A white very light-weight powder (and bag of this material is like a bag of feathers). It is practically insoluble in water. This is not the same material as magnesium carbonate (magnesite).

MagCarb is very refractory, it does not decompose and release its MgO to the glaze melt as readily as other sources. It is a good example of the need to consider mineralogy and material-level physics in addition to chemical makeup (when using glaze chemistry to adjust and fix glazes). Two glazes may have the same calculated chemistry, but the one using the MagCarb to source the MgO will not be melted as much.

MagCarb mattes glazes. MgO in sufficient percentages in glaze melts, is a classic matting agent. However, since magnesium carbonate is so refractory and does not readily release MgO, the matting mechanism is simply that it is refractory as a material and inhibits smooth-out of the melt (especially if added in larger percentages or employed at lower temperatures where making a good matte is more difficult).

Magnesium carbonate crawls glazes because it decreases melt mobility. The exceedingly small particle size of the material increases shrinkage. MgO, in the portion that does disperse into the melt, has a high surface tension, that definitely plays in the melt pulling itself into islands. Up to 35% can be found in some recipes.

MagCarb has a very high Loss on Ignition, this could cause glaze surface issues. Dolomite and talc more readily release their MgO to the glaze melt for a higher temperature glazes.