Lehr (Glassmaking)

In the manufacture of sheet glass and glassware, a lehr is a long kiln with an end-to-end temperature gradient, used for annealing newly made glass objects as they pass through that gradient on rollers or a conveyor belt. Rapid cooling of molten glass leaves an uneven temperature distribution through the material, which produces mechanical stresses severe enough to crack the glass immediately or leave it prone to breaking later; annealing in a lehr avoids this by first holding, or soaking, the glass at its annealing point long enough to equalize its temperature, then cooling it at a controlled rate, ranging from tens of degrees Celsius an hour for thin glass down to a fraction of a degree an hour for thick glass, so the finished piece comes out stronger and with far less internal stress than a piece cooled at room temperature.