Volume Relations in Na 2 O·B 2 O 3 and Na 2 O·SiO 2 B 2 O 3 , Melts at 1300·C

Density (and some viscosity) data are presented for binary sodium borate melts containing as much as 60 mole % Na 2 O and for ternary sodium silicoborate melts with B/Si <2.0 between 1000°C and 1300°C. The high‐temperature partial molar volume analysis of the binary sodium borate melts reveals about 50% BO 4 tetrahedra at the 40 mole % Na 2 O composition, in agreement with recent NMR estimates for the binary glasses. No “boron anomaly” was found near 18 mole % Na 2 O at high temperature. The synthetic partial molar volume model that agrees best with experiment for all ternary melts studied involves the presence of some BO 4 tetrahedra, the percentage of which varies with composition. This ternary model involves a high degree of internal consistency. No tendency toward extensive micro‐immiscibility was observed for ternary melts near the SiO 2 ·B 2 O 3 binary.