INGENUITY AND LUXURY 



to the glass. For example, sodium silicate gives the 

 glass a greenish tint; potassium silicate adds to the 

 brilliancy and fusibility of the glass; and lead silicate 

 increases the ductility, as well as the fusibility and 

 brilliancy of the product. An excess of lime renders 

 the glass too brittle for practical purposes. 



Since sand is the principal source of the silica used 

 in glass-making, it is this substance which comes in for 

 closest scrutiny and most careful examination in the 

 preliminary preparations of glass-making. Generally 

 speaking, the quality of the sand determines the quality 

 of the product, and as all sand contains many injurious 

 impurities, a course of preliminary preparation and puri- 

 fication is necessary before it is used. This preparation 

 consists of the various processes of washing, burning, and 

 sifting. In the process of washing, the heavier grains 

 of pure sand settle to the bottom, while many of the 

 lighter impurities float at the surface where they may 

 be skimmed off. The burning removes the moisture 

 and destroys whatever organic matter may be clinging 

 to the sand grains, while the final process of sifting 

 through copper gauze reduces the grains to uniform 

 size, and removes, besides, the impurities still further. 



By far the most troublesome impurity found in sand 

 one that can be neither sifted, burned, nor washed 

 out is iron. Indeed, this substance is so troublesome 

 that sands containing large quantities of iron are not 

 suitable for glass-making, and the value of any sand 

 is determined largely by the amount of this impurity 

 it contains. 



One reason for the large amount of inferior glass 



[286] 



