Zinc oxide is a fluffy white to yellow/white powder with a very fine particle size coupled with high surface area. It can be an active flux in smaller amounts. It generally promotes crystalline effects and matteness/softness in greater amounts.
If too much is used (10%) the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Also at high levels 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). The use of zinc in glazes is limited by its price, its hostility to the development of certain colours and its tendency to make glazes more leachable in acids.
Zinc oxide will produce opacity or whiteness, especially at low temperatures, if the calcium content is low. It does not opacify as well in boron glazes. It works well in combination with tin.