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CaO (Calcium Oxide, Calcia)
Notes-Together with SrO, BaO, and MgO it is considered one the Alkaline Earth group of oxides. It has a cubic crystal structure.-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 must be used with care in high-fire bodies because its active fluxing action can produce a body that 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 resist melting even at high pottery temperatures, but when soda and potash are added, calcia becomes very active in both oxidation and reduction. At higher temperatures, CaO contributed by wollastonite is more readily fusible than that contributed by whiting (calcium carbonate). This synergy between CaO and other fluxes and differences in the mechanism of its fluxing action generates some disagreement among experts regarding the nature of CaO since it does not appear to the a 'stand-alone' flux compared to others. -Hardness, stability, and expansion properties of silicates of soda and potash are almost always improved with the addition of CaO. -CaO 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%). -High CaO glazes tend to devitrify (crystallize). This occurs either because of the high melt fluidity imparted by CaO at higher temperatures or because of the readiness with which CaO forms crystals. Fastfire glazes can contain more CaO because the quick cooling does not give the crystallization a chance to occur. -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. -- -- ------- 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. Mechanisms
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Pictures At cone 10 CaO is a powerful flux. 10% calcium carbonate added to the under-melted bubble-laden celadon on the left transforms it into an fluid super-gloss with no bubbles.  |
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