GLASS AND GLASS-MAKING 



The ordinary annealing-oven is so arranged with 

 draughts of hot and cold air that the temperature can 

 be maintained indefinitely at any desired degree, or 

 cooled gradually to that of the surrounding atmos- 

 phere. There is another and more recent type of 

 oven, however, known as the "lehr," made in the form 

 of a tunnel some two hundred feet in length. The 

 heating of this long tunnel is done mostly at one end, 

 the temperature diminishing gradually and uniformly 

 toward the other end. Running the length of this is 

 an endless-chain arrangement, on which the plates 

 are passed from the heated extremity to the cooler one, 

 being timed so that a single passage through the lehr com- 

 pletes the annealing. In such ovens, which are becom- 

 ing rapidly popular, particularly in America, the time 

 required for annealing is reduced from days to hours. 



Aside from window glass, and coarse bottle glass, 

 glass used for most purposes is flint glass. French 

 "crystal" is the same as English flint, and this glass 

 is distinguished by its weight and brilliancy. Cut 

 glass, optical glass, and all the best blown and pressed 

 glassware for houshold use is of this material. 



In working this glass, all three methods of working 

 blowing, pressing, and molding are used. Cut glass 

 is first blown roughly into the desired shape in the open 

 air, and then subjected to the cutting process. It could be 

 cut after being molded instead of blown, but such glass 

 lacks something of the brilliant luster of glass blown in 

 the air. The cutting is done on grindstones moistened 

 by streams of wet sand, and by emery wheels, the 

 finest polish being given by putty powder. 



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