[IronJack]

I learned it in a physics class in college.

[imardmd1]

I, too, was taught that in undergraduate classes in ceramic college, but disabused of it when standing closer to the actual research work going on in the field. Under gravity forces alone, window glass compositions do not flow at ambient temperature.

Auto safety glass is placed in severe compression by the manufacturing process such that stress relief through scratching with a sharp tool causes disintegration. But in normal use, these stresses are not dissipated by relaxation through viscous flow.

Likewise, the clear glazes on dinnerware protect the underlying ceramic substrates by remaining in appreciable compression through the life of such articles. Time does not appreciably relieve these stresses built in by differential thermal contraction on cooling from their kiln treatment.

The same is true for glazed ceramic bakeware, like beanpots and casseroles, which see temperatures in use up to, say, 250 deg C.

Given a little challenge, the layman will think this quandary through. Any other questions?