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.