GLASS-MAKING. 



615 



quite unfit for use. To avoid this settling and secure a clear, 

 homogeneous glass, that is the problem. 



At the present time the best optical glass is probably made in 

 France, and the methods there in use are consequently most wor- 

 thy of examination. During the melting process the crucible is 

 placed in the center of a domed furnace. The flames play around 

 the crucible on all sides, making an intense heat possible. The 

 hood prevents the furnace 

 gases from acting upon the 

 compounds of lead and re- 

 ducing them to the metallic 

 state. The well-mixed batch 

 is introduced in small quan- 

 tities into the thoroughly 

 heated crucible, and the 

 charging process continued 

 until the pot is completely 

 filled. This will require 

 from six to ten hours. The 

 heat is then continued for 

 perhaps four hours, at the 

 end of which time the molt- 

 en glass is vigorously stirred 

 with a wrought-iron rod in- 

 cased in a fire-clay cylinder. 

 Then comes a second period of quiet heating and a second stirring. 

 After this the stirrings succeed each other at every hour. When 

 these hourly stirrings have been repeated perhaps half a dozen 

 times, the crucible is allowed to cool down for a couple of hours. 

 It is then heated to the utmost that the furnace will permit. As 

 the result of this intermittent treatment, the glass is very liquid 

 and is quite free from bubbles and striations. During the gradual 

 cooling which succeeds this firing, a constant stirring is main- 

 tained for at least a couple of hours. When the stirring becomes 

 too difficult, it is discontinued, and crucible and furnace are 

 allowed to cool during a period of ten days or more. 



The secret of making fine optical glass lies in this stirring. 

 It was first carried out by Guinand, in Switzerland, in the 

 early part of the century, and was introduced in Paris by Bon- 

 temps. 



When the cooling is accomplished, the crucible is removed 

 from the furnace and broken, so as to free the mass of flint glass 

 which it incloses. In spite of all this care, it must not be sup- 

 posed that a mass of perfect glass is the result. On the contrary, 

 it is full of flaws and imperfections, and only a part of it can be 

 used. It is customary to grind and polish parallel faces on the 



Fig. 1. Flint-glass Furnace. 



