A glass composition consisting essentially of, based on mole percent, 46-56% B.sub.2O.sub.3, 0.5-8.5% P.sub.2O.sub.5, SiO.sub.2 and mixtures thereof, 20-50% CaO, 2-15% Ln.sub.2O.sub.3 where Ln is selected from the group consisting of rare earth elements and mixtures thereof; 0-6% M'.sub.2O where M' is selected from the group consisting of alkali elements; and 0-10% Al.sub.2O.sub.3, with the proviso that the composition is water millable.