Glass annealing

Annealing is the process of slow cooling of the glass , necessary to relieve internal stresses in the glass after its creation. The process can be carried out in a temperature controlled oven. ^[1] Non-annealed glass cracks or breaks with small temperature changes or weak mechanical stresses. Annealing glass is of great importance for its durability. Many thermal stresses caused by tempering will remain in the annealed glass, which will significantly reduce the overall strength of the glass.

The glass heats up until the temperature reaches the stress relief point, at which the annealing temperature (also called the annealing point ) is related to a viscosity of 10 ¹³ P = 10 ¹² Pa · s, and the glass is still too hard to deform, but soft enough to smooth out the stresses. Then comes the stage in which the temperature becomes the same throughout the volume of the glass. The time required for this stage varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a given speed until the temperature is below the voltage point (η = 10 ^14.5 P). Then the glass can be cooled to room temperature at a rate limited only by heat capacity, density, thermal conductivity and coefficient of thermal expansion of the glass. After annealing, the material can be cut to size, drilled or polished.

While at the annealing point (η = 10 ¹³ P), the stresses are smoothed out within a few minutes, and at the voltage point (η = 10 ^14.5 P) this occurs within a few hours. ^[2] The stresses remaining after passing the voltage point are constant.