In the manufacture of

float glass Float glass is a sheet of glass made by floating molten glass on a bed of molten metal of a low melting point, typically tin, although lead was used for the process in the past. This method gives the sheet uniform thickness and a very flat surfa ...

, 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, chemical changes. Kilns have been used for millennia to turn objects m ...

with an end-to-end temperature

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

, 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, 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 phenomenon characterized by a rapid change in temperature that results in a transient mechanical load on an object. The load is caused by the differential expansion of different parts of the object due to the temperature chang ...

. 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