Category: windows and glass

Why is a car’s rear window put and kept under stress, and what has this to do with polarization? — BD, Leuven, Belgium

The rear window of a car is made of tempered glass — the glass is heated approximately to its softening temperature and then cooled abruptly to put its surface under compression, leaving its inside material under tension. That tempering process makes the glass extremely strong because its compressed surface is hard to tear. But once a tear does manage to propagate through the compressed surface layer into the tense heart of the glass, the entire window shreds itself in a process called dicing fracture — it tears itself into countless little cubes.

The stresses frozen into the tempered glass affect its polarizability and give it strange characteristics when exposed to the electromagnetic fields in light. This stressed glass tends to rotate polarizations of the light passing through it. As a result, you see odd reflections of the sky (skylight is polarized to some extent). Those polarization effects become immediately apparent when you wear polarizing sunglasses.

I have read that very old panes of glass become thicker at the bottoms than the tops. Doesn’t that show that glass flows? — MJ

While it is sometimes noted that old cathedral glass is now thicker at the bottom than at the top, such cases appear to be the result of how the glass was made, not of flow. Medieval glass was made by blowing a giant glass bubble on the end of a blowpipe or “punty” and this bubble was cut open at the end and spun into a huge disk. When the disk cooled, it was cut off the punty and diced into windowpanes. These panes naturally varied in thickness because of the stretching that occurred while spinning the bubble into a disk. Evidently, the panes were usually put in thick end down.

Modern studies of glass show that below the glass transition temperature, which is well above room temperature, molecular rearrangement effectively vanishes altogether. The glass stops behaving like a viscous liquid and becomes a solid. Its heat capacity and other characteristics are consistent with its being a solid as well.

A center punch is a device used in extricating people from cars. You put tape over the glass window you want to pop, place the center punch in the corner of that window, and simply press inward. This does something that causes the glass to crack in a spider web pattern. The glass sticks to the tape, you push in enough glass to get your hand through, and knock the rest outward. This technique works on any window except the front windshield, which is 3 layers(glass, plastic, glass). Can you explain it? — RSG, Boston, Massachusetts

A center punch is a common tool used to dent a surface prior to drilling. The drill bit follows the pointed dent and the hole ends up passing right through it. But in the situation you describe, the center punch is being used to damage the surface of a car window. When you push the handle of the center punch inward, you are compressing a spring and storing energy. A mechanism inside the center punch eventually releases that spring and allows it to push a small metal cylinder toward the tip of the punch. This cylinder strikes the tip of the punch and pushes it violently into the glass. The glass chips.

In normal glass, this chipping would be barely noticeable. But the side and rear windows of a car are made of tempered glass—glass that has been heat processed in such a way that its surfaces are under compression and its body is under tension. Tempering strengthens the glass by making it more resistant to tearing. But once an injury gets through the compressed surface of the tempered glass and enters the tense body, the glass rips itself apart. The spider web pattern of tearing you observe is a feature of the tempered glass, not the center punch. Any deep cut or chip in the tempered glass will cause this “dicing fracture” to occur.

Can you get a suntan or sunburn through glass? — SD, Farmington, Utah

Yes, but not as quickly as without the glass. While glass absorbs short wavelength ultraviolet light, it does pass 350 to 400 nanometer ultraviolet. While this longer wavelength ultraviolet is less harmful than the shorter wavelength variety, you can still tan or burn if you get enough exposure. Glass is like sunscreen—it protects you pretty well but it isn’t perfect.

Is glass in a gaseous state, a liquid state or a solid state? If I remember back to my freshman college year, it seems my prof said it was in a highly viscous state; therefore a liquid. — GC, Garland, Texas

The answer to that question is complicated—glass is neither a normal liquid nor a normal solid. While the atoms in glass are essentially fixed in place like those in a normal solid, they are arranged in the disorderly fashion of a liquid. For that reason, glass is often described as a frozen liquid—a liquid that has cooled and thickened to the point where it has become rigid. But calling glass a liquid, even a frozen one, implies that glass can flow. Liquids always respond to stresses by flowing. Since unheated glass can’t flow in response to stress, it isn’t a liquid at all. It’s really an amorphous or “glassy” solid—a solid that lacks crystalline order.

Sometimes on television a high pitched noise breaks the windows in a house. I know that tubular objects such as wine glasses will break when the frequency corresponds to the natural frequencies of the glass, but does flat sheet glass such as windows experience this same effect? — RF, Jackson, Michigan

In real life, only explosive sounds will break normal glass. That’s because normal glass vibrates poorly and has no strong natural frequencies. You can see this by tapping a glass window or cup—all you hear is a dull “thunk” sound.

For an object to vibrate strongly in response to a tone, that object must exhibit a strong natural resonance and the tone’s pitch must be perfectly matched to the frequency of that resonance. A crystal wineglass vibrates well and emits a clear tone when you tap it. If you listen to the pitch of that tone and then sing it loudly, you can make the wineglass vibrate. A crystal windowpane would also have natural resonances and would vibrate in response to the right tones. But it would take very loud sound at exactly the right pitch to break this windowpane. A few extraordinary voices have been able to break crystal wineglasses unassisted (i.e., without amplification) and it would take such a voice to break the crystal windowpane.

I can understand that the strings of bubbles from the side of a glass of champagne are due to nucleating dirt or other imperfections in the glass surface, but what causes those strings of bubbles in the center of the fluid? They are quite persistent. Are they just dust? — BM, Tehachapi, CA

If there were no impurities or imperfections in a glass of champagne, bubbles would only form through statistical fluctuations—random effects would occasionally bring enough gas molecules together to form (nucleate) a bubble and that bubble would grow and rise to the surface. But such spontaneously nucleated bubbles are extremely rare and form randomly throughout the fluid, rather than in chains of steady bubbles. In fact, bubbles would be so rare in this impurity-free liquid that you would probably not even notice them—the champagne would slowly go flat by losing gas molecules from its surface alone.

In real champagne, chains of bubbles do rise upward from the center of the fluid. These bubbles are clearly forming at suspended impurities. All it takes is a tiny piece of dust to trigger bubble formation. If you swirl the champagne slightly, you should be able to see these suspended chains of bubbles move, indicating that the impurities that are triggering them are also moving with the fluid.

What is the difference between crystal and glass?

The “crystal” that’s used in fine glassware is actually a glass, but it is chemically different from the glass that’s used in more common glassware. Both materials are formed by melting together a mixture of silicon dioxide (also called quartz or silica) and other chemicals and both are glasses, meaning that their atoms are arranged haphazardly and not in the crystalline lattices of such materials as salt or sugar. The chemicals that are added to silicon dioxide to make normal glassware—sodium oxide and calcium oxide—make the glass easier to melt and work with at the expense of strength and increased damping. That’s why normal glassware is relatively soft and emits a dull sound when you rap it; it experiences lots of internal friction. The chemicals added to silicon dioxide to make “crystal” glassware include lead oxide, which makes the glass easier to melt and soft enough to cut and shape easily. However, lead “crystal” glassware has less internal damping than ordinary glassware and emits a ringing tone when you rap it because it experiences very little internal friction.

How is glass made?

Common window glass is made by melting a mixture of quartz sand (silicon dioxide), soda (sodium oxide), and lime (calcium oxide). The quartz is the network forming material that forms the basic structure of the glass. The soda makes it much easier to melt and work with—along with making the glass weaker and more temperature sensitive. The lime prevents the soda-rich glass from dissolving in water.

How do you create sculptures out of glass? – RD

While I know how to work with glass in principle, I’m certainly not able to make sculptures. Although anyone can shape glass, doing so with artistry and precision requires great skill. In effect, glass is a frozen liquid. Its microscopic structure is very similar to that of a liquid and it softens with temperature rather than melting abruptly. If you heat a piece of glass carefully with a propane torch, it will begin to flow as a thick liquid (like cold honey). In that state, it can be reshaped rather easily. But making it take the shapes you want is a whole other story and something I know little about. I have bent lots of glass tubes in my day, but I often kink the tubing or smash it flat by accident. A skilled glassblower can do seemingly impossible things with glass. I should also note that glass can be cut or shaped by a water-cooled abrasive wheel. Again, anyone can slice and dice glass but it takes great skill to do something attractive. I usually chip the glass pieces that I try to cut.