An aluminum-free borosilicate glass includes the following glass composition (in weight-% on oxide basis): 70-80 SiO2; 8-14 B2O3; 0 Al2O3; 0-4 Na2O; 3-10 K2O; 0 Li2O; 3-14 sum Na2O+K2O; 0-1 CaO; 0-1 MgO; 0-1 BaO; 0-1 SrO; 0-2 sum CaO+MgO+BaO+SrO; 0-1 ZnO; 3.6-14 ZrO2; 0-10 TiO2; and 0.01-2.0 one or more refining agents, the weight ratio of ZrO2:K2O being in a range of 1.2:1 to 1.4:1.