as well as a total of 0-5.0 wt % of further components,wherein the proportions of the components are chosen in such a way that a weighted crosslinking index, that is, the mean number n of constraints per atom, given by
has a value greater than 2.9,wherein N is a total number of constraints per mole and NA is Avogadro's number and c(SiO2), c(Al2O3), and c(B2O3) refer to the molar concentrations of SiO2, Al2O3, and B2O3, the c(Mi2O) values refer to the molar concentrations of the alkali oxides Mi2O present, and the c(MiO) values refer to the molar proportions of the alkaline earth oxides present, and wherein the ratio
where the K(M-O), M=Si, Al, B are the degrees of covalence of the respective M-O bonds of the components SiO2, Al2O3, B2O3, where:
a degree of covalence of 0.5527 for SiO2,
a degree of covalence of 0.6126 for B2O3, and
a degree of covalence of 0.4329 for Al2O3, and wherein the ed(M-O) values are the respective, cation-related dissociation energies of the components SiO2, Al2O3, B2O3 and are given as: 1864 kJ/mol for SiO2, 1572.5 kJ/mol for B2O3, and 1537 kJ/mol for Al2O3.