Stress‐intensity‐factor calculations were used to account for variations in the shapes of fracture mirrors caused by surfaces and by nonuniform stress fields (flexure). The validity of the stress‐intensity criterion for crack branching was confirmed. The stress‐intensity factor at crack branching, KB, for flint glass was determined to be 2.55 MN m−3/2. Also, the stress‐intensity‐factor calculations account for the transition from mist to mir‐rorlike regions as a neutral axis is approached, in cases in which this is observed. The results are compared with earlier results.