The raw material for artificial ochre is always a ferrous salt, which can be purchased in large quantities and at very low prices, namely green vitriol, which, in the pure state, consists of ferrous sulphate, FeSO{4} + 7H{2}O. This substance forms sea-green crystals, which are readily soluble in water and impart an objectionable inky flavour thereto. On exposure to the air, green vitriol turns an ugly brown colour, and is no longer completely soluble in water, passing gradually into the condition of basic ferrous sulphate. This is because ferrous oxide is a highly unstable substance, which attracts oxidation and changes into ferric oxide. This latter, however, requires for the production of soluble salts a larger quantity of acids than does ferrous oxide, and therefore the oxidation of ferrous sulphate in the air leads only to the formation of salts that are imperfectly saturated with acid, namely basic salts. When a solution of green vitriol is left exposed to the air, basic ferric sulphate is also formed, which settles down to the bottom of the vessel as a rusty powder. If, however, a corresponding quantity of sulphuric acid be added to the solution at the outset, the resulting ferric sulphate remains in solution. On treating the green vitriol solution with one of caustic potash, caustic soda or quick lime, the ferrous oxide is thrown down as the corresponding hydroxide, forming a voluminous greyish-green precipitate. This hydroxide still possesses a great affinity for oxygen, and when the precipitate is brought into contact with air, its colour rapidly changes to a rusty red, through the transformation of the ferrous hydroxide into ferric oxide. The ferrous hydroxide can also be precipitated by alkali carbonates, the deposits behaving in exactly the same manner as that thrown down by the caustic alkalis. Various methods can be adopted in the preparation of artificial ochre, the selection depending on the properties desired in the finished product. To obtain an ochre with particularly good covering power, the method must be different from that employed to furnish a cheap product, in which low price is more important than covering power. In the former case, the ferrous hydroxide is mixed with substances which, in themselves, possess fairly high covering power, such as chalk or white clay; in the second, gypsum, which is of low covering power, is used. The preparation of the cheapest kinds of artificial ochre will be described first, followed by that of the higher grades which belong to the most valued artists’ colours. For cheap artificial ochres, the ferrous hydroxide is thrown down by caustic lime from a solution of green vitriol. According as a lighter or darker shade is required, two to three parts of ferrous sulphate are dissolved in water, care being taken to select crystals of a pure green colour, since those that have a rusty look are only imperfectly soluble, because they contain basic ferric sulphate.