Manganese Green, Rosenstiehl’s Green.—This pigment, which is handsome but difficult to produce, consists of barium manganate. It can be obtained by several methods, all of which produce a good green, but with very unequal properties. Manganese green made from barium nitrate has little durability; when made from barium hydrate it is more costly, but very durable.
Barium manganate is most easily obtained by precipitating a boiling solution of potassium manganate by barium chloride. The almost blue precipitate becomes nearly white when washed and dried, but when it is gradually heated on a porcelain plate to a dark red heat it acquires a fine green colour. The heating must be carefully conducted; if the temperature rises too high, the colour changes to a dirty greyish brown, owing to the reduction of the manganic acid.
This pigment is also made by heating 14 parts of manganic oxide, 80 parts of barium nitrate and 6 parts of barytes with access of air until the desired shade appears. The mass is then ground in a continuous current of water until it is changed into a very fine powder and nothing more is dissolved by the water. The best result is obtained by Rosenstiehl’s process: 4 parts of barium hydrate, 2 parts of finely-powdered barium nitrate and 0·5 part of artificial manganese dioxide are rapidly mixed; the mixture is moistened and heated to a dark red heat. The fused mass is boiled out with water and the residue dried under a bell jar, under which are dishes of sulphuric acid and caustic potash, the former of which absorbs the water, whilst the latter keeps the air free from carbonic acid, which would injure the shade of the moist substance.
Manganese green is amongst the pigments more recently discovered. On account of its high price it has found but limited use up to the present. It may be used in any vehicle, and is distinguished by great permanence.
In regard to the preparation of this colour by Rosenstiehl’s method, which of all gives the best results, it should be observed that the shade largely depends on the quantity of barium hydrate used; the greater this is the more the shade inclines to blue. If it is desired to produce a colour of a greener tone than is produced by the fusion, this can be accomplished by boiling for a very long time with very weak hydrochloric acid, which extracts a portion of the base from the compound and produces a deeper shade.
Böttger’s Barium Green.—A beautiful green is obtained by the process given by Böttger, as the author has found. It consists of barium manganate. The process is, however, somewhat costly, so that the pigment would only be available for artists’ purposes.
A solution of potassium manganate is first made by gradually adding 2 parts of very pure pyrolusite, finely powdered, to a fused mixture of 2 parts of caustic potash and 1 part of potassium chlorate, bringing the mass to a low red heat after the introduction of the whole of the pyrolusite, and finally extracting with water, in which the potassium manganate dissolves to a fine emerald green solution. This process is unattended by difficulty if the pyrolusite contains a sufficient quantity of manganese peroxide and is in a sufficiently fine powder, which is most important. Potassium manganate is a very unstable substance. Cold water must be used to dissolve it, and the solution should not long stand exposed to the air, but should at once be used to obtain barium manganate.
When the solution of potassium manganate is mixed with a solution of a barium salt, a handsome violet precipitate immediately forms. This is washed with water and quickly ground with ¾ to 1 part of its weight of barium hydrate. The mixture is heated in a copper dish with constant stirring to a low red heat; the colour changes gradually to a very fine green. When the mass has reached the proper shade it is treated with cold water to remove excess of barium hydrate, until the washings show no trace of alkaline reaction.
Manganous Oxide is occasionally used as a green pigment, especially for painting metal work; it is obtained in the following manner: Manganese sulphate solution is precipitated by soda solution, and the manganous carbonate obtained strongly ignited in a crucible. In order to avoid oxidation of the manganous oxide, which would readily occur in the process, it is necessary to prevent air from reaching the substance. With this object the crucible in which the heating is conducted is covered by another from which the bottom has been removed and which is filled with coal. The air entering the crucible on cooling must pass through the layer of glowing coals, by which it is deprived of oxygen.
Manganese Blue is obtained, according to G. Bong, by igniting a mixture of 3 parts of quartz, 6 parts of soda ash, 5 parts of limestone and 3 parts of manganese oxide, or of 3 parts of quartz, 8 parts of barium nitrate and 3 parts of manganese oxide, air being admitted and reducing gases excluded. All materials must be free from iron. The quantity of manganese oxide regulates the depth of the colour, but not its shade; by increasing the soda a greener, by increasing the quartz a more violet, shade is obtained.