Annealing of glass is a process of slowly cooling hot glass objects after they have been formed so as to relieve residual internal stresses introduced during manufacture.
The glass is heated until the temperature reaches a stress-relief point, which is known as annealing temperature or annealing point .This is a temperature at which the glass is still too hard to deform, but soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout. Soaking is a process of subjecting glass to a steady temperature. The higher the temperature the glass is soaked at, the shorter the period the glass needs to be exposed to such a temperature. Glass exposed to very high temperatures requires longer time to cool down.
One must not expose the glass to a temperature that can adversely affect its structure. On the contrary, when glass is annealed at lower temperatures, it takes longer soaking time but requires commensurately less cooling time. The type of soak a glass should be subjected to depends on the type of glass.
Annealed glass is generally used in general windows and is also popularly known as float glass. Glass that is not properly annealed retains thermal stress which in turn decreases the strength of the glass and hence it is more likely to crack.