A core glass and a fiber-optic light guide made from it are described. The core glass is in the alkali-zinc-silicate system and contains, in Mol % on an oxide basis: 54.5-65, SiO2; 18.5-30, ZnO; 8-20, Σ alkali oxides; 0.5-3, La2O3; 2-5, ZrO2; 0.02-5, HfO2; 2.02-5, ΣZrO2+HfO2; 0.4-6, BaO; 0-6, SrO; 0-2, MgO; 0-2, CaO; 0.4-6, Σ alkaline earth oxides; 0.5-3, Li2O, but no more Li2O than 25% of the Σ alkali oxides; >58.5, Σ SiO2+ZrO2+HfO2. A molar ratio of Na2O/K2O is from 1/1.1 to 1/0.3. A molar ratio of ZnO to BaO is greater than 3.5.