Glass Expert Discusses Exploding Tempered Glass

Classic Symetrical Temper Stresses

Three Photomicrographs of

Poor Temper Fracture Patterns

There has been some concern about glass oven wear made from tempered soda lime glass exploding while in use. A Glass failure analysis expert witness performed fractography on a thick glass jar that had been "tempered". Because this is a prototype piece, the temper was not done correctly. When the glass jar surface was abraded prior to thermal shock testing, it exploded violently. Recent tests by this glass expert on properly tempered glass had shown no "explosion". The glass merely cracked in place. Failure analysis of the two types of glass revealed some interesting differences. The non-exploding tempered glass showed symetrical stresses in the glass. The photomicrograph on the top left is representative of the stress pattern for standard tempered glass. In this case, the outer surfaces were under approximately the same compression, and the center of the glass was in tension; this is demonstrated by the "mist hackle" in the center of the fracture surface. Also, there are two sets of symetrical Wallner lines on either side of the center mist hackle. In the case of the exploding glass jar, the temper was found to be non symetrical and not uniform. In some regions the fracture surface had the mist hacklemuch nearer one surface; in addition, one surface does not appear to have been in compression. Other areas on this jar showed weak or very weak surface compressive stresses. The other three photomicrographs demonstrate thiese results. At this point the glass failure expert believes that improperly tempered glass oven ware can "explode". This manufacturing defect may be a product liability and could pose some danger to the user.