Comparing decomposition of studied glazes the differences in the intensity of the bands associated with the stretching vibrations of Si–O and the bridging vibration of Si–O–Si and Si–O–Al were especially visible. Additionally,intensity of the (PO4)3 asymmetric stretches of P–O–P bond also increased with a simultaneous increase of the band intensity. It could be concluded that the increasing firing temperature led to the break of (SiO4)4 and (PO4)3- tetrahedra in theseparative-phase glazes, destroying the covalent amorphous network. As a result, the viscosity of molten glazes decreased with the increasing of firing temperature