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Annealing (glass)

 

 
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Annealing (glass)
 
 
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Annealing is a process of slowly cooling glass
Glass

Glass generally refers to a Hardness, brittle, transparency amorphous solid, such as that used for windows, many Glass Bottles, or eyewear, including, but not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovite , or aluminium oxynitride....
 to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr
Lehr (glassmaking)

A lehr is a temperature-controlled kiln for annealing objects made of glass. The name derives from the German verb lehr meaning to learn and is cognate with the English lere also meaning to learn or acquire knowledge of ....
. 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.

The glass is heated until the temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
 reaches a stress
Stress (physics)

In continuum mechanics, stress is a measure of the average amount of force exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces....
-relief point, that is, the annealing temperature (also called annealing point) at a viscosity
Viscosity

Viscosity is a measure of the Drag of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness"....
, ?, of 1013 Poise
Poise

The poise is the unit of dynamic viscosity in the centimetre gram second system of units. It is named after Jean Louis Marie Poiseuille.The analogous unit in the SI is the pascal second :...
 = 1012 Pa·s, at which the glass is still too hard to deform, but is soft enough for the stresses to relax.






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Annealing is a process of slowly cooling glass
Glass

Glass generally refers to a Hardness, brittle, transparency amorphous solid, such as that used for windows, many Glass Bottles, or eyewear, including, but not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovite , or aluminium oxynitride....
 to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a Lehr
Lehr (glassmaking)

A lehr is a temperature-controlled kiln for annealing objects made of glass. The name derives from the German verb lehr meaning to learn and is cognate with the English lere also meaning to learn or acquire knowledge of ....
. 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.

The glass is heated until the temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
 reaches a stress
Stress (physics)

In continuum mechanics, stress is a measure of the average amount of force exerted per unit area. It is a measure of the intensity of the total internal forces acting within a body across imaginary internal surfaces, as a reaction to external applied forces and body forces....
-relief point, that is, the annealing temperature (also called annealing point) at a viscosity
Viscosity

Viscosity is a measure of the Drag of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness"....
, ?, of 1013 Poise
Poise

The poise is the unit of dynamic viscosity in the centimetre gram second system of units. It is named after Jean Louis Marie Poiseuille.The analogous unit in the SI is the pascal second :...
 = 1012 Pa·s, at which the glass is still too hard to deform, but is soft enough for the stresses to relax. The piece is then allowed to heat-soak until its temperature is even throughout. The time necessary for this step 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 (? = 1014.5 Poise). Following this, the temperature can safely be dropped to room temperature
Room temperature

Room temperature is a common term to denote a certain temperature within enclosed space at which humans are accustomed.Room temperature is thus often indicated by general human comfort, with the common range of 10celsius to 23?C , though climate may acclimatize people to higher or lower temperatures....
 at a rate limited by the heat capacity, thickness, thermal conductivity
Thermal conductivity

In physics, thermal conductivity, , is the List of materials properties of a material that indicates its ability to conduct heat. It appears primarily in Heat conduction#Fourier's law for heat conduction....
, 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 (? = 1013 Poise) stresses relax within several minutes, while at the strain point (? = 1014.5 Poise) stresses relax within several hours. Stresses that are still present below the strain point are permanent.

See also

  • Annealing (metallurgy)
    Annealing (metallurgy)

    Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness....
  • Float glass
    Float glass

    Float glass is a sheet of glass made by floating molten glass on a bed of molten tin. This method gives the sheet uniform thickness and very flat surfaces....
  • Tempered glass
  • Fabrication and testing (optical components)
    Fabrication and testing (optical components)

    Optical fabrication and testing spans an enormous range of manufacturing procedures and optics test configurations.The manufacture of a conventional spherical lens typically begins with the generation of the optic's rough shape by grinding a glass blank....


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