Pyrex Glass Failure Analysis: Glass Baking Dish Failure Analysis

California glass expert witness performed a failure analysis on a pyrex glass baking dish. This dish was made from pyrex borosilicate glass, and the glass failure occured as the dish was being removed from the oven. The failure initiated at an internal glass defect. The defect appears to be a colapsed bubble (seed) in the bottom of the dish. The upper left (Mag 8X) photomicrgraph is an overview of the failure origin. Upper right is a higher power (Mag 40X) magnification of the origin. It appears as if the bubble colapsed during hot forming of the pyrex glass baking dish. Because the surfaces of the bubble did not re-weld, a stress riser was created that caused later failure as a result of thermal stresses created during normal use.

Glass Expert Discusses Termpered Glass Cookware

Modern Pyrex glass cookware is now made from tempered soda lime glass. As a result there is a potential that the tempered glass failure will create dangerous projectiles. In one case it was found that if the temper glass stresses are not symetrical, flying glass pieces will occur. In the present case, tempered glass cookware lids were caused to fail; the resulting failure was completely contained by the stainless rim. The rim performs two functions. It protects the vulnerable edges of the tempered glass lids from glass damage. In addition, it appears that it contains any glass particles that could become projectiles.

Glass Expert Discusses Glass Ovenware

A glass expert in California compares Pyrex glass ware from two eras. The original glass ovenware was developed by Corning Glass Works. They introduced annealed borosilicate glass that had a lower thermal expansion coefficient than the more common soda lime glass. The lower expansion coefficient reduced the level of thermal shock and allowed this glass to be used for ovenware. For a long period Pyrex was synomonous with borosilicate glass. Later the decision was made to make glass ovenware from heat strengthened soda lime glass and to continue to call it pyrex. This is the present approach to this type of cookware. Above, the upper right image is an edge stress measurement performed on an older piece of glass cookware. This measurement shows that this glass is annealed. This is an older piece of traditional pyrex cookware. On the lower right is the same measurement performed on a recently purchased piece of pyrex cookware. The presence and number of the color bands indicates that this ovenware is made from heat strengthened glass.

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.