There is only 0.35% iron oxide in this recipe, as much as ten time less than normal. But this is just enough to provide a darker shadow (where the glaze is thicker) at the edges of the design. This is porcelain fired at cone 10 reduction by Janel Jacobson.
A type of stoneware glaze normally fired in a high temperature reduction atmosphere kiln. It is transparent and stained green or blue by the presence of iron oxide.