Pyrex Glass Bakeware

Glass expert discusses recent changes in glass bakeware:
Traditionally glass bakeware was made from Pyrex type glass (i.e. borosilicate) glass. Borosilicate glass has a lower coefficient of thermal expansion than soda lime glass, and therefore, stresses generated by thermal gradients are less. This has changed. Now glass bakeware is made from tempered soda lime glass. Thermal tempering increases the strength of the bakeware by generating a compressive layer on the outer surface of the glass. This stress must be overcome before the surface can experience tensile stresses. Thus, the glass piece is stronger. Recently Read Consulting was asked to perform a failure analysis of a supposed tempered glass baking dish that had broken. The packaging advertised that this tempered glass is safer because it will break into small pieces instead of the large pieces that normal Pyrex glass would make.

The broken dish is shown directly above.

The fracture pattern is not that of tempered glass. Therefore an edge stress meter was used to look at the glass edge on the failed part and on an exemplar. The top left photo is the stress meter measurement taken of the fractured baking dish.

The next photo is of asimilar measurement on a tempered edge. The color fringes indicate that the glass edge is tempered.

Obviously,the subject dish was not tempered.

Analysis of Failed Glass Baking Dish

Glass failure analysis expert analyzes failed glass baking dish"Pyrex". Traditionally, glass baking dishes have been made from annealed borasilicate glass (i.e. Pyrex glass). Now baking dishes are made from tempered glass. The glass is still called pyrex but it is really tempered glass (soda lime). As a result of this change, the newer dishes are labeled to be safer because they are made from tempered glass. Given this, a person was injured by a failed baking dish that was sold as tempered glass. A failure analysis was performed on the subject dish.
The failed dish is shown above . This dish failed as a result of a blow to the region indicated by the arrow in the figure. The fracture pattern and the size of the resulting pieces proves that this dish is made from annealed glass; therefore, it is not made as advertised. It is not made from tempered glass.

Manufacturing Engineer and Product Liability

Product Liability, Design Defects and Manufacturing Defects are obvious arenas for an experienced manufacturing engineer or process engineer. He has the insight to best evaluate manufacturing defects in product liability cases. Naturally, he must first perform a complete root cause failure analysis to determine if there was product misuse, a manufacturing defect or a design defect. Naturally it is a benifit if the chosen engineer has experience with similar products, materials and processes in question. This is an asset because he can relate the defect back to a specific process step. In addition, he will be familiar with what is necessary for correct product development. An example is a case where a product failed after a short time in service. He is best qualified to determine if the product or process development procedures were thorough and proper. For example, he can determine if the proper accelerated life tests were performed to determine the product performance over its expected lifetime. Product development and process development usually occur in four stages: concept, bread board, pilot production and full production. An important aspect of this cycle is to determine if there are potential circumstances during normal use that might cause product failure and/or personnel injury. Documented tests must be performed to prove that the hypothetical degradation mechanisms do not cause premature product failure. If this is not done, an unreliable or dangerous product (i.e. poor product safety) may be put on the market. An experienced manufacturing engineer is the best prepared to determine if the product was properly developed to allow for proper product quality. This insight can only be gained from direct hands on manufacturing experience.