90 bowen: significance of glass-making processes 



true case of liquid immiscibility and that the persistence of two 

 layers is due entirely to the slowness of diffusion is shown by 

 the fact that appropriate stirring will completely eliminate this 

 layering and give a single homogeneous liquid. When real im- 

 miscibility occurs in the glass pot, as it does under certain cir- 

 stances, it is quite a different matter. If the alkaline carbonates 

 used in the batch contain a considerable amount of chloride or 

 sulfate, these salts form a separate liquid layer which floats on 

 top of the glass, forming the ''salt water" of the glass-maker. 

 No amount of stirj-ing, however vigorous, will render such a 

 mass homogeneous. This immiscibility between silicate, on the 

 one hand, and sulfate or chloride, on the other, serves but to 

 emphasize that immiscibility between silicate and silicate is not 

 encountered in the whole range of glass compositions. 



This process of settling down of heavy liquid through the 

 porous mass of the batch can take place only at a stage when the 

 mass is mostly solid. A factor tending to produce a closely re- 

 lated result comes into play at the stage when the mass is mostly 

 liquid. Of all the ingredients of the batch the sand is usually 

 the last material to dissolve. The sand grains tend to rise in 

 the liquid and thus to render the upper parts more siliceous and 

 of lower density. This action results in a continuous density 

 gradient rather than in a sharp division into two layers. That 

 it is not a spontaneous arrangement of the liquid according to 

 the Gouy-Chaperon phenomenon is shown by the fact that as 

 time goes on diffusion tends to lessen the gradient rather than 

 to increase it. 



Figure 1 is a photograph of a fragment of glass taken from 

 such a pot, the straight edge being part of the original upper 

 surface of the glass. Two parallel plane faces were cut normal 

 to this surface and the specimen was photographed in a bright 

 light close to a white screen. Under these conditions heavy 

 shadows are cast by the globules of low refracting glass sur- 

 rounding the silica grains and by the tails of similar material 

 pointing downward from them. It is obvious that silica is 

 continually being transferred towards the top. 



There can be no doubt of the correctness of this explanation 

 of the density gradient as a result of the floating of sand grains, 



