The severity of surface flaws in a sodium aluminosilicate glass was assessed by subjecting flat glass plates to successively higher thermal downshock and determining the temperature range that causes failure. The temperature data are then translated to short-lived tensile stresses, which, in turn, provide an estimate of flaw depth using Griffith's fracture criterion. The data show that the failure occurs from the edge zone indicating a more severe flaw population associated with edge finishing than that with manufactured flat surfaces. Etching of the edge region reduces flaw severity and increases the failure temperature nearly twofold. The observed fragmentation density appears to vary with stored elastic energy. The estimated thermal stresses, flaw depth, and elastic energy during thermal downshock are discussed.