As in the case of aureolin, we have had a prolonged personal experience of this new yellow, an experience which justifies us in asserting 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 passed 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 masses, 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 arsenic, 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 arsenic 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 arsenic, 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 substitute 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.