I am wracking my brains to figure out how this could have happened. Obviously, this former bowl (and two others in the same firing) was made out of a lower temperature clay than the Cone 10 stoneware that this soda firing called for -- but how? I know all were made from recycled clay; I know I used a low-fire body in my studio briefly, about a year ago, and hubby has been experimenting with a local earthenware clay. But we were very careful to keep the earthenware separate (at least, we thought we were), and it's very visibly obvious in the green state that it is not stoneware. So...I don't know. Obviously we screwed up. The good news is that none of the pieces which I really needed from the firing were damaged, and we were able to save the shelves. More careful in future!