Pyrax (Pyrophillite) is a mineral having a very low thermal expansion. It stands to reason that if we can maximize its percentage in a body and not fire the body to a point that changes the crystal structure, it will be resistant to thermal-shock-resistant cracking. To that end I mixed it with only kaolin (ball clay would add some quartz that would increase thermal expansion) and made slip-cast pieces. I fired them to cone 2 (after finding that by cone 4 shock-resistant properties begin to decline). As you can see from the video, the addition of grog actually harms the performance! The higher the Pyrax, the better. Will this work for kiln shelves? Yes!
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.