324 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



Although it is a completely rigid nonreactive material below its 

 annealing temperature of 500 to 600 degrees, yet glass permits rapid 

 diffusion and exchange of certain monovalent ions such as those of 

 silver, copper, sodium, lithium, and potassium, well below the anneal- 

 ing temperature. 



THE NATURE OF GLASS 



Let's start our more detailed description of the newly evolving 

 teclinology of glass by discussing our present concei)ts of its chemical 

 and physical nature. Immediately it becomes necessary to say that no 

 two experts agree on the subject of the molecular structure of 

 glass. For example, the proceedings of the most recent xill-Eussia 

 Glass Congress report that the main emphasis of Russian glass re- 

 search for the preceding 5 years was on glass structure. So what 

 you read here should not be taken as a consensus of all glass scientists, 

 or even two of us. 



Molten silica is an extremely viscous liquid. It can be visualized as 

 a three-dimensional polymer whose basic structure is a network of 

 tetrahedral silicon dioxide molecules, figure 1, joined to one another 

 by oxygen bonds at the corners in random manner. Figure 2 shows 

 this random network in two dimensions; in the molten state, the 

 chemical bonds are continually breaking and re-forming. This 

 molten glass is a universal solvent, capable of incorporating almost 

 every chemical element in its structure. Some elements can replace 

 silica and become part of the network; but most (for example, sodium 

 and calcium) become an ionic plasma moving through holes in the 



SILICON ION. 



Q OXYGEN ION. 



Figure 1. — The silica tetrahedron, basic ingredient in glass, has four Ions of oxygen, one of 



silicon. 



