Zinc Oxide

Zinc Oxide (ZnO) is a white powder flux used in both high-fire and mid-fire glazes. It began to be used widely in ceramics in the late 19th century as potters discovered its unique effects on glaze surface and color.

Zinc has several personalities depending on how it is used. In small amounts (2 to 5 percent), it increases gloss, brightens colors, and widens the firing range. In large amounts (10 to 25 percent) with proper slow cooling, zinc produces dramatic macro-crystalline glazes, where visible crystals grow across the glaze surface during the cooling cycle, creating star-burst or flower-like patterns.

Zinc also affects colorants in distinctive ways. It intensifies cobalt blues, pushes copper toward turquoise, and dulls iron and chrome. In chrome-tin pink glazes, zinc must be avoided entirely, since it reacts with chrome to produce muddy browns instead of pinks.

Zinc should be calcined (pre-fired to 1000 degrees Celsius) before use in some recipes to prevent crawling, a defect where the glaze pulls away from the clay body in bare patches.

Typical usage: 2 to 8 percent in general glazes for gloss and flux. 10 to 25 percent in crystalline glazes. Avoid in chrome-tin pink recipes.