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.