During firing, a glaze applied on a ceramic body undergoes several transformations, including water loss from the clay components, glass transition and softening. During the softening phase, the glaze starts to melt, giving rise to a continuous liquid layer. As it cools, the glaze viscosity increases until the glaze becomes rigid and starts to contract simultaneously with the ceramic body (though not necessarily in the same way). The temperature required to achieve this locking phenomenon between the two materials during cooling is called the coupling temperature. Corresponding to the coupling temperature, the glaze softens during heating (absorbing tension) and solidifies during cooling (building up tension).
In glazed or double-layer tile, deformations may be generated from the different behavior of the two overlapped layers during both the heating and cooling phases. The coupling of materials with different thermal behaviors inevitably gives rise to a system of stresses due to the thermal incompatibility between the layers.