I had this happen on a glaze firing,,,, the kiln sitter cone broke. I called Orton and they said to check each cone by holding with fingers and pull... not twist. The cone should not break. I had about 15 in a 50 piece box break. Not sure why... defect in cones or rough handling in shipping??? But with the test pull I feel more confident it won't happen again. I had some interesting glaze melts which didn't quite correct in the refiring and never to be replicated.