If you've ever watched a glass blower at work, you've seen a material behaving in a very special way. As it cools, the viscosity of molten glass increases steadily but gradually, allowing it to be shaped without a mold. Physicists call this behavior a strong glass transition, and silica glass is the textbook example. Most polymer glasses behave very differently, and are known as fragile glass formers. Their viscosity rises much more steeply as temperature drops, and therefore they cannot be processed without a mold or very precise temperature control.
