By examining containers broken in thermal endurance tests, it is possible to arrive at the cause of the failure. The examination calls for some knowledge of the stress system set up during the test; use of markings on the fracture surfaces; and recognition of certain types of fracture origin. As part of the technique the homogeneity of the glass must also be taken into account. The analysis of the breakage is reduced to a table of procedure similar to those used in qualitative chemical analysis. -- AATA