Cobalt Red consists of cobalt phosphate. It was first recommended by Salvetat. It is obtained by precipitating a solution of a cobalt salt by sodium phosphate. After drying at the ordinary temperature the precipitate has a beautiful rose-red colour. By careful heating, the shade becomes more violet according to the temperature used.
Cobalt-Magnesia Red.—When magnesia is moistened with the solution of a cobalt salt and strongly heated, a rose-red mass is obtained which can be used as an extremely durable artists’ colour. It probably consists of a compound of cobalt oxide with magnesia. The method of preparation is as follows: Magnesium carbonate is mixed to a thin paste with a dilute solution of cobalt nitrate; this is heated and stirred until quite dry, when the residue is strongly heated in a covered crucible. In the manufacture of this and other cobalt pigments it is of particular importance to prevent the entry of the fire gases into the crucible. Unless this is done a product of good colour is not obtained.
Cobalt Arsenate.—By precipitating a cobalt salt with sodium arsenate, a violet-red precipitate is obtained which leaves a fine durable red on heating. The mineral erythrite has a similar composition and colour. This pigment has a bright shade, is permanent, and although it contains arsenic is not particularly poisonous, since after fusion it is little soluble.
Chromium Stannate.—This artists’ pigment, which may be used in oil and porcelain painting, is made in the following manner (Gentele): 1 kilogramme of tin is converted by strong nitric acid into metastannic acid, 50 grammes of potassium chromate are dissolved in 1 litre of water, the solution mixed with 2 kilogrammes of chalk and 1 kilogramme of powdered quartz, and finally with the metastannic acid. The mixture is dried and strongly heated. The temperature must be raised to a white heat, at which the mass sinters and acquires a dark rose-red colour. It is then completely extracted with boiling water, when a fine powder is left.
This pigment, which is known in the market as “pink colour,” may be used with advantage in oil painting in the place of rose madder lake, over which it has the advantage in durability.
Silver Chromate.—A solution of silver nitrate gives, with potassium chromate, a deep red precipitate of silver chromate, which is occasionally used as an artists’ pigment under the name of “purple red”. Not only is the price of this pigment high, but it has little durability, since, like all silver compounds, it is acted upon by sulphuretted hydrogen and turned dark, owing to the formation of black silver sulphide.
CHAPTER XXI.
PURPLE OF CASSIUS.
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.