How does calcined zinc oxide alter the crystal structure of zinc oxide, thereby affecting its fluidity in glazes?
Publish Time: 2025-08-20
In the modern ceramic industry, zinc oxide, as an important multifunctional flux and opacifier, is widely used in glaze formulations for architectural ceramics, artistic ceramics, household ceramics, and specialty ceramics. However, while zinc oxide produced by traditional direct or indirect methods boasts high chemical purity, in practice, it often suffers from loose particles, large surface area, and strong hygroscopicity, leading to poor glaze fluidity and large drying shrinkage, which in turn causes defects such as pinholes and cracking in the glaze surface. This is where calcined zinc oxide comes in. Through high-temperature heat treatment, the physical form and crystal structure of zinc oxide are significantly improved, significantly enhancing its processability and firing stability in ceramic glazes.
1. Restructuring the Crystal Structure of Zinc Oxide During Calcination
Calcinated zinc oxide is produced by heat-treating ordinary industrial-grade zinc oxide at temperatures between 900°C and 1200°C for a prolonged period of time. During this process, the fine, porous, and agglomerated nanoscale grains within the original zinc oxide undergo significant physical and chemical changes: internal stress is released, the crystal structure becomes more complete and densified, and the grains undergo sintering and fusion, forming larger and more stable hexagonal wurtzite crystals. This high-temperature recrystallization process effectively reduces the surface energy and specific surface area of the zinc oxide particles, while eliminating a large number of micropores and adsorbed water. More importantly, the crystal arrangement becomes more orderly and lattice defects are reduced, resulting in a "heavier" quality of the calcined zinc oxide—indicated by a higher bulk density, higher specific gravity, and enhanced fluidity. This structural optimization directly determines its superior performance in subsequent glaze preparation.
2. How Crystal Densification Improves Glaze Fluidity
Glaze fluidity is primarily reflected in the rheological properties of the glaze slurry and its viscosity behavior in the high-temperature molten state. When using ordinary zinc oxide, due to its fine particles, rough surface, and high water absorption, it easily forms a flocculent structure in the glaze slurry, resulting in increased viscosity and decreased fluidity, which affects the uniformity of the glaze during spraying or dipping. Calcined zinc oxide, due to its dense crystal structure, stable particle aggregation, and smooth surface, is less likely to disperse into ultrafine particles after being incorporated into the glaze slurry. Instead, it maintains excellent suspension and low viscosity. Furthermore, during the high-temperature firing stage, calcined zinc oxide's well-developed crystals allow for a more stable melting process. Unlike ordinary zinc oxide, which experiences sudden and violent reactions due to its loose structure, this can lead to localized overmelting or bubble accumulation. Its slow and even melting behavior helps reduce the overall viscosity of the glaze layer, facilitates gas discharge, and thus improves melt fluidity, reducing surface defects such as pinholes and orange peel.
3. Comprehensive Performance Advantages of Improved Flowability
Because calcined zinc oxide exhibits superior flow behavior in glazes, its benefits extend far beyond process convenience. First, glaze surface smoothness and gloss are significantly improved. Second, due to its uniform melting and consistent shrinkage, stress matching between the glaze layer and the body is improved, effectively suppressing cracking and flaking. Third, impurities in calcined zinc oxide are fully volatilized, reducing the risk of "zinc spot" or "fogging" caused by the reduction of ZnO to metallic zinc in a reducing atmosphere. Especially in the large-scale production of architectural ceramic wall and floor tiles, calcined zinc oxide has become a standard raw material for high-quality glazes. It's suitable not only for transparent and opalescent glazes, but also plays a key role in regulating crystal morphology and promoting glaze surface development in artistic glazes such as crystalline and matte glazes.
4. Color Change and Application Adaptability
It is noteworthy that zinc oxide calcined at high temperatures typically exhibits a pale yellow hue. This is due to the valence state changes of trace impurities (such as iron and manganese) at high temperatures, but this does not affect its chemical purity or functional properties. In glazes requiring extremely high whiteness, the addition level must be carefully controlled. In artistic ceramics, this slightly yellowish hue can even be utilized as a coloring aid.
Calcinated zinc oxide achieves this transformation from a lightweight powder to a heavy-duty functional material by reconstructing the original zinc oxide crystal structure at high temperatures. Its dense crystal morphology not only enhances physical stability and fluidity, but also optimizes melting behavior and glaze quality in ceramic glazes. It can be said that the calcination process is not only a means of physical modification, but also a key technical path to enhance the functional value of zinc oxide.
