Category: windows and glass

What is the chemical formula for glass? — GL, Birmingham, AL

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.

How do sound proof and bulletproof glasses work? – DH

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.

How do color-changing eyeglasses work?

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.

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?

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.