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Zinc oxide is an inorganic compound with the formula ZnO. ZnO is a white powder that is insoluble in water, and it is widely used as an additive in numerous materials and products including rubbers, plastics, ceramics, glass, cement, lubricants. Although it occurs naturally as the mineral zincite, most zinc oxide is produced synthetically.

Ceramic industry consumes a significant amount of zinc oxide, in particular in ceramic glaze and frit compositions. The relatively high heat capacity, thermal conductivity and high-temperature stability of ZnO coupled with a comparatively low coefficient of expansion are desirable properties in the production of ceramics. ZnO affects the melting point and optical properties of the glazes, enamels, and ceramic formulations. Zinc oxide as a low expansion, secondary flux improves the elasticity of glazes by reducing the change in viscosity as a function of temperature and helps prevent crazing and shivering. By substituting ZnO for BaO and PbO, the heat capacity is decreased and the thermal conductivity is increased. Zinc in small amounts improves the development of glossy and brilliant surfaces. However, in moderate to high amounts, it produces matte and crystalline surfaces. With regard to color, zinc has a complicated influence.

It can be an active flux in smaller amounts. While boron dominates as the key flux in middle-temperature glazes, for example, zinc is employed in some base glazes to augment the B2O3 or even replace it entirely. No combination of the common raw materials feldspar, kaolin, silica, feldspar, calcium carbonate, dolomite and talc will melt properly at cone 6, however, a 5% addition of zinc can transform the mix into a glossy glaze. 5% more and it will be a very fluid glossy glaze. The zinc can also significantly reduce the thermal expansion of the glaze it is fluxing.

Zinc generally promotes crystalline effects and matteness/softness in greater amounts. If too much is used the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Other surface defects like pitting, pinholing, blistering and crawling can also occur (because its fine particle size contributes to glaze shrinkage during drying and it pulls the glaze together during fusion).

Zinc oxide is thermally stable on its own to high temperatures, however, in glazes, it readily dissolves and acts as a flux. Zinc oxide sublimes at 1800°C but it reduces to Zn metal in reduction firing and then boils at around 900°C (either causing glaze defects or volatilizing into the atmosphere; note that electric kilns with poor ventilation can have local reduction). While it might seem that zinc would not be useful in reduction glazes when zincless and zinc-containing glazes are compared it is often clear that there is an effect (e.g. earlier melting, more crystallization, and variegation). Thus some zinc has either remained or it has acted as a catalyst.