WILLEMITE NUCLEATION AND GROWTH IN CRYSTALLINE GLAZES

Certain glazes for ceramics which have an excess of zinc-oxide and silica are known to produce large (1-2 cm dia.), plumose crystals when fired to high temperatures. Crystals of roughly the same composition occur in nature and are known as willemite. Several experiments were devised to test nucleation points, rates of crystal growth and crystallization textures within these crystalline glazes. A base glaze was used which contained Ferro Frit 3110, zinc-oxide, spodumene, talc, silica, and titanium dioxide. Six percent red copper was added to the base glazes as a colorant, allowing the crystals to be better observed. All of the experiments were fired to a peak temperature of 2210°F, allowing the glaze to completely liquify. Experiments were designed to “seed” the crystal growth, and nucleation points were encouraged by adding an excess of powdered zinc-silicate, or willemite crystal powders. Amounts of crystal growth were examined by varying hold times at peak temperatures, as well as cooling temperatures. Nucleation density and growth rates appear to be controlled by surface tension effects and glaze thickness. Additional experiments are ongoing, and further results will be discussed in greater detail.