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:— _Percentage of Gold._ _Shade of the Purple._ 33·5 Brownish-red (excess of gold). 25·0 Deep carmine-red. 20·0 Medium carmine-red. 10·0 Pale carmine. 5·0 A good rose-red. 3·0 Rose-red. 1·0 Pale rose. 0·2 Delicate rose. 0·1 Appreciable red tint. =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. CHAPTER XXII. BLUE MINERAL PIGMENTS—IRON-CYANOGEN PIGMENTS. The pigments occurring in commerce under the names of Prussian, Chinese, Berlin or Paris blue consist, when pure, of ferric ferrocyanide, Fe₄[Fe(CN)₆]₃. This compound is obtained as a deep blue precipitate when yellow prussiate (potassium ferrocyanide) is mixed with the solution of a ferric salt.