Barium manganate is an oxidizing agent. It is used for this property in some organic syntheses. It is an extremely stable compound, and can be stored dry for months.
Barium manganate is a very dark blue, insoluble compound. Because of this, it used to be used, usually mixed with barium sulfate to lighten the color, as a blue pigment, known as manganese blue. Now it is not used anymore, in favor of manganese blue hue pigments, which approximate the color of the classic pigment using phthalocyanine based pigments.
Barium manganate is only available in pure form from chemical suppliers, and only at a steep price. It may be found in the form of manganese blue pigment, as for the most part it is no longer used as a pigment. Do not be misled by pigments called "manganese blue hue," these are actually pthalocyanine pigments.
Barium manganate can be prepared by mixing solutions of potassium manganate and a soluble barium salt such as barium chloride. It is not quite as simple as it sounds though, as there are often side products that lead to unexpected results if the potassium manganate solution is not pure (and it never will be, since it must always be kept in a basic solution).
- Make other metal manganates
Although both barium and manganese are toxic, since this compound is insoluble, it is not as much of a safety concern. It might potentially be contaminated with soluble barium salts though, so still handle it with care. Manganates are oxidizers. Keep them away from combustible materials.
Personal experience handling and investigating manganates -zts16