JOHN A. WILLIAMS. 255 



ing the batch for continuous working is less than the aver- 

 age cost per ton for melting the batch which is worked 

 out in 6 to 10 hours, as is the case in pot furnaces. 

 There is no necessity for lost time, as the pure glass is 

 always ready for use and the delays occurring in pot fur- 

 naces due to broken pots and imperfect melts are un- 

 known. 



Greater economy in repairs, owing to the fact that in 

 tank furnaces an almost constant heat is employed, 

 thereby not subjecting the furnace to such changes of 

 temperature as are necessary in running pot furnaces. 



The greater durability of the tank furnace is largely 

 due to the fact that the batch is filled in, in such quan- 

 tities, as not to come in contact with the sides of the 

 furnace, thereby not subjecting them to the action of 

 the free alkali contained in the batch. 



The construction of a glass furnace is attended with 

 many difficulties, owing to the great heat required, 

 ranging from 2,500 to 3,000° Fahr., and to the destruc- 

 tive action of the alkali on the pots or tanks employed. 



It is necessary to select the most refractory fire clays 

 possible, and for those portions which come in direct 

 contact with the glass, clays especially free of iron and 

 alkalies are selected. The walls of the furnace are built 

 of fire-clay blocks, molded into special shapes as re- 

 quired, and the dome is usually built of silica fire-brick. 



The most important process in the manufacture of 

 glass is the annealing, and unless this is well performed 

 the goods manufactured are absolutely worthless. 



The process of annealing consists in allowing the 

 manufactured goods to cool gradually, so that the 

 particles may so arrange themselves that they will all be 

 in a state of uniform attraction to each other. 



When glass has been heated to the melting point and 

 allowed to cool rapidly it is very brittle ; when allowed 

 to cool gradually it is capable of resisting severe shocks 



211 



