A guy asked me to fire some pots for him and, unknown to either of us, the pots were not made of Stoneware or Porcelain clay. They were Earthenware clay. The melted stuff is all that is left of his pots. This was a very costly mistake!
There are different kinds of clay. The have different properties, like color, temperature range, plasticity, and absorbency. We use Groleg porcelain. It is white, strong, high fire and non-absorbent when fired.
Don’t ever bring on clay you got somewhere else. We fire only Fireborn porcelain, so we know it is safe in our kiln.
I had a similar issue once when someone brought in a little pot he made in Africa. It was small, but made a big, expensive mess. He didn’t think to tell me it was not Fireborn’s clay.