OPTICAL GLASS FOR WAR NEEDS in 



ing optical glass into thin sheets just as plate glass is made 

 something quite unheard of previously in optical glass manu- 

 facture. Some glass of nearly every variety has been treated 

 by this method, principally in the plant of the Pittsburgh Plate 

 Glass Company, and later elsewhere. 



Bubbles, likewise called seeds, air bells, boil, and pot bubbles, 

 are also a source of great annoyance and show lack of chemical 

 homogeneity. They come from many sources and no doubt are 

 entrapped mechanically. Some form during the reaction, cling 

 tenaciously to the sides and bottom of the pot, and loosen but 

 gradually during the stirring. Others doubtless represent dis- 

 solved gases, air stirred into the mass, and steam from leaking 

 water-cooled stirring rods, and under unusual circumstances, 

 vacuum bubbles resulting when a pot is cooled very quickly. 

 Among the methods to free the glass of these bubbles, can be 

 mentioned the use of arsenic in quantities not over 0.3 per cent, 

 and of antimony oxide which reacts at a high temperature with 

 the evolution of gas, which rising through the mass, literally 

 sweeps out the small bubbles. The Pittsburgh Plate Glass 

 Company developed the use of ammonium nitrate for this pur- 

 pose, the compound being introduced in the shape of a small 

 moulded stick. The nitrate is volatilized completely and gives 

 off large bubbles of gas when forced to the bottom of the pot 

 of glass at the high " fining " temperatures. These methods 

 are called " blocking " and sometimes a potato or block of wet 

 wood is forced to the bottom of the pot, the object again being 

 to sweep out the bubbles by the use of larger ones, and this 

 action also tends to mix the glass. However, some glass can- 

 not be entirely freed from bubbles. 



Stones refer to fragments of undissolved materials and more 

 often to pieces of the pot wall or the furnace crown which fall 

 into the pot. A good pot will cast very few, sometimes no 

 stones into the melt. Occasionally stones are introduced into 

 the glass during the pressing of the irregular pieces into desired 

 shapes. 



