130 BELL SYSTEM TECHNICAL JOURNAL 



Manufacture of Relatively Gas Free Glass 



In our first experiment we melted some ordinary commercial glass 

 in vacuo. The temperature within the vacuum furnace was kept 

 considerably above the melting point of the glass under examination 

 and was held there for a period of about one hour. For a portion of 

 this time huge quantities of gas were evolved. After this evolution 

 subsided the furnace was cooled to room temperature and the melt 

 was removed. Some of the glass obtained from this melt was later 

 reheated in vacuo at various temperatures up to 500°C. At the lower 

 temperatures from 20 C. to 200°C. a measurable amount of gas was 

 evolved, but above 200 C. there was practically no gaseous evolo- 

 tion. The results obtained from this and other similarly conducted 

 experiments make it seem probable that a vacuum furnace process 

 for the manufacture of certain kinds of glass is possible. The expense 

 incurred, however, in the manufacture of gas free glass by this method 

 would undoubtedly always prohibit its general use in the industries. 

 After realizing that owing to the difficulties of making it, a gas free 

 glass could be used only in a very limited field, we sought next to 

 determine if it would not be possible to modify the ordinary standard 

 procedure now pursued in glass making in such a way as to make a 

 relatively gas free glass. 



At our request, the Corning Glass Works agreed to undertake some 

 experiments to produce glass that would be more nearly gas-free than 

 that obtainable on the market. 



In one of the most successful experiments and the only one that will 

 be recorded here, materials that would produce a low melting point 

 glass were subjected to a heat treatment of between 1500°C. and 

 1600°C. for a period of one hour. For a chemical analysis of the glass 

 produced and for its behavior upon being heated in vacuo see Tables 

 I and II — Glass 6. 



Attention has been called to the fact that this glass behaved quite 

 differently in the above experiments from the other glasses of the 

 same type in two particulars (1) the total amount of gas given up by 

 the glass is very much less than for the other glasses of like composi- 

 tion and (2) of the gas given up by this glass, 90% is given up at 200 C. 

 indicating that this proportion of the gas is adsorbed gas. We may 

 conclude then that the special treatment of this glass in its manufac- 

 ture was very efficient in so far as the removal of the absorbed or 

 dissolved gas was concerned. It could not, however, prevent the 

 adsorption of gases by the glass on standing. The adsorbed gases, 

 however, are rather unimportant from the standpoint of vacuum 



