Many varieties of pyrolusite contain considerable quantities of manganic oxide which are of little use in the manufacture of chlorine, for which pyrolusite is chiefly employed. If such pyrolusite be digested with crude hydrochloric acid for several days, the manganic oxide is dissolved with the production of manganic chloride.
The oxidation of the white precipitate is performed in the usual way; after running off the liquid an excess of manganic chloride solution is added, as soon as two tests taken at an interval of a few minutes show no further increase in the depth of the colour, the reaction is ended and the solution of manganous chloride is drawn off.
Any powerful oxidising agent may be used to oxidise the white precipitate; chromic acid has been proposed for this purpose, but up to the present the chromates are too dear to be used in place of the cheap oxidising agents previously mentioned.
In regard to the present endeavours to utilise by-products of the chemical industries, a method for preparing this pigment may be mentioned which would certainly be very profitable on the large scale. Coal gas before purification contains some quantity of ammonium cyanide, which is decomposed by ferric oxide with the production of Prussian blue. This pigment might be made by passing coal gas over very finely divided ferric oxide, until it was completely converted into Prussian blue. The process is, however, not practicable on the large scale: the ferric oxide would have to be exposed in an extremely thin layer to be completely converted into Prussian blue, and the transformation would require a long time. A better process is to make yellow prussiate instead of Prussian blue. A mixture of ferric oxide with coarse sawdust is spread out in thin layers, over which the impure coal gas passes. When the mass has acquired a blue colour owing to the Prussian blue produced, it is treated with a mixture of quicklime and potassium sulphate. Potassium ferrocyanide is formed; it is obtained in crystals by the evaporation of the solution and can be used in the preparation of Prussian blue.
The “Laming’s mass,” now generally used in gas works, contains, after use, a considerable quantity of cyanogen compounds, and it would only require a short trial to decide whether it was more advantageous to make this mixture by exposure to air again suitable for the purification of coal gas, or to use it to obtain yellow prussiate.
Turnbull’s Blue.—When a solution of a ferrous salt is precipitated by potassium ferrocyanide, a fine blue precipitate is formed, identical in physical properties with Chinese blue, but differing in chemical constitution. It is ferrous ferrocyanide, Fe₃[Fe(CN)₆]₂. The manufacture of Turnbull’s blue is expensive; the pigment has no specially advantageous properties and is therefore seldom made.
In preparing this pigment, a solution of yellow prussiate is transformed into red prussiate by passing chlorine through it so long as it is absorbed by the liquid; the mother liquors from which red prussiate has been crystallised may be used with advantage. When to the solution of red prussiate green vitriol solution is added so that the former remains in excess, a soluble Turnbull’s blue is obtained, but when excess of green vitriol is used the insoluble form is produced.
Chinese blue and Turnbull’s blue are used both as oil and water colours, and these handsome but not particularly durable pigments are also employed in colouring wall papers.
Antwerp Blue is a mixture of Prussian blue with zinc cyanogen compounds. It is obtained by precipitating a solution of 2 parts of zinc sulphate and 1 to 2 parts of green vitriol (according as pale or deep blue is required) by a dilute solution of yellow prussiate.