GASES EVOLVED FROM GLASSES 



123 



The main gases held by the glass are carbon dioxide (from the 

 carbonates and possibly from the furnace gases), water (from the 

 materials and the furnace gases), with smaller quantities of sulphur 

 dioxide, oxygen and nitrogen. The amounts of the gases may cor- 

 respond to a real equilibrium under the condition of melting; but 

 more usually the viscosity of the melt has been such as to prevent 

 the gases present in solution from escaping completely during the 

 melting period. Commerical glasses consequently retain some gas 

 which escapes slowly when the glass is reheated. 



In most of the investigations published in the past the gases have 

 been divided into three fractions: (a) condensable above -78°C. (b) 

 condensable between -78°C. and -190°C, (c) not condensable at 

 -190°C. These fractions represent with fair accuracy (a) water vapor, 

 (b) carbon dioxide, (c) the permanent gases; oxygen, nitrogen, hydro- 

 gen. In general water vapor is the most abundant, followed by 

 carbon dioxide; but, owing to lack of the necessary data, it is not 

 possible to correlate these findings with the composition of the glass, 

 still less with the mode of its melting. 



Apparatus and Method 

 The apparatus used for determining the gases evolved from the 

 glass on being heated is shown in Fig. 1. 



-•-TO PUMP 



The water vapor was removed by surrounding the condensation 

 trap by a mixture of frozen and liquid acetone contained in a Dewar 



