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 symmetrical, 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.