These are fired bars of Barnard Slip going from cone 04 (bottom) to cone 6 (top). It is melting at cone 6. Porosity is under 3% and the fired shrinkage above 15% from cone 1 upward. Drying shrinkage is 4% at 25% water (it is very non-plastic). The darkness of the fired color suggests higher MnO than our published chemistry shows.
During drying, clay particles draw together and shrinkage occurs. During firing the matrix densifies and shrinkage continues. More vitreous bodies shrink more.
The term Slip can have various meanings in traditional ceramics.