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Annealing (glass)

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	Topics Annealing (glass) Topic Home Annealing is a process of slowly cooling glassGlass Glass is a uniform amorphous solid material, usually produced when the viscous molten material cools very rapidly to below i... to relieve internal stresses after it was formed. The process may be carried out in a temperature-controlled kiln known as a LehrLehr (glassmaking) A lehr is a temperature-controlled kiln for annealing objects made of glass.... . 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 the durability of glass. 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 temperatureTemperature Overview In thermodynamics, temperature is a measure of the tendency of an object or system to spontaneously give up energy.... reaches a stressStress (physics) Overview A mature tree trunk may support a greater force than a fine steel wire but intuitively we feel that steel is stronger than wood.... -relief point, that is, the annealing temperature (also called annealing point) at a viscosityViscosity Viscosity is a measure of the resistance of a fluid to deform under shear stress.... , ?, of 10¹³ PoisePoise The poise is the unit of dynamic viscosity in the centimetre gram second system of units.... = 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 (? = 10^14.5 Poise). Following this, the temperature can safely be dropped to room temperatureRoom temperature Room temperature, in common usage, is taken to be roughly 20–25 degrees Celsius.... at a rate limited by the heat capacityHeat capacity Heat capacity is a measurable physical quantity that characterizes the ability of a body to store heat as it changes in temp... , thickness, thermal conductivityThermal conductivity In physics, thermal conductivity, k, is the intensive property of a material that indicates its ability to conduct heat.... , 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 (? = 10^14.5 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 the microstructure of a material is altered, cau... Float glassFloat glass Float glass is sheet glass made by floating the molten glass on a bed of molten tin.... Tempered glass Fabrication and testing (optical components)Fabrication and testing (optical components) The subject of optical fabrication and testing spans an enormous range of manufacturing procedures and optical test configur... External links