Temper of Non-Flat Glass Cooking Pot Lid:
Because of economics, Pyrex, a glass with a low coefficient of thermal expansion, is being replaced in cookware by tempered soda lime glass. The idea is that the compressive outer glass surface will protect the glass object from thermal shock. An example of this is see through cooking pot lids. Ordinarily these lids are dome shaped; thus, the level of temper on one lid surface will be different than that of the other. In this example, the outer surface is convex, and the interior surface is concave. In this example the compressive layer on the convex surface is much thicker this implies that this surface is more robust. It is believed that the side to side difference results from a more rapid air quench rate on the convex surface.