Notes
The term "lime" encompasses several different minerals and manufactured products which are used to introduce CaO into ceramic mixtures.
-The term "Whiting" traditionally refers to a specific variety of calcium carbonate produced by the grinding of chalk from the cliffs of England, Belgium and France. However, this title is typically used in a much more general sense to refer to any ground calcium carbonate material (i.e. those processed from marble and calcite ores).
-Ground limestone and calcined limestone (burned lime) are used in the glass industry. The purest forms found in North America are in Missouri and Kentucky.
-Dolomite (magnesium carbonate) is a mineral which supplies some magnesia in addition to its CaO complement. It is preferred in many situations because it more readily fluxes and the magnesia imparts desirable properties.
-Wollastonite is a calcium silicate which is more expensive than other sources of calcium, but is used bodies, glaze, porcelains, enamels and frits for its many superior properties.
CaO is not found in nature. Minerals containing it are abundant (i.e. calcite, aragonite, limestone, marble) but vary greatly in their purity (impurities usually include magnesia, iron, alumina, silica, sulfur). A material is considered rich and pure if it has less than 5% impurities. Most of the impurities are also useful in the formulation, however iron and sulfur can be troublesome where clarity is important in glass. Lime minerals vary in the degree of crystallization and cohesion of the crystalline mass and the homogeneity of the matrix.
See Calcium Carbonate, Whiting.
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