Calcite as known as Whiting, Limestone or Calcium Carbonate
Whiting has traditionally been a source of CaO in raw glazes and glass (however whitings also typically contain some dolomite as a contaminant). Well known deposits are the chalk cliffs of England, France and Belgium. Marble and calcite ores are abundant in many places.
Inexpensive non ceramic grades of whiting tend to lack the quality and consistency needed for use in glazes (especially for industrial use). Also whiting produces a very large volume of gases while decomposing, it loses more than 40% by weight. While these gases should be gone well before 1100C (and therefore should not disturb the glaze melt), in low or fast fire they can contribute to imperfections and faults in the glaze surface. With the advent of faster firing schedules in recent years whiting has been replaced by wollastonite and frits as a source of CaO in many applications (CaO oxide is advantageous in fast fire because it does not lower the melting point as much as the alkalies). Since LOI is a good indicator of variation in chemistry it may be practical to do an LOI test on shipments by firing a specimen of powder in a thin bisqued bowl to confirm the consistency of shipments.
There are many alternate no-LOI sources of CaO (e.g. wollastonite, frits) and incorporating one of them to source the CaO instead is a classic application of glaze chemistry calculations. However, remember that CaO is not an active melter below about cone 8, so particle size can make a big difference in its willingness to enter the glaze melt. A 325 mesh material could create a glossy glaze whereas a 200 mesh could create a silky matte, exclusively because of the difference in particle size. A 325 mesh material may have a mean particle size of only 10 microns (or even less), whereas a 200 mesh grade might be two or three times that (yet both powders feel the same).
In glazes that contain both calcium carbonate and silica it is preferable to source as much as possible of the SiO2 from wollastonite instead. This is because the SiO2 in wollastonite is taken into solution in the melt much more easily than from highly refractory quartz particles.
In low-fire bodies, calcium carbonate is sometimes added in small amounts as a filler to reduce fired shrinkage and act as a whitener. It is also common to see 5% whiting included in porous earthenware body recipes to prevent moisture expansion (which causes glazes to craze).