CaO

Notes

-Together with SrO, BaO, and MgO it is considered one the Alkaline Earth group of oxides.

-Quicklime is pure calcia, but it reacts with water to produce calcium hydroxide or slaked lime. Calcium oxide, on the other hand, is an extremely stable compound.

-Calcium oxide is the principle flux in medium and high temperature glazes, beginning its action around 1100C. It lacks usefulness in high-fire bodies because its active fluxing action produces a body which is too volatile (melting if slightly overfired).

-Calcia usually hardens a glaze and makes it more scratch and acid resistant. This is especially so in alkaline and lead glazes. Its expansion is intermediate.

-Calcia and silica alone will not melt even at high pottery temperatures, but when soda and potash are added, calcia becomes very active in both oxidation and reduction. Hardness, stability, and expansion properties of silicates (of soda and potash) are almost always improved with the addition of CaO.

-It is not effective below cone 4 as a flux in glazes but in small amounts (less than 10%) it can dissolve in earthenware glaze melts especially with lead, soda, potash) to add hardness and resistance to leaching. In non-lead mixes it can also help reduce crazing. In larger amounts, it encourages the growth of crystals which can give decorative effects to glossy glazes and produce matteness (i.e. 30%).

-It reduces viscosity in glazes which have high silica, but if the melt is too fluid, devitrification may take place.

-Calcia is a moderate flux in the cone 5-6 range, but a very active one at cone 10.

-High calcia glazes tend to have good (although sometimes unexpected) color responses. For example, in oxidation iron glazes calcia likes to form yellow crystalline compounds with the Fe2O3 producing a 'lime matte'. Without the calcia, glossy brown glazes are the norm.

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CaO is not found pure in nature but rather is contained in various abundant minerals (i.e. calcite, aragonite, limestone, marble) but vary greatly in their purity (impurities usually include magnesia, iron, alumina, silica, sulfur). Of these iron and sulfur are most troublesome (i.e. 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.

The term "lime" encompasses several different minerals and manufactured products.

-The term "Whiting" traditionally refers to calcium carbonate produced by the grinding of chalk from the cliffs of England, Belgium and France. However this title also refers 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.

-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.

See Calcium Carbonate, Whiting.

Properties
Surface Modifier	High molar amounts of calcia combined with adequate silica and preferably lower alumina will form a calcium silicate crystal matte (lime matte). The presence of zinc will increase the size of crystals.