=Prussian Blue.=—As we have already stated, Prussian blue is identical in chemical nature with Chinese blue, from which it differs only by certain additions which are made in order to reduce the price. Prussian blue should therefore be regarded as a more or less impure Chinese blue; less pure materials are used in its preparation than for the fine Chinese blue. Instead of pure potassium ferrocyanide, the crude salt, which is sold by the prussiate makers at a lower price, is used for Prussian blue; in addition to the ferrocyanide this crude salt contains a considerable proportion of other salts, including a quantity of potassium carbonate. The ferrous sulphate used for Prussian blue is mixed with alum, which neutralises the potassium carbonate, precipitating alumina, by which the precipitate formed from the yellow prussiate and ferric salt is made paler. If the pale blue precipitate were treated with acids they would first dissolve out the alumina; this should be avoided, since the alum has been added with the intention of increasing the quantity of precipitate and making the pigment paler. The precipitate can therefore only be changed into the blue pigment by spontaneous oxidation; this is accomplished by the oxygen of the air in the manner previously described. The oxidation is more rapid according to the quantity of ferric salt contained in the green vitriol at the commencement; it is convenient to prepare a large quantity of green vitriol solution and expose it in shallow vessels to the air so that the oxidation may proceed. But if the solution of green vitriol were exposed alone to the action of the air a rust-coloured precipitate of basic salt would be deposited at the bottom of the vessels. To prevent the formation of this basic salt, sulphuric acid is added in small quantity, and, as the oxidation proceeds, further small quantities of acid are added. In this way a solution of ferric sulphate is slowly formed, which gives with yellow prussiate a blue precipitate requiring little further oxidation. =Mineral Blue, Brunswick Blue.=—This pigment, the most reduced of the colours of this class, is obtained by mixing white pigments, barytes, pipe clay, etc., with the already impure Prussian blue.