Due to its amorphous structure, a glass at room temperature is essentially a very viscous liquid. Viscous flow takes place in glass but so slow as to be on a geological time scale.
The viscosity of glass changes with temperature. There are four standard temperature points that are used in glass working to define the viscosity. These points are known as strain, annealing, softening, and working.
The strain point is the maximum temperature at which a glass can be used for structural applications without undergoing creep.
The majority of glass forming operations occur between the softening point and the working point. At the softening point the glass will yield with a small amount of force while at the working point the glass has the viscosity of honey.
The annealing point is the temperature to which a glass may be heated after working to relieve any internal stresses that arose as a result of the forming process.
As well as an increase in viscosity, glass undergoes a thermal contraction when it is cooled down.
When cooling at high temperatures the glass undergoes a rapid decrease in volume. Glass at this stage has a soft rubbery texture. At some point the glass transforms to a hard brittle texture and the glass contracts at a slower rate.
The temperature at which this transformation occurs is known as the Glass Transition Temperature, Tg.