If the glass is fired at a temperature where its viscosity is very low, bubbles may not be found as these are easily
allowed to diffuse out of the glass. For example, bubbles were found to be trapped in a borosilicate glass
(70.4wt% SiO2, 2.1wt% Al2O3, 17.5wt% B2O3, 10wt% BaO) in SiCf/SiC composites below 1573K [8]. Therefore,
there exists a range of viscosities where bubbles can be trapped in the glassy network. above 100MPa have been used [20]. Moreover, subsequent testing of the composites was carried out at temperatures that were much lower than the softening
point of the glass, which would not allow the formation of bubbles due to a too high viscosity.