The oven used for annealing glassware. Early lehrs were connected to the furnace by flues, but the difficulty of controlling heat and smoke made this arrangement impracticable. Later lehrs were long, brick-li ned, separately heated tunnels through which the glass objects were slowly pushed; the glass remained in the lehr for several hours, while it was gradually reheated and then uniformly cooled. Today, lehrs work on a conveyor belt system.
Source: Glass: A Pocket Dictionary of Terms Commonly Used to Describe Glass and Glassmaking compiled by David Whitehouse of The Corning Musueum of Glass.