The proportion of materials in which the least loss is involved is here given. Nine kilogrammes of green vitriol are dissolved in 100 litres of water, 15 kilogrammes of strong sulphuric acid are added, and then a solution of 15 kilogrammes of yellow prussiate in 100 litres of water. The solutions are kept in constant motion during the mixing. As soon as the decomposition is finished, without waiting for the precipitate to settle, steam is led in and 20 kilogrammes of nitric acid of 1·3298 specific gravity added in small portions. The heating by direct steam is continued until red vapours are no longer evolved from the liquid, which is a sign that the oxidation is finished.
Although the use of direct steam is very convenient for heating, since wooden vessels can be used and the heating quickly accomplished, it may be dispensed with, and the solutions heated in pans or even not at all. In this case the oxidation lasts considerably longer than when the liquid is heated. In the process recommended by Gentele 109 parts of yellow prussiate are used to 20 parts of green vitriol, each dissolved in much water. The precipitate is heated for a short time with 51 parts of nitric acid of 1·2285 specific gravity and 16 parts of sulphuric acid. The mixture is allowed to stand for several days to complete the oxidation before the precipitate is separated from the liquid.
In Hochstätter’s method, solutions of 6 parts of yellow prussiate in 15 parts of water and of 6 parts of green vitriol in 5 parts of water are mixed. To the precipitate 24 parts of strong hydrochloric acid and 1 part of sulphuric acid are added, and then a solution of bleaching powder in 80 parts of water until the liquid smells distinctly of chlorine. Apart from the fact that this method does not employ equivalent quantities of iron salt and yellow prussiate, it is also to be regarded as unsuitable, since the addition of sulphuric acid produces calcium sulphate, which, being soluble with great difficulty, will be precipitated along with the blue, making the shade lighter. The addition of sulphuric acid should be omitted and only hydrochloric acid used, the calcium chloride formed by the decomposition of the bleaching powder being very soluble in water. Many formulæ given for the manufacture of Chinese blue are said to produce a pigment of special beauty. This, however, is incorrect. Chinese blue is a compound of fixed composition, which is always formed when equivalent quantities of the salts are used and the precipitate converted into the blue compound by one of the oxidising agents given. It is most simple to use nitric acid as the oxidising agent; for this purpose the presence of chlorine in the acid is harmless. When sufficient space and time are allowed, a special oxidising agent may be omitted and the oxidation of the white precipitate accomplished by atmospheric air.
In working in this manner the precipitate is washed several times with water, and spread in a thin layer upon boards which are placed in rows one above the other. In a short time the mass turns blue on the surface, but the pale colour remains underneath much longer, since the access of oxygen to the interior is difficult. The boards must be left until the mass has a uniform colour throughout. In order that the paste shall not dry too thoroughly whilst on the boards it is desirable to make it very fluid. If the precipitate has been completely washed before the oxidation the product is finished after drying, but if a portion when completely dried shows a crystalline incrustation, salts are still present, which must be removed by careful washing.
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.
Soluble Prussian Blue.—The modification of ferric ferrocyanide easily soluble in water is obtained by adding a solution of a ferric salt to a solution of yellow prussiate, the latter being in considerable excess. Brücke gives the following method: a solution is prepared of 217 grammes of yellow prussiate in 11 kilogrammes of water. Another solution contains 100 grammes of ferric chloride in 1 litre, 1 litre of which is mixed with 2 litres of a saturated solution of Glauber’s salt and the mixture added to the prussiate solution so long as a blue precipitate forms. The Glauber’s salt plays no part in the formation of the colour; it is added to prevent the solution of the precipitate, which is insoluble in salt solutions. The precipitate is brought on to a filter and washed with water until the washings begin to be blue, which is a sign that the salts have been almost completely removed. The precipitate cannot be further washed without the loss of a large quantity which would be dissolved by the water; without further washing it is slowly dried in the air.
Soluble Prussian blue has but a limited application; anatomists use its solution to inject specimens, and it was formerly used for blue writing inks, for which purpose aniline dyes are now preferred. It may also be remarked that ordinary Prussian blue readily dissolves in a solution of oxalic acid; the solution was also used as a writing ink, but since it attacks steel pens, it has now been replaced by solutions of dyes which have no action on steel.
Special Processes for the Manufacture of Chinese Blue.—In place of chlorine or nitric acid manganic chloride can be used to oxidise the white precipitate obtained from yellow prussiate and green vitriol. This process is based upon the property of manganic chloride to readily give up part of its chlorine and be changed into manganous chloride (the ordinary chloride of manganese). The conversion of the white precipitate into the blue is thus also in this case indirectly effected by chlorine.