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Annealing (glass)
From Wikipedia, the free encyclopedia
Annealing, in glassblowing and lampworking, is the process of heating, and then slowly cooling glass to relieve internal stresses. The process, carried out in a temperature-controlled kiln known as a Lehr[1], makes the glass much more durable. Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or other shock.
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 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 thickness of the thickest section. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 1014.5 Poise). Then 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.
Tempered glass must be converted to annealed glass before cutting or grinding.
- ^ E. F. Collins (1921) Electrically heated glass annealing lehr. Journal of the American Ceramic Society 4 (5), 335–349.
- ^ Werner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg GmbH & Co. K; 2nd revised edition (November 1994), ISBN 3540575723
