Annealed glass is glass produced without internal stresses imparted by heat treatment, i.e., rapid cooling, or by toughening or heat strengthening. Glass becomes annealed if it is heated above a transition point then allowed to cool slowly, without being quenched. Glass is treated with heat in order to change its properties by the annealing process. Annealed glass is the most common glass used in windows. Annealed glass is also known as a standard sheet of float glass.

Annealing is actually a process of slowly cooling glass to relieve internal stresses after it is formed. The glass, formerly annealed on shelves in a melting furnace, is now usually carried on rollers through temperature-controlled kiln known as a Lehr (annealing ovens). The shaped glass is annealed to relieve stresses caused by manipulation, then is slowly cooled.

Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass.

Annealing Process

The glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at a viscosity, η, of 1013 Poise = 1012 Pa•s, 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. Of course, glass exposed to very high temperatures requires longer time to cool down.

Caution should be taken to 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.

The time necessary for soaking varies depending on the type of glass and its maximum thickness. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 1014.5 Poise). Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. After the annealing process the material can be cut to size, drilled or polished.

At the annealing point (η = 1013 Poise) stresses relax within several minutes, while at the strain point (η = 1014.5 Poise) stresses relax within several hours.[2] Stresses that are still present below the strain point are permanent.

Float glass is annealed during the process of manufacture. However, most toughened glass is made from float glass that has been specially heat-treated. Annealed glass breaks into large, jagged shards that can cause serious injury, thus considered a hazard in architectural applications.

Care should be taken when choosing locations to install annealed glass. Building codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, indoor panels, fire exits and at low heights in schools or domestic houses.

Annealed glass has the surface strength that provides the wind-load performance and thermal-stress resistance needed in most architectural applications. In areas of high wind loads, or in conditions where higher-than-normal thermal stresses occur, heat-treated glass may be required.

Curved Annealed Glass

Curved annealed glass is used in applications that do not require the use of safety glass. This includes shop windows and display counters. Curved annealed glass can be produced for custom designs and has the advantage of being able to be cut and processed after it has been bent to its desired shape/form.