Stresses in glasses

Question 1

What happens to a glass bottle formed in a steel water-cooled mold? The interior and exterior?

Answer 1

~ exterior: cools quickly to liquid like LOW density, puts TENSILE stress on interior

~ interior: insulated by exterior, cools slowly and condenses to HIGH density, puts COMPRESSIVE stress on exterior

Question 2

How do we prevent these bottles from spontaneously bursting on store shelves?

Answer 2

~ annealing: reheating bottle to annealing point and holding for short time

~ enough thermal energy is provided for RELAXATION of atomic strains, but viscosity is high enough so part doesn’t slump

Question 3

What are polariscopes used for? How do they work?

Answer 3

~ checking for residual stresses in a glass part

~ unstrained glass = isotropic, appears dark

~ strained glass = birefringent, appear as colors