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Windows and Glass
Page 1 of 2 (14 Questions and Answers)

442. I thought that glass could move, that is supposed to be a check for antique glass (It has flowed downward). Does glass move over hundreds of years?
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People used to think so. They thought that glass was simply a very, very viscous liquid. However, it now appears that something happens below the glass transition temperature that stops all flow. In effect, the viscosity goes to infinity. While it might be a liquid in principle, it simply doesn't flow, even in terms of geological time scales. Antique glasses have non-uniform thicknesses because of how they were made. The earlier techniques involved stretching blown glass and tended to make sheets with irregular thicknesses. Antique glass exhibits these irregularities.

486. How do color-changing eyeglasses work?
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These eyeglasses are made from a special photochromic glass that contains about 0.01% to 0.1% silver halide crystals. These crystals are transparent and so small that they leave the glass almost perfectly clear. But when the glasses are exposed to bright sunlight, which contains substantial amounts of ultraviolet light, the silver ions in those crystals are reduced to silver atoms and begin to form tiny silver particles inside the glass. Like the particles that form in black and white photography, these silver particles are so jagged and imperfect that they're light absorbing rather than shiny. The glasses thus darken when exposed to sunlight. But when the eyeglasses are returned to the dark, the halogen gas atoms recombine with the silver atoms and reform the silver halide crystals. The eyeglasses once again become clear. Incidentally, the glasses can also be rendered clear by exposing them to elevated temperatures, so a short time in the oven should help to clear them up if darkness alone doesn't do the trick. That assumes, of course, that you don't melt the frames, overheat the glass, or expose the glass to sudden thermal shocks.

628. How do sound proof and bulletproof glasses work? - DH
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Sound proof glass uses several separate layers of glass to make it difficult for sound to move from one room to another. Each time sound passes through a surface and experiences a change in speed, some of the sound reflects. Sound travels much more slowly in air than in glass, so with each transition into or out of a glass pane, most of the sound is reflected backward. If two rooms are separated by 3 or 4 sheets of glass, each carefully sealed into place so that there are no holes for sound to leak through, the amount of sound that can make it through the overall window will be very small. Most of the sound will be reflected.

Bulletproof glass is actually a multi-layered sandwich of glass and plastic—it's like the front windshield of a car, but with many more layers. When a bullet hits the surface of the sandwich, it begins to tear into the layers. But the bullet loses momentum before it manages to burrow all the way through to the final layers. The bullet's energy and momentum are transferred harmlessly to the layers of glass and plastic.


643. What is the chemical formula for glass? — GL, Birmingham, AL
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Glass isn't a simple molecule that can be represented by a normal chemical formula. It's a network solid in which the atoms are joined in one gigantic non-crystalline structure. In effect, a piece of glass is a single enormous molecule. Window glass is called soda-lime-silica glass and consists mostly of silicon, oxygen, sodium, and calcium atoms. Silicon and oxygen are considered to be network-forming atoms and bind to one another in long atomic linkages that form the backbone of the glass. The sodium and calcium atoms are added to terminate the linkages. This network termination softens the glass, lowers its softening and melting temperatures, and generally makes the glass easier to work with. Harder glasses such as lead "crystal" replace the sodium and calcium with other materials (e.g. lead oxide) that don't weaken the glass as much and produce harder or stronger glasses. Pyrex cookware contains boron instead of sodium and calcium, and is a borosilicate glass.

670. How do you create sculptures out of glass? - RD
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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.

1030. How is glass made?
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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.

1102. What is the difference between crystal and glass?
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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.

1216. 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
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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.


1297. 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
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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.


1298. Can you get a suntan or sunburn through glass? — SD, Farmington, Utah
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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.

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