are obtainable ranging from the richest crimson purple to the most delicate violet. We have produced them by wet and dry methods, varying in brilliancy and beauty, but characterised generally by want of body, and frequently by a smalt-like grittiness. Chemically, good and stable colours, they are not received with favour on the palette, and certainly may be very well replaced by mixtures of cobalt blue and madder red. When a permanent compound is obtainable equal in colour to an original pigment, and superior in its physical attributes, no objection can fairly be taken to its artistic preference. There are other things to be considered in a pigment besides permanence, or even permanence and colour combined. The two together do not constitute a perfect pigment, that is, a material of practical utility and value. In the last chapter, allusion was made to a green which possesses both the one and the other, and yet is—at present, at least—quite unfitted for artistic use. Hence, with a strong partiality for simple original pigments, we are bound to confess there are cases where mixtures are justifiably preferred. All we contend for is, that each constituent of such mixtures should be stable, and neither give nor receive injury by being compounded.
219. Gold Purple,
Purple of Cassius, or Cassius's Purple Precipitate, was discovered in 1683 by Cassius of Leyden. It is a compound of tin and gold, best formed by mixing aqueous perchloride of iron with aqueous protochloride of tin, till the colour of the liquid has a shade of green, and then adding this liquid, drop by drop, to a solution of perchloride of gold, which is free from nitric acid and very dilute: after twenty-four hours the purple is deposited. When recently prepared, the colour is brightened by boiling nitric acid. Not brilliant, but rich and powerful, this purple varies in hue according to the mode of manufacture from deep crimson to murrey or dark purple: it also differs in degrees of transparency. Working well in water, it is an excellent though costly pigment, once popular in miniatures, but at present rarely, if ever used, as purple madder is cheaper, and perfectly well supplies its place. Retaining its colour at a high red heat, it is now confined to enamel and porcelain painting, and to tinging glass of a fine red. If, whilst in its hydrated state, it be washed with ammonia, a bright purple liquid results, from which a violet colour, somewhat less expensive, can be produced, by combining the gold purple with alumina, and calcining the product in the same way that is practised with cobalt. This compound may be exposed to the action of the sun's rays for a year without being sensibly affected.
220. Prussian Purple.
A prussiate of iron is obtainable of a violet hue, affording good shadow tints and clear pale washes. It has not, however, been introduced as a pigment, as ordinary Prussian blue tinged with red furnishes a similar colour.
221. Sandal Wood Purple.
Sandal wood contains about 1616 per cent. of colouring matter, soluble with difficultly in water, but readily dissolved by alcohol. From the latter solution, chloride of tin throws down a purple, and sulphate of iron a deep violet precipitate; neither of which is remarkable for permanence.
222. Tin Violet.
By heating chromate of stannic oxide to bright redness, a dark violet mass is obtained, which is better adapted to enamel painting than to the palette. It communicates in glazings a variety of tints, from rose-red to violet.