Glass failure analysis expert discusses interesting window failure, that of a piece of antique window glass. The window expert performed a failure analysis that determined the failure initiated at crush damage from the scribing operation, and it started as a bending crack. It then transformed into a “thermal crack”. That is the crack front was driven by thermally generated stresses ( either residual stresses or temperature differential generated stresses). Termal cracks tend to wander, and after this crack initiatiated it wandered and almost created a semi-circle (crack growth is indicated by the arrows in the above photographs). This window failure is interesting because this glass was made in the 1930’s, and it was ground and polished. Present flat glass (i.e. float glass) is solidified on liquid tin and does not require glass machining (i.e.grinding and polishing). Also, modern float glass processing is better controlled than the previous roll forming processes.