Windshield and car glass: what are they?

We don’t spend much time in our lives thinking about one of the most ubiquitous products that surrounds us every day: Glass. Glass is everywhere! Look around you and you’ll see it in your home, office, glasses (unless they’re plastic), ceramics like toilet bowls and plates, bottles, light bulbs, TV screens, and of course, your car.

What is glass? It is a combination of materials, most commonly silicon dioxide (sand), lime, and potash. There are specialty glasses made from other materials, but the type we see all the time, soda lime glass, is made up primarily of the three materials mentioned above. For a detailed, easy-to-understand explanation of glass, visit Wikipedia and type in the search term glass.

The two common types of glass we interact with on a daily basis, annealing and tempered, are basically the same products that have undergone different manufacturing treatments to give them individual characteristics. These features are designed to fulfill certain functions.

Tempered glass is known as safety glass because it breaks into hundreds of small blocks, instead of long, knife-sharp shards that can inflict terrible wounds. Small blocks of broken tempered glass are much less likely to cut into human flesh. Tempered glass is used in automotive side and rear windows, as well as commercial glass, sliding doors, and floor-length windows.

Annealed glass, such as in your kitchen window, is used to make windshields. You may be wondering how a kitchen window can be used in front of a driver. That’s crazy! Yeah, that would be crazy, unless the annealed glass was laminated together with a piece of strong 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 glass and one plastic. Glass sticks to plastic when it breaks. No doubt you’ve seen a shattered windshield and noticed that it’s hanging together in one big cracked piece. This is what makes the windshield different from your kitchen window.

Tempered glass is very strong and hard to break unless hit with a sharp object. You may have seen a tool advertised on TV used to escape a car by breaking the tempered side window. The tool is like a small hammer with a sharp point. Unfortunately, criminals have also discovered a use for it and keep it in their criminal toolbox. If he’s ever had a window broken during a carjacking, it was probably done with one of these tools.

When manufactured, tempered glass is just like annealed glass. It then goes through a heating process to “temper” it, making it strong and resilient. The glass cools quickly so the outside contracts and forms a compressed outer shell. It puts the entire piece of glass under tremendous stress and this is why it seems to explode when it breaks. The stress is released with such force that the glass breaks into small harmless pieces.

Why isn’t tempered glass used in windshields? Many years ago it was used for the windshields of some cars. Laminated safety glass is simply a better and safer product for the front of your car because it stays in place, even when broken, and prevents debris from flying out of your face. It also helps keep occupants inside the car.

The glass we know is an unusual and interesting substance. Other materials can also be classified as glass. It could be said 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 is 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 constantly produces glass. People who live in volcanic areas are familiar with obsidian, a glass used by early Americans to shape arrowheads, spearheads, and knives.

Just remember that when you’re replacing your car glass, the material you’re looking at has an interesting history and unique characteristics that few other materials can match. Glass has been around for centuries and will endure for the foreseeable future.