Relaxation of Thermal Stress in Phase‐Separated Borosilicate Glasses

Although the “apparent” viscosity of borosilicate glasses is known to increase with the progress of phase separation, temperatures at which the thermal strain is relieved are seen to decrease by the increase of phase separation. This behavior is explained by the difference in the “real” viscosity of each phase in the phase‐separated glass and by the very low thermal expansion of the higher viscosity SiO 2 ‐rich phase. The “annealing point” based on ASTM designations loses its meaning for phase‐separated glasses of this type.