The fracture toughnesses of fused SiOt and float glass were measured at high temperatures. In both glasses, low-temperature regions of elastic fracture were identified and correlated with the elastic moduli and their temperature dependence. A viscous flow contribution to the fracture toughness was identified in the fused Si02 at T>800°C. Similar indications of viscous flow were also noted in the float glass, although at much lower temperatures.

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The elastic moduli and fracture toughnesses of a series of PbO‐ZnO‐B2O3 glasses were measured for different PbO/ZnO ratios and for B2O3 contents from 30 to 70 mol%. Substituting ZnO for PbO increased both the elastic modulus and fracture toughness at all B2O3 levels with the fracture toughness being related to the elastic modulus. Structural effects on these properties are discussed.