Production of birefringent glass

Claim 1

THE PROCESS OF PRODUCING AN ANNEALED GLASS SHEET HAVING PERMANENT BIREFRINGENT PROPERTIES WHICH COMPRISES HEATING A BOROSILICATE GLASS SHEET CONTAINING BY WEIGHT FROM 50-75% SIO2, 5-25% B2O3, AND LESS THAN 5% AL2O3, TO A FIRST TEMPERATURE AT WHICH THE VISCOSITY OF SAID GLASS RANGES BETWEEN 1010 AND 1013 POISES. APPLYING AN EXTERNAL MECHANICAL FORCE TO SAID GLASS SHEET AT SAID FIRST TEMPERATURE, REDUCING THE TEMPERATURE OF SAID GLASS SHEET TO A SECOND TEMPERATURE AT WHICH THE VISCOSITY EXCEEDS Z1013 POISES WHILE CONTINUING THE APPLICATION OF SAID EXTERNAL FORCE, AND THEREAFTER COOLING SAID GLASS GRADUALLY TO ROOM TEMPERATURE IN SUCH MANNER AS TO INHIBIT THE RETENTION THEREIN OF PERMANENT INTERNAL STRESSES RESULTING FROM TEMPERATURE GRADIENTS CREATED IN THE GLASS DURING THE COOLING STEPS, WHEREBY AN ANNEALED GLASS SHEET IS OBTAINED EXHIBITING PERMANENT BIREFRINGENT PROPERTIES, SAID GLASS BEING COOLED IN SUCH MANNER SA TO INHIBIT THE REACTION THEREIN OF PERMANENT INTERNAL STRESSES RESULTING FROM TEMPERATURE GRADIENTS CREATED IN THE GLASS DURING THE COOLING STEPS.