Author: Harold Sharon
In the early thirties along came a revolutionary invention: "safety" glass. Two flat layers of glass were bonded under heat and pressure to a very tough, and quite transparent, layer of plastic. The bond was so strong that as the glass flexed in a collision it broke into lots of small pieces, many of which stuck to the plastic. The passenger might have 500 fragments of glass imbedded in his skin, but no severed arteries. Quite an improvement. By the forties the industry learned how to make tooling to produce curved panels for, ugh, streamlined cars. It was expensive but immediately adopted by all manufacturers, so it was worth the cost.
The next development was an economical way to make distortion-free flat glass. "Float" glass is made by pouring molten glass onto a vat of molten tin. As long as the floor doesn't shake and the tin has a clean surface this is a less expensive way to make flat glass, as clear as ground plate. This glass isn"t really flat. It follows the curvature of the earth, (Have you heard his question, "does a train track flex when a fly lands on it...) It still has to be laminated for safety.
Next came "tempered" glass, which has been exposed at high temperatures to chemicals which affect a few thousandth of an inch on the surface, causing the molecules to expand. The thin layer of larger molecules is not strong enough to stretch the core much. Both sides are trying to expand. The core is in a mild state of tension. The skins are quite strongly in compression, because of their constrained urge to expand. When any point on the surface fails the core cracks through and the crack propagates in all directions at lightening speed. So, flat or curved, the glass shatters into tiny blunt pieces. If they fall on you no cuts, but if they stay in place you cant see through the cracks, which is why it ain't so good for windshields.,
The current windshield technology comes from Europe, It is laminated safety glass with another plastic skin on the inside to contain the fragments,
The other recent development is not exactly high tech, although "tempered" glass helped to bring it along. This great invention is thinner, therefore, lighter glass, It gives a lower center of gravity, better gas milage. Your old car glass should be 1. safe, 2 clear and free of distortion, 3 light weight, if your horsepower is as scant as mine ( That last comment from a man who never had a windshield) Replacing the original glass on a Model T sedan would make it safer, faster, more economical, less tippy. How could you not do it?
For more of Harold's Great technical knowledge, check out His Book