A clay deflocculant used to thin slips and slurries. It is a source of the flux sodium oxide in glazes but it has limited use because of its solubility in water. Instead, soda is typically sourced from nearly insoluble frits or feldspathic minerals which ensures even distribution of the oxide throughout the glaze. Chemically, dense and light soda ash are the same.
For more information on sodium carbonate, see the Digitalfire.com Reference Library of ceramic materials.