Sulfur trioxide (SO3) additions, up to 3.0 mass%, were systematically investigated for effects on the physical properties of sodium borosilicate glass melted in air, with a sulfur‐free composition of 50SiO2–10Al2O3–12B2O3–21Na2O–7CaO (mass%). Solubility measurements, using electron microscopy chemical analysis, determined the maximum loading to be ~1.2 mass% SO3. It was found that measured sulfur (here as sulfate) additions up to 1.18 mass% increased the glass transition temperature by 3%, thermal diffusivity by 11%, heat capacity by 10%, and thermal conductivity by 20%, and decreased the mass density by 1%. Structural analysis, performed with Raman spectroscopy, indicated that the borosilicate network polymerized with sulfur additions up to 3.0 mass%, presumably due to Na2O being required to charge compensate the ionic SO42- additions, thus becoming unavailable to form non‐bridging oxygen in the silicate network. It is postulated that this increased cross‐linking of the borosilicate backbone led to a structure with higher dimensionality and average bond energy. This increased the mean free paths and vibration frequency of the phonons, which resulted in the observed increase in thermal properties.

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