It is proposed that the hardness of aluminosilicate glasses correlates with the melting temperature as predicted by the phase diagram. To evaluate this hypothesis, a broad range of CaO–Al2O3–SiO2 compositions was evaluated, verifying the glass formation region, and allowing for a systematic comparison of Vickers indentation hardness with melting temperature. Candidate glass forming compositions were restricted to the normal glass formation region (SiO2 > CaO) and ranging from SiO2:CaO ratios of 1:1 to 9:1 and Al2O3:CaO ratios ranging from .2:1.0 to 1.3:1.0. Glass forming compositions were X‐ray amorphous. The correlation of hardness was nonlinear but showed reasonable correlation with the melting behavior, with the lowest hardness glass obtained for the eutectic composition. The maximum hardness was obtained in the narrow glass chemistry range of 1.0:1.0:1.5 to 1.0:1.2:1.5 (CaO:Al2O3:SiO2) in the vicinity of anorthite (CaO·Al2O3·2SiO2).

Vickers hardnesses of RO–Al2O3–SiO2 glasses were measured over a broad range of compositions, ranging from ternary endpoints to mixed ratios of RO (CaO with MgO, SrO, or BaO), with the systematic variation of Al2O3 and SiO2 levels. The hardnesses of CAS and MAS glasses are similar, ranging from 6.7 to 7.2 GPa, with the replacement of CaO with MgO producing a marginal increase in hardness. The substitution of SrO or BaO for CaO generally produced in a decrease in hardness down to 4.5 GPa with BaO. The sensitivity to alumina and silica levels, however, was much greater ranging from a minimum of 4.5 GPa to a maximum of ∼8.2 GPa. The correlation of the Vickers hardness with melting temperature was observed in the CAS system but generally not in the RO‐blended glasses. Overall, a combined cation field strength of modifier cations determined the hardness above the critical RO blending ratio.