Annealing Process
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. |

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