We don’t spend much time in our lives thinking about one of the most ubiquitous products surrounding us every day: Glass. Glass is everywhere! Look around and you will see it in your home, office, eyeglasses (unless they’re plastic), ceramics like toilets and dishes, bottles, light bulbs, tv screens, and of course in your car.
What is Glass? It’s a combination of materials, most commonly silicon dioxide (sand), lime, and potash. There are specialty glasses made of other materials but the kind we see all the time, soda-lime glass, is mostly composed of the three materials mentioned above. For an easy-to-understand detailed explanation of glass, visit Wikipedia and type in the search term glass.
The two common types of glass we interact with daily, annealed and tempered, are basically the same products that have undergone different manufacturing treatments to give them individual characteristics. These characteristics are designed to serve certain functions.
Tempered glass is known as safety glass because it shatters into hundreds of tiny blocks, instead of into long sharp-edged, knife-like shards that can inflict terrible wounds. The small blocks of broken tempered glass are much less likely to cut human flesh. Tempered glass is used in the side windows and back glass on cars, as well as in commercial glass, sliding doors, and windows that reach to the floor.
Annealed glass, like in your kitchen window, is used to make windshields. You might wonder how in the world a kitchen window can be used in front of a driver. That’s crazy! Yes, that would be crazy, unless the annealed glass was laminated together with a strong piece of plastic and another piece of glass to make a sandwich called laminated safety glass.
When you look through your windshield, you are looking through all three pieces, two of glass and one of plastic. The glass sticks to the plastic when it breaks. No doubt you’ve seen a shattered windshield and noticed that it hangs together in one big cracked piece. This is what makes the windshield different from your kitchen window.
Tempered glass is very tough and is difficult to break unless it gets punched with a sharp object. You may have seen a tool advertised on TV that’s used to escape from a car by breaking the tempered side window. The tool is like a small hammer with a sharp point. Sadly, criminals have also discovered a use for it and keep it in their criminal toolbox. If you’ve ever suffered a broken window during a car break-in, it was probably done with one of these tools.
When it’s manufactured, tempered glass is just like annealed glass. Then it goes through a heating process to “temper” it, making it tough and resilient. The glass is cooled quickly so the outside contracts and forms a compressed outer layer. It puts the whole piece of glass under tremendous stress and that’s why it seems to explode when broken. The stress is released so powerfully that the glass breaks into harmless little chunks.
Why isn’t tempered glass used in windshields? Many years ago it was used for windshields in some cars. Laminated safety glass just makes a better, safer product for the front of your car because it stays in place, even when broken, and keeps flying debris out of your face. It also helps to keep occupants inside the car.
The glass that we know is an unusual and interesting substance. Other materials can also be classed as glass. You might say that glass is not a substance but a quality or characteristic. A scientist might refer to glass as a state, much like a gas, liquid, or solid. Water makes a good example. Water can be gaseous (steam), liquid, or solid (ice), however it is not seen in a glassy state like liquid rock (lava) or molten metal.
Nature produces glass constantly. People who live in volcanic areas are familiar with obsidian, a glass used by early Americans to shape arrowheads, spear tips, and knives.
Just remember that when you are replacing your auto glass, the material you are looking through has an interesting history and unique characteristics that few other materials can match. Glass has been around for centuries and will endure into the foreseeable future.