The manufacture of earth colours

by Josef Bersch

The advantage, however, of the direct employment of a solution of ferric oxide is that it gives at once a colour that can be dried straight away; whilst at the same time the colour undergoes no change in drying, whereas it does when ferrous oxide solution is used.

The method of producing ochres from this ferric solution varies according as the product is to be used without any further treatment than drying, or is to be modified by firing.

In the former event, caustic lime is again used as the precipitant, but in only just sufficient quantity to throw down all the ferric oxide in the solution. This amount can be calculated exactly, 36·84 parts by weight of pure burnt lime being required for every 100 parts of pure ferrous sulphate taken. The actual quantity, whether larger or smaller, will depend on the relative purity of the sulphate and lime; and this can readily be ascertained by a simple trial.

The lime is used in the form of milk of lime, as already described. If lime alone is employed, the precipitate will consist of pure ferric hydroxide and the calcium sulphate thrown down at the same time. The resulting colour, when dried, will be an intensely brown mass, which can be used in place of the very dark natural ochres.

In order to obviate entirely the disadvantages resulting from the presence of a large amount of caustic lime in the precipitate, fine levigated chalk or white clay is added in the preparation of the lighter shades of ochre, the addition being made as soon as the two ingredients have been brought into contact; and the mixture is thoroughly stirred, to ensure uniform admixture with the ferric hydroxide. The colour of the settled deposit will be lighter or darker in proportion to the amount of chalk or clay employed; and in this way the whole range of shades from pale yellow to bright brown can be obtained without the application of heat.

Ochre that has been made with chalk is unsuitable for toning by heat, because this treatment would causticise the lime, and the ochre could not be mixed with other colours, since these would be affected by that substance. On the other hand, when white clay is used in preparing the ochre, the latter can be more easily toned by firing, provided care be exercised in the process. The ochre must be dried completely in the air, and either spread out in thin layers on iron plates, for the burning process, or else put into a drum, of the kind already described, in which the mass is moved onward by a worm.

The clay remains unaltered in firing, but the gypsum parts with its water of crystallisation. In order to restore the latter, the ochre issuing from the drum is discharged direct into a vessel of water, in which it can be kept in constant motion by a stirrer. The water is soon warmed by the heat of the mass, and absorption by the gypsum proceeds at a rapid rate. When the whole charge has been fired and collected in the vessel of water, the stirrer is stopped and the precipitate dried, being then ready for use.

In certain circumstances, ochre can be made by other methods. In large towns, ammonium salts are sometimes obtainable at a moderate price, being manufactured in large quantities as a by-product in gasworks. For our purpose, crude gas liquor might be used, since it contains ammonia for the precipitation of the ferric hydroxide. In most cases, however, this gas liquor contains only very small quantities of ammonia, and, therefore, in a works of any size, very large vessels would be needed for the production of a comparatively small quantity of ochre. On this account, preference is given to crude carbonate of ammonia, which is also obtainable at low prices.

On bringing a solution of this salt into contact with one of ferric oxide, ferric hydroxide is precipitated, and the sulphate of ammonia resulting from the reaction remains in solution. By stirring white clay into the liquid at the same time, the ochre can be correspondingly lightened in shade.

The precipitates obtained in this way can be dried at once, and converted into any shade obtainable with natural ochre, from brown to red, by strong firing. The sulphate of ammonia still remaining in the air-dried product is completely volatilised by the heat, and the resulting ochres are even superior to the natural varieties in beauty and permanence.