Lehr (glassmaking)

In the manufacture of glass, a lehr oven is a long kiln with an end-to-end temperature gradient, 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.[1]

The rapid cooling of molten glass unevenly distributes the temperature throughout the ribbon of glass material. The uneven rate of cooling (temperature differential) results in the occurrence of mechanical stresses throughout the ribbon of molten glass, which are sufficient to: (i) cause the material to crack, before the molten glass has cooled to ambient temperature; or (ii) result in a brittle material, glass susceptible to cracking during later use, usually by thermal shock. To prevent such material weaknesses, objects made from molten glass are annealed — gradually cooled in a lehr oven, from a temperature just below the solidification temperature of the glass.[2] In the process of annealing glass, the rate of cooling depends upon the casting thickness of the slab of glass, ranging from tens of degrees-Celsius per hour (for thin-cast slabs of glass) to fractions of a degree-Celsius per hour (for thick-cast slabs of glass).[3]