Used in small amounts as a flux in glaze, it often helps promote craze resistance. In larger percentages (3%+) may act as an opacifier and promotes matting. Zinc used at 18 - 28% produces crystals when cooled slowly. Zinc has a strong effect on many colorants-primarily in oxidation. Because of its high shrinkage ratio, calcining is often helpful when used in glazes formulated for bisqueware. Uncalcined. 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 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).