The Elasticity of Glass
Obviously some materials are more bendable/flexible than others—rubber is more flexible than concrete.
The flexibility of solids was first put on a mathematical basis by Thomas Young (1773–1829), physicist, physician and Egyptologist1 (the first to completely translate the enchorial (demotic) text of the Rosetta Stone).
The basic idea is this: suppose you're bending a piece of metal—a plastic ruler, for example. If you bend it just a little, then release it, it returns to its original shape. But there is a point beyond which you can bend the spoon and it is permanently bent — it doesn't return to its original shape when you release your pressure on it. That 'point of no return' is called the 'elastic limit'. (In the case of glass, when you exceed the elastic limit you break the glass.)
As long as you stay short of the elastic limit, the amount the spoon bends is proportional to the force applied. That ratio—how much the spoon bends per amount of force applied—is called "Young's Modulus". This may seem like a simple enough idea, but it has proven so useful and powerful that every bridge you drive over and every high rise you enter is designed depending on this "simple" mathematical model that Young devised.
Young's Modulus is a ratio — it's a number. The bigger the number, the stiffer the material.
A related number is Poisson's Ratio. Basically the idea is this: imagine you are stretching a piece of rubber. As you stretch it, it 'pinches' in the middle. If you are stretching a material East/West, say, Poisson's Ratio is how much the material 'pinches' North/South.
Here is a table of Young's Modulus and Poisson's Ratio for various types of glass, as well as a few other materials for comparison:
Material Young's Modulus
Poisson's Ratio Fused Silica ('quartz') 71 0.17 Soda-lime glass (the most common type of glass) 69 0.24 Aluminosilicate glass 88 0.25 Borosilicate glass (aka 'Pyrex') 63 0.20 High-lead glass 51 0.22 Rubber 0.01–0.1 Nylon 2–4 Oak wood (along grain) 11 Aluminium Alloy 69 Wrought Iron & Steel 190–210 Tungsten Carbide 450–650 Diamond 1050–1200
So, for example, from the above chart, you can see that aluminium alloy is about as flexible as glass.
For the mathematically inclined, see the article "Elastic Behavior of Materials: Continuum Aspects", from:Encyclopedia of Materials: Science and Technology, ISBN 0-08-0431526, pp. 2398-2404
1An earlier attempt at translating Egyptian heiroglyphics was made by Athanasius Kircher (1601–1680). Kircher also played a role in the development of the 'musical glasses', the precursor of the glass armonica.