MODERN GLASS — STOOKEY 

 Stress (1000 psi) 



331 



M 



^ 



■80 

 I 



M 



i 



Stress (1000 psi) 



-20 —40 —60 —80 

 J L— L-.L-J 



Figure 4. — Two ways to strengthen glass both depend on canceling tensile stress on surface 

 by pre-stressing in compression. In diagram, dotted line is the pre-stress. Dashed line 

 is stress created by external bending load M. Solid line shows net stress. Note that 

 surface stress in crystallized glass stays much farther into compression range than in 

 chill-tempered glass. 



mentioned earlier both involve forming transparent poly crystalline 

 layers within the surface of glasses of appropriate chemical compo- 

 sition. The crystallized skin, which is grown at a high temperature, 

 has a negative thermal expansion and thus expands as it cools. The 

 shrinking of the internal glass induces an extremely high compres- 

 sion of the skin, resulting in high strength. Figure 4 shows how this 

 technique compares with chill-tempering. 



One of the new methods employs surface-nucleated crystallization 

 of a lithia-alumina-silica glass. The other, more complex, begins with 

 a soda-alumina-titania-silica glass, replaces the sodium ions in the 

 surface by lithium ions at high temperature — by immersion in a 

 molten lithium sulfate bath, and ends with titania-nucleated crystalli- 

 zation of eucryptite crystals at the surface of the glass. 



