Refired to 1950F. The recipe is very flux-heavy (high feldspar) dolomite:spodumene matte, zero silica, 4% tin oxide and less than 1% iron oxide. Sounds like crystallization territory. The plate on the left is the way it normally fired. On the right the way it started firing. The mug on the bottom looked like the plate on the right, but look what happened after refiring at 1950F in oxidation! The tin is likely a catalyst for the crystallization that occurred in the original result. Could be a fragile mechanism. This underscores the need for a period of oxidation at the end of a normal reduction firing.
Glaze is Off-Color
Questions to ask and strategies to try in dealing with glazes that do not fire the expected color or have stains or discolored areas
Refiring pottery and porcelain comes with many issues that can ruin ware, knowing about these will help you do it successfully.
A high temperature reduction glaze made by adding a small amount of iron oxide to a magnesia matte base glaze
A method of firing stoneware where the kiln air intakes and burners are set to restrict or eliminate oxygen in the kiln such that metallic oxides convert to their reduced metallic state.