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 that way it started firing. The mug on the bottom has been refired at 1950F in oxidation and the color is back. The tin is likely a catalyst for the crystallization that occurred in the original result. Could be a fragile mechanism. Anyway, a period of oxidation at the end of the reduction firing should solve this.
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
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.