Corningware three-layer glass-ceramic

Modern Corningware: three glass layers rolled together with the center having higher coefficient of thermal expansion than the skins, producing compressive surface stress on cooling. Stamp-forged into ramekins and casseroles. Replaces the earlier low-CTE glass-ceramic body.

Corningware originally used a glass ceramic that had very low coefficient of thermal expansion, but that was fairly expensive. Modern Corningware — and Corningware has sold this process to another company; Corning likes to get out of businesses after a while — has three different layers of glass that they bring in and roll together. The two outer layers have a different coefficient of expansion than the center one, and when it cools down, the center contracts more than the two on the top, and you end up with a tensile residual stress in the center and a compressive residual stress on the surface. So modern Corningware — you go buy a little white Corningware dish — is actually made by putting three layers of glass together, the one in the center having a larger coefficient of thermal expansion based on its composition, and you basically come in with a little forging press and stamp out the product, whether a little ramekin or a casserole bowl. It now has scratch resistance — if you scratch it, it's not going to shatter, because it's got compressive residual stresses. You can go through the fracture mechanics, but if the compressive residual stresses exceed the stresses that would be on it, you never get to tensile stresses at your crack tip, and the Griffith criterion for brittle fracture isn't satisfied. If you take it and put it in cold water after it's been hot, it won't shatter, because you've got compressive residual stresses, even though you've got microscopic flaws on the surface.