This iron oxide stain on porcelain disappeared during firing?

This porcelain is 43% kaolin, 20% silica, 36% nepheline syenite and 1% Veegum. I cast it in a test mold that had been used for a very dark red burning body. The inner unstained section looks no different than the outside after firing to cone 6. This porcelain develops a vitreous surface, it is apparently able to dissolve and absorb the thin film of iron (unlike a stoneware). Kaolins in even very white burning porcelains always contain a small percentage of iron oxide as a contaminant, but as long as it is below about ~0.5% the fired color is not visibly darkened.

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This is how much iron is in 50lb of the cleanest plastic porcelain you can make!

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The recipe: 50% New Zealand kaolin, 21% G200 Feldspar, 25% silica and 3% VeeGum (for cone 10R). These are the cleanest materials available. Yet it contains 0.15% iron (mainly from the 0.25% in the New Zealand kaolin, the VeeGum chemistry is not known, I am assuming it contributes zero iron). A 50 lb a box of pugged would contain about 18,000 grams of dry clay (assuming 20% water). 0.15% of 18,000 is the 27 grams of iron you see here! Even more surprising: This mug is a typical Grolleg-based porcelain using 5% of a standard iron-bearing raw bentonite. A box of it contains four times as much iron. Enough to fill that cup half full!

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