Differences in density of the inner and outer surfaces occur due to moisture loss. In the physical sciences there are only four changes that can be directly measured. Force is that which tends to change the state of motion of the clay. There can be no change without force. In ceramics, forces are physical and chemical.
Nature usually applies force as heat.
Steady State is the process for a system of given mass whereby there is a constant exchange of new material such that the system mass remains constant. As moisture is drawn from within to the clay body's surface, the piece becomes drier and stress will be imposed on the system. Closed system is one in which the mass remains constant and only energy is exchanged. In a sealed container, the mass of a piece of moist clay will remain constant even though there are changes in the distribution of moisture within. Through this, energy can be exchanged between the system and its environment without compromising the integrity of a piece.