Manganese Compounds

Manganese dioxide, the black oxide, MnO2, is used in the form of a powder obtained by grinding the mineral pyrolusite. As the effectiveness of this compound is made complete only by the use of oil of vitriol, which needs subsequent neutralization with lime, it cannot be recommended as a material for rendering linseed oil intended for painting, or for making picture-varnish, more drying. The difficulty of preparing the manganese hydrates above mentioned constitutes an objection to their employment for this purpose. But the borate, the linoleate, and the oxalate of manganese may be obtained in commerce in a state of sufficient purity for our present purpose, and it is to them that we wish to direct attention. Borate of manganese may, moreover, be so easily prepared, that it is worth while to give here the necessary directions. One pound of pure manganese sulphate is dissolved in six pints of distilled water, the solution being filtered if cloudy. A few drops of the liquid are now to be tested with caustic soda solution - the precipitate formed should be white; if it show a greenish, yellowish, or greyish hue, iron is probably present, and it will be necessary to treat the whole of the solution with caustic soda until a white precipitate falls, and then to filter it again.

In order to produce manganese borate, a boiling saturated solution of pure borax is added to the manganese sulphate solution until no more precipitate falls. The precipitate is collected on a filter and washed with hot distilled water until the wash-waters show no turbidity when a solution of barium chloride and a few drops of dilute hydrochloric acid are added to the last portion coming through the paper. The borate of manganese is then dried in a warm place, and finally in the water-oven. One grain of it, warmed with linseed oil, is sufficient to render an ounce of the latter highly drying (see Chapter V.). The oxalate or the linoleate of manganese may be used in the same way, and there are also met with certain manganese 'resinates' which may be employed similarly. Cobalt resinate is also a powerful dryer.

¶ The quantities of lead or manganese compounds necessary to confer the drying character on oil are very small. With .2 to 1.0 percent of the weight of the oil the maximum effect is so nearly attained that any further addition is unnecessary. If the manganese in the form of resinate or linoleate be dissolved in ten times its weight of turpentine, a siccative having very little colour but possessed of great power is obtained; this, when added to the oil in the proportion of from 1 to 20, 1 to 50, or even 1 to 100, confers the drying character upon it without any further heating. Moreover, if this oil be allowed, under carefully adjusted conditions, to absorb enough additional oxygen for it to enter on the second period (p. 53), it dries with great rapidity. Oil of this kind, prepared with the smallest possible amount of a manganese salt, is much to be commended for the use of the artist, since it assures to the finished painting a longer life than any other preparation of this kind.