Purple of Cassius, or gold purple, is not a pigment for painters’ use; it is employed for colouring glass, for porcelain painting and for coloured glazes. For these last purposes we know nothing which can completely replace purple of Cassius, so that its preparation will be described here. Its costliness prevents its general use.

As regards the chemical constitution of purple of Cassius very diverse views are held. Chemists have not yet succeeded in explaining it; some regard it as aurous stannate, others as stannic acid, in which a particular form of gold of a red colour is contained in an extremely finely divided condition. There are reasonable grounds for both opinions. Theoretical considerations are of little interest to the manufacturer, who is chiefly interested in the method by which he can prepare a product which satisfies the demands of the consumer.

Purple of Cassius has been known for nearly two hundred years, hence there are many formulas for its preparation. Before these are enumerated it is necessary to give an account of the conditions under which this substance is formed. In order to obtain gold purple, a solution is required which contains both stannous and stannic chlorides; if this solution is mixed with a very dilute solution of gold chloride a precipitate is obtained which is generally brownish red, and only acquires its fine red shade on igniting. The colour shown by the purple after precipitation is no guide to the shade of the finished product; a purple which shows a very fine colour whilst wet often produces a pigment of much less beauty than an actually ugly precipitate. These differences are to be ascribed to the varying molecular condition of the purple, for the different varieties show very small differences in chemical composition.

According to the directions of Fuchs, a fine purple is obtained by mixing a solution of stannous chloride with sufficient ferric chloride solution to give the mixture a green colour. Part of the ferric salt is then reduced to ferrous salt, whilst the stannous chloride is partially oxidised. At the same time a solution of gold chloride is made; this must be quite free from nitric acid and contain 1 part of the salt in 400 to 500 parts of water. The tin solution is added drop by drop to the gold solution with constant stirring; the mixture becomes turbid, but the precipitate requires a long time to settle on account of its fine state of division. The depth of colour shown by the purple varies according to the strength of the solutions.

M. Müller states that the following process gives the best results: The quantity of stannic chloride equivalent to 9 grammes of stannic acid is dissolved in 200 cubic centimetres of water; potassium carbonate is added to alkaline reaction, then 1 gramme of gold in the form of chloride. Grape sugar is added to the mixture, which is made up to 300 cubic centimetres by water, and warmed until the brightest shade is reached. When a very gelatinous mass is formed after the addition of the potassium carbonate it is heated for a short time before adding the gold solution and grape sugar.

Wächter gives directions for obtaining pale and deep shades. To obtain the pale purple 5 grammes of tin are dissolved in aqua regia, the solution evaporated to dryness on the water bath, the solid residue mixed with a solution of 2 grammes of stannous chloride and dissolved in 10 litres of water. To this liquid is added a solution of 0·5 gramme of gold in aqua regia, and immediately 50 grammes of ammonia to neutralise free acid. The precipitate separates spontaneously from the dark red liquid; its formation may be hastened by adding sulphuric acid. The washed precipitate is mixed whilst moist by means of a silver spatula with 20 grammes of lead flux, 2 grammes of red lead, 1 gramme of quartz sand, and 1 gramme of calcined borax. The mixture is then dried; it produces a purple red colour. If 3 grammes of silver carbonate are added, a clean, pale purple pigment results.

The deep purple is made by mixing a solution of 0·5 gramme of gold in 10 litres of water with 7·5 grammes of stannous chloride solution (specific gravity, 1·7) whilst stirring, and adding a few drops of sulphuric acid. The washed precipitate is mixed with 10 grammes of lead flux and 0·5 gramme of silver carbonate.

A rose-red shade of gold purple is obtained by dissolving 1 gramme of gold and mixing the solution simultaneously with solutions of 50 grammes of alum in 20 litres of water, and of 1·5 gramme of stannous chloride (specific gravity, 1·7). Ammonia is now added until all the alumina is precipitated. In order to prepare from the dried precipitate the mixture which will produce the colour on fusion, it is mixed with 50 grammes of lead flux and 2·5 grammes of silver carbonate.

Magnesia Gold Purple.—According to M. Müller, the colour of gold purple is produced by covering the particles of a very finely divided white substance with metallic gold. Thus, when calcined magnesia is stirred up with water, gold chloride solution added, and the mixture warmed to 100° C., the gold is precipitated upon the magnesia; the wet, yellow powder acquires a reddish shade on drying, which at a red heat becomes so beautiful a carmine-red that it surpasses purple of Cassius in fineness of shade. A purple which contains 20 grammes of gold as chloride to 84 grammes of magnesia has the most pure carmine tint; the shade varies with the proportion of gold to magnesia. The different shades of purple contain the following percentages of gold:—

Alumina Gold Purple is obtained, according to Müller, by adding gold chloride and excess of potassium carbonate to alum solution and boiling. The purple obtained in this way is as deep in colour with 10 per cent. of gold as a magnesia purple with 20 per cent. of gold, but has a more bluish violet shade. If, in the preparation of purple, the alumina is precipitated by potassium carbonate and the gold chloride reduced by grape sugar, a different shade of purple is obtained to that in the precipitation of which ammonia is used. The alumina gold purple is specially adapted for the production of porcelain enamels.