bowen: sigxificaxce of glass-making processes 89 



Optical Company, where we went hoping not merely to be of 

 assistance in solving these urgent problems but expecting also 

 to learn something of more general interest concerning the be- 

 havior of silicate liquids when handled on the comparatively 

 large scale of the glass plant. 



One of the principal requirements of optical glass is homo- 

 geneity. A fragment of glass to be used for a lens or prism 

 must have the same refractive index and therefore the same 

 chemical composition in all its parts, and from every pot of glass 

 made a considerable proportion is rejected because it fails to 

 fulfil this requirement. Naturally, the causes of inhomogeneity 

 are diligently sought for, with the hope of removing them or re- 

 ducing them to a minimum, and it may be stated that these causes 

 are now pretty well understood. To those interested in the causes 

 of inhomogeneity (differentiation) in masses of silicate rocks the 

 factors that lead to inhomogeneity in these artificial silicate 

 melts are perhaps of sufficient interest to merit description. 



Optical glass is made in a great many varieties with a wide 

 range of composition. SiOj and B2O3 are the principal acidic oxides, 

 and the alkalies with CaO, PbO, BaO, and ZnO are the princi- 

 pal basic oxides, though a number of other oxides enter into the 

 composition of special glasses. The alkalies, lime, and baryta 

 usually go into the batch in the form of carbonates; lead and 

 zinc as oxides; and silica as quartz sand. The carefully mixed 

 batch is usually fed in several instalments into the pots, which 

 have already been heated to the melting temperature. Factors 

 tending to produce inhomogeneity immediately set to work. 

 Some constituents of the batch are readily fusible, others, es- 

 pecially the sand, are quite refractory. The more fusible por- 

 tions quickly form a liquid which tends to filter downward 

 through the porous structure formed by the grains of the more 

 refractory material. This action is especially marked in the 

 heavy glasses rich in lead. 



As typically developed the result may give every appearance 

 of liquid immiscibility and the formation of two liquid layers. 

 The two layers may be sharply marked off from each other and 

 may so persist throughout the run. But that we have here no 



