Made and fired this pot atleast a month ago, I had mixed in some Franklin NJ fluorescent mineral chips which are mostly calcium carbonate. That doesn't melt or frit at bisque temperatures but apparently this stuff does? So one hot and humid day I go to look at it and about 50 or so chips popped off and in the center were the graphite looking rocks I put in that shouldnt have done what they did. They must have absorbed the moisture from the air and expanded. There doesn't appear to be any structural cracks on the pot so it should still be salvagable. I'm definitely not mixing those large chunks into any more clay though. I am going to let it sit for a while on the shelf to make sure all of the rocks expand. Wouldn't want any explosions when I glaze it.
My teacher was curious about the rocks also so she took a bunch and put them in the last bisque firing in a bowl. I think we are going to explore glazing possibilities with the minerals at some point.