Why is barium carbonate added to Yixing clay for purple clay teapots?

Why is barium carbonate added to Yixing clay for purple clay teapots?
Barium carbonate serves a specific technical role in Yixing clay processing: it neutralizes soluble and slightly soluble sulfates in the raw clay. When clay is acid-washed—especially white clay substituted for more expensive bentonite-based green clay—residual sulfuric acid reacts with cobalt oxide to form soluble sulfates. These salts migrate to the surface during firing, causing 'wind glaze' or efflorescence defects when the pot meets air after kiln discharge. Adding barium carbonate precipitates sulfate ions as insoluble barium sulfate, preventing this issue. After firing at over 1000°C, the barium is locked in an inert, non-leachable state, so properly processed pots pose no health risk when used for tea. Reputable clay suppliers keep additive levels within safe limits, and buyers concerned about safety can request leach testing on finished ware.