Lehr (glassmaking)

A lehr is a temperature-controlled kiln for annealing objects made of glass. The name derives from the German verb "lehr" meaning to learn and is cognate with the English "lere" also meaning to learn or acquire knowledge of (something)Shorter Oxford English Dictionary, Oxford University Press]

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 resulting from thermal shock. To prevent this, objects manufactured from molten glass are annealed by cooling very gradually in a lehr from a temperature just below the solidification point of the glass.

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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