"The oxide ions go into the network and they break the oxygen bonds in a process we call scission. And then lastly in certain glasses we add a compound of a type called an intermediate. And these are actually network formers, but they form a different number of bonds than the parent network.

Typically a larger number of bonds. And, in doing so, they create some free volume which enhances certain properties, sometimes the mechanical properties, sometimes the thermal properties. Specifically, thermal shock resistance is enhanced by the addition of network formers..."