The sodium disilicate-barium disilicate join of the ternary system, Na2O-BaO-SiO2, has been studied by means of the quenching method, and the phase-equilibrium diagram has been determined. The two compounds form a binary system without intermediate compounds and without appreciable solid solution. The eutectic occurs at approximately 32% of barium disilicate by weight and at a temperature of 797° C. The melting point of barium disilicate was determined as 1418° C. The glasses in the field of stability of barium disilicate show a marked tendency toward devitrification.
The refractive indices of the glasses for sodium light were measured by the microscopic immersion method. These varied regularly from 1.505 for sodium disilicate glass to 1.610 for barium disilicate glass. The refractive index-composition curve plotted on a weight percentage basis is definitely convex downward, whereas on a mol percentage basis it is slightly convex upward.
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