Lime, which is calcium oxide (CaO), has a specific gravity of 3.4, a melting point of 2572°C, boils at 2850°C, and is soluble. It is introduced into ceramic mixtures in several different forms. In pottery bodies and glazes it is bought as whiting (calcium carbonate) or dolomite (calcium carbonate and magnesium carbonate). In glass batches it is introduced by limestone, burned lime (calcined limestone), and dolomite. In the enameling industry it is used in the form of whiting.
Lime as CaO is not found in nature. Calcium carbonate, which is the chief source of lime, is found in the form of the minerals calcite and aragonite.
The chemical requirements of lime used in glass vary with the type of ware produced. The combined CaO and MgO should be at least 89% for bottle glass, 91% for sheet glass, 93% for blown glass, 96% for rolled glass, and 99% for optical glass. The iron oxide should be practically zero for optical glass, whereas in bottle glass as much as 0.5% is permissible, with nearly the same limits for blown or sheet glass. The silica or alumina may run as high as 15% for bottle glass, but should be vented much less for other grades.
Limestone
A number of terms are in general use for the different varieties of limestone based on differences of origin, texture, composition, etc. Marble is a limestone that is more or less distinctly crystalline. Chalk is a fine-grained ara-gonite limestone composed of finely divided calcium carbonate usually from marine shell sources.
It may be said, regardless of the impurities that are found in limestone, that lime is in all cases practically the only base found in a pure theoretical limestone. In glass, lime is one of the most important of the common batch ingredients.
Lime gives to glass, when added in proper quantities, stability or permanency, hardness, viscosity, and tenacity, and facilitates melting and refining. Lime decreases the viscosity at high temperatures but increases the rate of setting in working range.
Magnesium lime or dolomitic lime is largely used because of its low iron content. Dolomitic lime seems to have a more powerful fluxing action and a glass using dolomitic lime is said to fine or plain up quicker than one using lime from another source.
Calcium carbonate is not used to the same extent in enamels as it is in glasses and glazes. This is probably because the average burning range of enamels is lower than that of either glasses or glazes, and calcium carbonate exerts strong fluxing action only at high temperatures.
