Magnesium Carbonate Light
Magnesium carbonate otherwise known as light magnesium carbonate. A white very light-weight powder, practically insoluble in water.
When added to glazes magnesium carbonate often mattifies them, simply because it is refractory (it is useful at lower temperatures where making a good matte can be quite difficult). Magnesium carbonate can also induce crawling because of the decreased mobility of the glaze melt and the exceedingly small particle size of the material increases shrinkage.