260 METHODS OF GLASS MANUFACTTJRHSTG. 



greater the proportion of these bases present, the less 

 heat and time are required to make the fusion complete. 

 An abnormal amount of bases present is detrimental to 

 the quality of the glass, as reagents react readily upon it, 

 removing the metal and leaving silicic acid deposited. 



At an intense heat pure glass is tolerably fluid, while 

 at a brisk red heat it is in a semi-fluid condition, and it 

 is due to this property that it can be worked into forms 

 so commonly seen. 



The chemical properties of glass are various, depending 

 entirely upon its chemical constitution. Dry air or oxygen 

 exert no action whatever. Deoxidizing bodies, by aid of 

 heat, reduce the oxides of lead, manganese and iron if 

 present. 



Water acts on all glasses to a greater or less extent, 

 particularly on those rich in alkalies. Alkaline solutions 

 and acids exert a decided action, decomposing it, in the 

 first case by forming soluble silicates, in the latter case 

 by combining with the metal and learing silicic acid 

 free. 



Glasses which may be called normal, that is those in 

 which the amount of silicic acid present is approximate- 

 ly that required to form definite compounds with the 

 bases present, are scarcely acted on by water, alkaline 

 solutions or acids. 



As the quality of glass depends on the purity of the 

 ingredients employed, great care must be taken in their 

 selection, if glass of a fine quality is to be manufactured. 

 The principal ingredients employed are sand, lime, and 

 soda ash, or carbonate of soda. 



Sand and quartz rock are the chief sources from which 

 is obtained the silicic acid required for the operation. 

 Sand is usually used in the manufacture of window and 

 ordinary light green bottle glass. A sand should be se- 

 lected which is comparatively free of oxide of iron and 

 also one, the grains of which are not too hard or flinty. 



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