All glass must be annealed, or slowly cooled, after it has been made. The annealing process allows the atomic structure of glass relaxes as it cools and thus reduces internal streses that can otherwise weaken the glass. In most cases, annealing is achieved by holding the glass at approximately 5° C (9° F) above its annealing point for 5 to 15 minutes, followed by slowly cooling it through the glass-transition range and the strain point and finally to room temperature. Some glasses, such as tempered glass, are strengthened by flash cooling the glass from its annealing point. Optical glasses are held for a long-time at their annealing point then so slowly cooled that they have almost no detectable internal stresses.
Synonyms and Related Terms
tempered glass; heat-strengthened glass
Sources Checked for Data in Record
- Encyclopedia Britannica, http://www.britannica.com Comment: Glass. Retrieved June 6, 2003, from Encyclopædia Britannica Premium Service.
- Dictionary of Building Preservation, Ward Bucher, ed., John Wiley & Sons, Inc., New York City, 1996