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" at which the glass is still too hard to deform, but is soft enough for the structure of the material to flow together. 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 a critical point when the internal stresses are balanced with the surface tension on the face of the glass. Then the temperature can safely be dropped to room temperature. After the annealing process the material can be cut to size, drilled or polished.
Tempered glass must be converted to annealed glass before cutting or grinding.
See also
References
- ^ E. F. Collins (1921) Electrically heated glass annealing lehr. Journal of the American Ceramic Society 4 (5), 335–349.
External Links
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