Hydrothermal treatment of abraded silica glass in water produces a sig
nificant increase in fracture strength and fracture toughness. This wa
s attributed earlier to crack tip blunting. The so-called 'mirror cons
tant' of the fracture surface of hydrothermally treated silica glass w
as investigated. It was found that mirror size changed while the crack
length remained unchanged with hydrothermal treatment. At the same ti
me, the 'mirror constant' increased with hydrothermal treatment. The v
ariation of 'mirror constant' was analogous to that of the apparent fr
acture toughness value due to crack tip blunting.