Rapid cooling of molten glass generates an uneven temperature distribution in the body of the glass, which results in mechanical stress sufficient to cause cracking before the object has reached ambient temperature, or to result in susceptibility to cracking in later use, often triggered by thermal shock. To prevent this, objects manufactured from molten glass are annealed by cooling gradually in a lehr from a temperature just below the solidification point of the glass. Anneal cooling rate depends on the thickness of the glass and can range from several tens of degrees Celsius per hour for thin sections to a fraction of a degree Celsius per hour for thick slabs or castings.
In glass manufacture, a lehr is typically a long kiln with a temperature gradient from end to end, through which newly made glass objects such as glasses or vases are transported on a conveyor belt. However, the same effect can be obtained in a small kiln by controlling the cooling rate with an electronic temperature controller.
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