Partially crystallizable glass compositions

An amorphous partially crystallizable alkaline earth zinc silicate glass consisting essentially of a composition falling within the area defined on a weight points g-l of FIG. 2 of the drawing, in which:

• (1) alpha is SiO.sub.2 in admixture with a glass former or conditional glass former selected from the group consisting of no more than 3% Al.sub.2 O.sub.3, 6% HfO.sub.2, 4% P.sub.2 O.sub.5, 10% TiO.sub.2, 6% ZrO.sub.2 and mixtures thereof, with the proviso that the composition contains at least 0.5% ZrO.sub.2 ;
• (2) beta is an alkaline earth selected from CaO, SrO, MgO, BaO and mixtures thereof, with the proviso that the composition contain no more than 15% MgO and no more than 6% BaO;
• and
• (3) gamma is ZnO, the loci of points g-l being as follows:
• ______________________________________ Point Alpha Beta Gamma ______________________________________ g 48.0 32.0 20.0 h 46.0 34.0 20.0 i 40.0 34.0 26.0 j 40.0 24.0 36.0 k 46.0 18.0 36.0 l 48.0 19.0 33.0 ______________________________________