Chrome Oxide (Dry)
Is the most common source of calcium in glazes. It is a high temperature flux that gives durability and hardness to glazes.
Calcium Carbonate alone is quite refractory but when mixed with feldspar or silica will start fluxing around 1100°C. It is often used in low fire clay bodies as an addition to help with glaze fit. It is rarely used as a flux in stoneware or porcelains.