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What is Annealing?

Updated May 17, 2024
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Annealing is the process by which both metal and glass are treated with heat in order to change their properties. If the process is applied to glass it is held at a high temperature to rid the item of any stresses that it underwent in the manufacturing process. Once the glass has been subjected to a consistently high temperature for the required length of time, it is then slowly cooled, which acts to seal in the piece’s strength and durability. As with the glass annealing process, when metal is annealed it is first introduced to a very high temperature before being cooled to seal in the metal’s ductility and strength.

The process of subjecting glass to a steady temperature is termed "soaking." 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 to cool down. Caution should be taken not expose the glass to a temperature that can adversely affect the glass’s 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. General guidelines suggest that Bullseye, Lausha and Effetre glasses should under go an annealing temperature of approximately 940ºF (504ºC); Satake glass approximately 890ºF (476ºC); and Borosilicate glass approximately 1050ºF (566ºC).

The annealing of metal is a similar process to that of glass. For a full annealing of the metal it must be introduced to an approximate temperature approximately 90ºF (50ºC) above the Austenitic temperature. This temperature is to be maintained for a period that ensures that the entirety of the material involved is transformed into Austenite or Austenite-Cementite. Upon completion of this process the metal goes on to be slowly cooled at a temperature of approximately 36ºF/hr (20ºC/hr) in a furnace to about 90ºF (50ºC) into the Ferrite-Cementite range. Once it achieves this temperature, the metal can then complete the annealing process by cooling at regular room temperature.

The term annealing is also used in another context. In the science of genetics it refers to the process wherein DNA and RNA pair via hydrogen bonds to a sequence that is complimentary with its own in order to form a double-stranded polynucleotide.

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