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.

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