Why does the same 99.5% zinc oxide perform differently in ceramic glazes?

Why does the same 99.5% zinc oxide perform differently in ceramic glazes?
Ceramic glaze buyers often compare two lots of zinc oxide with identical assay sheets showing 99.5% purity and similar prices, yet kiln results diverge. The reason is that glaze applications demand more than a single content number. Key variables include batch-to-batch consistency, loss on ignition, particle morphology and dispersion behavior, and compatibility with the specific firing curve and recipe. Calcined zinc oxide grades are often specified separately for glaze use because calcination alters material state and reactivity. Experienced purchasers therefore ask about product type, target glaze systems, technical parameters, and request sample trials before committing. Relying solely on a certificate of analysis misses the factors that actually determine production stability.