In the manufacture of float glass, a lehr oven is a long

kiln A kiln is a thermally insulated chamber, a type of oven, that produces temperatures sufficient to complete some process, such as hardening, drying, or chemical changes. Kilns have been used for millennia to turn objects made from clay int ...

with an end-to-end temperature

gradient In vector calculus, the gradient of a scalar-valued differentiable function of several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "direction and rate of fastest increase". If the gr ...

, which is used for annealing newly made glass objects that are transported through the temperature gradient either on rollers or on a conveyor belt. The annealing renders glass into a stronger material with fewer

internal stresses In continuum mechanics, stress is a physical quantity. It is a quantity that describes the magnitude of forces that cause deformation. Stress is defined as ''force per unit area''. When an object is pulled apart by a force it will cause elonga ...

, and with a lower probability of breaking. The rapid cooling of molten glass results in an uneven temperature distribution throughout the material. This temperature differential results in mechanical stresses throughout the molten glass, which may be sufficient to cause the material to crack as it cools to ambient temperature or to make it susceptible to cracking during later use, either spontaneously or due to mechanical or

thermal shock Thermal shock is a type of rapidly transient mechanical load. By definition, it is a mechanical load caused by a rapid change of temperature of a certain point. It can be also extended to the case of a thermal gradient, which makes different pa ...

. To prevent such material weaknesses, objects made from molten glass are annealed by gradual cooling in a lehr oven, from the annealing point, a temperature just below the solidification temperature of the glass. In the process of annealing glass, the temperature is first equalised by holding or "soaking" the glass at the annealing point for a period of time that depends on the maximum thickness of the glass. The glass is then slowly cooled at a rate that depends upon the maximum thickness of the glass, ranging from tens of degrees Celsius per hour (for thin slabs of glass) to fractions of a degree Celsius per hour (for thick slabs of glass).

References

Glass engineering and science Glass art Kilns {{Glass-engineering-stub

See also

References