tints, and more transparent. It is also brighter and much less opaque than yellow ochre. It approaches somewhat the character of clear bright raw sienna, though more pure and brilliant, serving for strong semi-transparent greens and sunny effects.

45. OXFORD OCHRE

Is a native pigment from the neighbourhood of Oxford, semi-opaque, of a warm yellow colour and soft argillaceous texture, absorbent of water and oil, in both of which it may be safely employed. It is one of the best of yellow ochres.

46. STONE OCHRE

Has been confounded with the last variety, to which, as well as to Roman ochre, it is frequently similar. True stone ochres are found in b.a.l.l.s or globular ma.s.ses of various sizes in the solid body of stones, lying near the surface of rocks among the quarries of Gloucestershire and elsewhere. These b.a.l.l.s are smooth and compact, in general free from grit, and of a powdery fracture. They vary exceedingly in colour, from yellow to brown, murrey, and gray, but otherwise do not differ from ordinary ochres.

In enamel they may be used for browns and dull reds.

47. DI PALITO

Is a light yellow ochre, with no special distinguishing quality, except that its tints are rather purer in colour than most ochres.

48. ORIENT YELLOW

Is an entirely new preparation of absolute permanence, and perfectly unexceptionable in all respects, both in water and oil. We can give it no higher praise than by saying it equals aureolin in stability, as well as in neither injuring, nor being injured by, other colours. Not possessed of the same amount of transparency, it is distinguished by greater richness and depth. Of a soft golden hue, l.u.s.trous and luminous, it resembles a brilliant and somewhat opaque Indian yellow. A gorgeous and durable subst.i.tute for that fugitive pigment is produced by compounding the orient with aureolin, or by using the latter as a glaze.

Being more transparent than cadmiums and less obtrusive, the new yellow is adapted for mellow sunset and sunrise clouds, or for sunshine on distant mountains. With French blue it affords a beautiful sea green; and, mixed with aureolin, gives fine foliage tints. It is also eligible for draperies and illumination. For enamelling it is inadmissible, the colour being destroyed by great heat; but in fresco it may safely be employed.

As in the case of aureolin, we have had a prolonged personal experience of this new yellow, an experience which justifies us in a.s.serting that there is none more permanent. In the whole range of artistic colours there is no pigment less affected by chemical or physical agents. Acid and ammoniacal fumes, foul gases, and exposure to damp, air, light, or sunshine, equally fail to injure it. The perfect impunity with which it bears the action both of sulphuretted hydrogen and sulphide of ammonium is remarkable. The former gas may be continuously pa.s.sed into the colour suspended in water, or a strong solution of the latter sulphide be poured upon it, and the yellow remains unchanged. Submitted to the direct rays of the sun during an entire summer, its lightest and faintest tints have preserved their original hue.

In a preceding chapter we remarked that, provided the colour be stable, the more colour a pigment possesses the better. The "latent colour"

there alluded to, is one of the advantages of orient yellow. The more it is looked into, the more colour is seen--there is no suspicion of a base coloured, the pigment is colour itself.

49. ORPIMENT,

Also called _King's Yellow_, _Chinese Yellow_, _Yellow Orpiment_, &c., was known in ancient times: the Romans called it _auri pigmentum_ or gold colour, whence, by corruption, its present name is derived. It is found in the native state in China and elsewhere, the best quality being in ma.s.ses, consisting of plates of a fine golden hue, intermixed with portions of a vermilion or orange-red colour; the inferior kinds are yellow or greenish yellow. Of orpiment, or sulphuret of a.r.s.enic, which is produced artificially, there are two distinct varieties; one of a bright pure yellow tint, in which the sulphur predominates, and one of an orange hue, in which the a.r.s.enic is in excess. The former is the most lasting, but it is not durable in water, and still less so in oil, although not discoloured by impure air. Compounded with white lead it is soon destroyed, nor can it be mixed with any colours into which lead enters, such as chrome yellow, the old Naples yellow, &c. The sulphur in combination with the a.r.s.enic, having less affinity for that metal than for lead, lets it go, and forms a sulphuret of lead of a dark greyish hue. Moreover, as orpiment is apt to deprive other pigments of their oxygen, and therefore to change and be changed by all pigments whose colour depends on that element--metallic pigments especially--it is probable that the orpiment after some time withdraws the oxygen from the lead; and this would be an additional cause for the darkening of the tint composed of the two colours. With sulphides or pigments containing sulphur, orpiment may be used with less danger. If employed at all, however, it had better be in a pure and unmixed state. We are far from recommending orpiment as an eligible colour, and it is highly poisonous.

Brick dust and yellow ochre are sometimes found as adulterants.

50. RAW SIENNA,

Known likewise as _Raw Sienna Earth_, _Terra di Sienna_, &c., is a ferruginous native pigment, firm in substance, of a glossy fracture, and very absorbent. It is of rather an impure yellow colour, and much used in landscape, being very serviceable both in distance and foreground.

Unless proper skill is exercised in its preparation, the sienna has the objection of being somewhat pasty in working. Being little liable to change by the action of either light, time, or impure air, it may safely be employed according to its powers, in oil, water, and other modes of practice. It possesses more body and transparency than the ochres; and by burning becomes deeper, orange-russet, as well as more transparent and drying.

Raw sienna compounded with cobalt, indigo, or Prussian blue, and a very little bistre, yields good sea greens, that with indigo being the most fugitive. Alone, it is adapted for shipping, sails, baskets, decayed leaves, brooks and running streams.

51. STRONTIAN YELLOW,

To justify its name, should be a chromate of strontia, a compound very slightly soluble in water, and not more stable than the zinc chromate.

The pigment, however, now sold as strontian yellow is usually formed by admixture, and contains no strontia whatever. Its absence cannot be considered a disadvantage, for the subst.i.tute possesses a durability to which the original could lay no claim. Other things being equal, we prefer an original pigment to one compounded, but a good mixture is decidedly better than a bad original. A light primrose, clear and delicate.

The foregoing comprise those yellows more generally employed, advisedly or not, as the case may be. The following are for the most part not commercially obtainable, a remark that will apply in ensuing chapters to all numbered colours printed in italics. As a rule, these have become obsolete as pigments, or have never been introduced as such. The former could not well be omitted in a work of this kind, and the latter deserve notice as being at least suggestive. At present, many of them must be regarded as mere curiosities, being obtainable only from materials of excessive rarity. In time, however, the sources whence they are derived may possibly be found in greater abundance, and these now fancy products prove of value to the palette. The new metal indium, for instance, furnishes a bright yellow sulphide, like that of cadmium. The colour could not be affected by foul air, and might possess other advantages which would render indium yellow a desirable pigment. With regard to those compounds available for artistic use, but which have not to our knowledge been adopted, several are quite ineligible. It may be thought that they are needlessly referred to, but they are mentioned as a warning and a guide. Strange preparations have been offered as pigments, and sometimes accepted, witness turbith mineral, iodine yellow, &c. In these days of chemistry there is less chance for them, but they are continually submitted to one's notice, their merits being enlarged upon in proportion to their worthlessness. Through an exceptional ignorance they may still gain a place, and it has been deemed, therefore, not superfluous to allude to them. At the same time we do not pretend to exhaust the list, any more than we claim to note all substances possessing colour, but yet not admissible as pigments. Some there are which do not retain that colour on drying; others, whose preparation involves processes too nice, complicated, or expensive, for manufacturing purposes. There are many colours, again, which exist only on paper. We have too often found the imaginations of chemical writers far more vivid than the colours they describe. Gorgeous yellows turn out dingy drabs; dazzling scarlets dirty reds; and brilliant blues dusky slates. As respects colours, most books of science need revising.

52. _a.r.s.enic Yellow_,

Called also _Mineral Yellow_, has improperly been cla.s.sed as an orpiment, from which it differs in not being a sulphide, and in containing lead. It is prepared from a.r.s.enic fluxed with litharge, and reduced to powder. It is much like orpiment in colour, dries better, and not being affected by lead, is less liable to change in tint. The presence of the litharge, however, renders it subject to be blackened by sulphuretted hydrogen. Of course it is poisonous.

53. _Bis.m.u.th Yellow_,

Or chromate of bis.m.u.th, may be obtained either as a lemon or an orange yellow, sparingly soluble in water. The colour is not permanent, and turns greenish-brown even when excluded from light and air.

54. _Copper Yellow_,

Or chromate of copper-pota.s.sa, is of a bright yellow tint, not insoluble in water. It is discoloured both by foul gas and exposure.

55. _Gelbin's Yellow_,

Or chromate of lime, is a pale whitish yellow, poor in colour, partly soluble, and not at all to be depended on.

56. _Indium Yellow._

Whether the new metal indium will ever be found in sufficient quant.i.ty to render it practically useful remains to be seen. The most abundant source at present known is the Freiberg blende, 100,000 parts of which only yield from twenty-five to forty parts of indium. The metal is chiefly interesting in an artistic sense on account of its sulphide, a fine bright yellow resembling cadmium, and best obtained by precipitating an acetic acid solution with sulphuretted hydrogen, or sulphide of ammonium. In the latter, the yellow dissolves on being heated, but deposits again on cooling of a rather paler tint. With one modification, what was said in a former edition of this Treatise concerning cadmium yellow may be repeated of indium yellow. "The metal from which it is prepared being hitherto scarce, it has not been employed as a pigment, and its habits are not therefore ascertained."

All we can tell is, that the colour does not suffer from impure air.

Indium is likewise distinguished by a straw-yellow oxide.

57. _Iodine Yellow_,