It is a source of lithia which is a strong high temperature flux. Lithium carbonate improves the brightness of glazes and increases the firing range. Also reduces thermal expansion. MnCO3 A very weak coloring agent. In an alkaline glaze, a blue-purple or plum color can be obtained.
Manganese carbonate is more useful as a flux.
Manganese Dioxide MnO2 A black powder which will produce brown, purple or black tones to clay bodies and glazes, depending on composition. Manganese carbonate may be substituted by using 1.5 times the carbonate. A colorant to give specks to clay bodies and low fire glazes. It tends to over flux after cone 8. White firing porcelain grog suitable for use in any white clay body.