A glaze colorant typically giving browns, blacks and, at high concentrations, metallics. It decomposes to MnO at 200º C, picks up O2 to become the dioxide and above 1080º C changes back to MnO, an active flux; glazes that are molten at 1080º C may show defects due to liberated O2 gas.
For more information on manganese carbonate, see the Digitalfire.com Reference Library of ceramic materials.