The present invention relates to a glass composition which is resistant to alkalis and to acids, in particular for the preparation of reinforcing glass strands, which comprises the following constituents within the limits defined below, as percentages by weight: TABLE-US-00001 SiO.sub.2 ≧58%, preferably ≦65% ZrO.sub.2 15-20% R.sub.2O (R = Na, K or Li) ≧14% K.sub.2O ≦0.1%, preferably ≦0.05%, RO (R = Mg, Ca or Sr) 2.5-6% MgO ≦4% TiO.sub.2 >1 and ≦4% the composition additionally being devoid of F, comprising less than 1% of impurities (Al.sub.2O.sub.3, Fe.sub.2O.sub.3 and Cr.sub.2O.sub.3) and satisfying the following relationships:
ZrO.sub.2+TiO.sub.2≧17%
ZrO.sub.2/TiO.sub.2≧6 It also relates to the use of the glass strands obtained in the reinforcing of inorganic materials, for example cementitious materials, or organic materials, for example plastics, and to the composites including such strands.

In a unit gravity environment, a glass preform is encased in a material to generate an encased glass preform. The material remains solid at the glass preform's crystal melting temperature and is inert with respect to the glass preform. The encased glass preform is placed in a microgravity environment and heated to a temperature above the crystal melting temperature until the glass preform melts and is free of crystals, wherein a crystallite-free glass preform is encased within the material. The crystallite-free glass preform is then cooled in the microgravity environment to generate a solid crystallite-free glass preform encased within the material. While still in the microgravity environment, the material encasing the solid crystallite-free glass preform is removed in the microgravity environment and the solid crystallite-free glass preform is polished. A glass optical fiber is then drawn from the solid crystallite-free glass preform in the microgravity environment.