330 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



a bending strength of 100,000 p.s.i, compared to less than 10,000 p.s.i. 

 for commercial annealed glass, and maintains most of its strength 

 after abrasion. We believe that these glasses can belie the reputation 

 for fragility and brittleness that now is deserved by glass. 



Since lack of strength has been the Achilles' heel of glass, it is 

 worth examining the reasons for it to help us understand the new 

 cures. The key to both the strength and the weakness of glass is in its 

 amorphous structure. We can regard any piece of glass as a single 

 molecule, a three-dimensional polymer whose strength is equal to the 

 interatomic bond strengths. Therefore, as long as the surface is free 

 from flaws, glass is fantastically strong. Fibers have been measured 

 at one million p.s.i., quarter-inch diameter rods at 400,000 p.s.i. in ten- 

 sion. By contrast, the strongest steel alloys have tensile strengths in 

 the 200,000 to 400,000 p.s.i. range. Compressive strength of glass is 

 also of the order of hundreds of thousands of pounds per square inch. 



Why then is glass so weak ? Unfortunately, any contact with solid 

 surfaces produces surface scratches. These become sites of highly con- 

 centrated stress when the surface is put into tension; and since the 

 glass does not flow to relieve the local stress, a relatively low overall 

 tension is sufficient to extend a scratch into a catastrophic crack. 



An obvious way to maintain high strength is to protect the surface 

 from abrasion, by coating with rubbery plastics or with slippery 

 silicones before it has become scratched. These methods are in fact 

 being employed for some types of glass containers, and for "armored" 

 industrial pipe. Such plastic coatings serve another useful purpose, 

 in preventing loss of the contents if the glass breaks. 



A most promising principle for strengthening glass has been known 

 for many years, and is practiced in the form of "chill tempered" glass. 

 The principle, stated simply, is that tensile strength increases propor- 

 tionately with the previously induced compressive stress in the sur- 

 face layer of glass. 



In chill tempering, an object is cooled rapidly from just below its 

 softening point. Since the inner portion cools more slowly than the 

 surface, it continues to contract after the surface is essentially rigid. 

 Thus, compressive stresses develop in the surface layer with compen- 

 sating tensile stresses in the interior. 



Chill-tempering is capable of inducing compressive stresses up to 

 about 20,000 p.s.i. under favorable circumstances, but is limited to 

 relatively thick glass and simple shapes because of heat-flow problems, 

 and its strengthening effect is permanently lost if the glass is re- 

 heated above 400° C. 



New^ methods of inducing stress are free from these limitations, and 

 in addition can induce much higher compressive stress, well over 

 100,000 p.s.i. The two methods of chemical tempering or armoring 



