0- 10MgO 0- 15Al2 O3 0- 30______________________________________ wherein the ratio of the molar sum of Al2 O3 + MgO + CaO to the molar sum of ZnO + CuO + TiO2 + Fe2 O3 present in the composition is equal to or less than 0.5, in which metallic oxides selected from the group consisting of As2 O3, Sb2 O3, V2 O5, B2 O3, PbO, CdO, SnO2 and M2 O wherein M is an alkali