Glasses are uniform amorphous solid materials. A glass is usually produced when molten material cools very rapidly, not giving enough time for a regular crystal lattice to form.
Common glass is mostly amorphous silicon dioxide (SiO2), which is the same chemical compound as quartz, or in its polycrystalline form, sand. One of the most obvious characteristics of glass is that it is transparent to visible light. This transparency is due to the fact that there are no atomic line transition states with the energy of visible light in the material that makes up glass. Extremely pure glass can be made so transparent that hundreds of kilometers of glass can be 'seen through' at infrared wavelengths in fibre optic cables.
Most common glass has other ingredients added to change its properties. Leaded glass is more brillant, because of its increased refractive index, while boron may be added to change the thermal and electrical properties. Adding barium will also increase the refractive index, and cerium is used in glass that absorbs infrared energy. Other metal oxides are added to change the color. Soda or potash are sometimes added to lower the melting point, and manganese can be added to remove unwanted colors.
It is sometime created naturally from volcanic flows in the form of obsidian.
History of Glass
Naturally occurring glass, such as obsidian, has been used since the stone age. The first documented glass making is in Egypt around 2000 B.C., where sheets of glass were created by the melting of sand on molten lead, which is now called "float glass" as the glass floats to the surface. During the Roman Empire many forms of glass were created, but until the 1100s stained glass (which is glass with some metals added for color) was not widely used. Also used up to the mid-1800s was the Crown glass process, in which a glassblower would spin around 9 lbs. of molten glass at the end of a rod until it flattened into a disk approximately 5 feet in diameter. The disk would then be cut into panes. Venetian glass was highly prized between the tenth and fourteenth centuries as they kept the process secret. Around 1688, the process for casting glass was developed, which led to it being a much more commonly used material. The invention of the glass pressing machine in 1827 allowed the mass production of inexpensive glass articles.
Glass is transparent, strong, hard-wearing, largely chemically inert, and biologically inactive and can be formed with very smooth and impervious surfaces, hence the very many uses of glass.
The word glass, Latin glacis (ice) German Glas, M.E. glas, A.S. glaes was also used by the Aesti-Old Prussians. They used the word glaes to describe amber, recorded by Roman historians as glaesum. Angle-Saxons used the word glaer for amber. Another German word for amber Bernstein (english translation : burning stone) came into use because of its tranparency as glass, to shine (glare) and its ability to melt.
Fine glassware
Even with the availablity of common glassware, there remains place for hand blown glassware. Some artists in glass include Sidney Waugh, René Lalique, Dale Chihuly, and Louis Tiffany, who were responsible for extraordinary glass objects. The term "crystal glass", derived from rock crystal, has come to denote high-grade colorless glass, often containing lead, and is sometimes applied to any fine hand-blown glass.
Toughened glass
Glass is a tremendously strong material, but very brittle. Glass articles intended to survive rough handling, or rapid temperature changes, are sometimes toughened by rapid and localised cooling of their surfaces during the manufacturing process (called "tempering"). This prestresses the material and reduces its tendency to crack at the surface when stressed. When tempered glass does shatter, it tends to break into rounded granules that are not as dangerous as ordinary glass shards. The pattern of cooling is revealed by observing the glass with polarised light.
Laminated glass
Large sheets of glass can cause a serious hazard when broken, as they tend to form shards with very sharp edges. This risk is unacceptable in applications such as automobile windscreens, or large shop windows. This hazard can be reduced by laminating the glass with layers of plastic material. Laminated glass tends to hold together when shattered, the shards bound to the flexible plastic layer where they are less likely to cause injury. Often, large sheets of glass will be both toughened and laminated. The laminate can be either within the bulk of the material, or applied to the surface.
Does glass flow?
It is sometimes claimed that glass may show some of the properties of liquids that flow at room temperature, albeit very slowly. As an example, it is sometimes claimed that old windows are often thicker at the bottom than at the top, and that this might be due to flow. It is a bit unclear where this belief came from, or if there was ever any evidence to support it.
One possible source of this belief is that when panes of glass were commonly made by glassblowers, the technique that was used was to spin molten glass so as to create a round, mostly flat and even plate (the Crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk would be thicker due to centrifugal forces. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability. There is anecdotal evidence that occasionally such glass has been found thinner side down, as would be caused by carelessness at the time of installation.
The "glass flows" issue has been discussed at great length in the alt.folklore.urban newsgroup, and the consensus there (supported by citations from glass experts) is that glass does not flow at room temperature. Note, however, that glass can and does 'creep' , just like crystalline solids do, in response to a load. Furthermore, in some applications (such as some laboratory thermometers), glass gets heated above the transition temperature at which it actually does become a supercooled liquid. This can cause the calibration of thermometers to change slightly over the course of many years of use.
Arguments against glass flow:
- if medieval glass has flowed perceptibly, then ancient Roman and Egyptian objects should have flowed proportionately more - but this is not observed.
- if glass flows at a rate that allows changes to be seen with the naked eye after centuries, then changes in optical telescope mirrors should be observable (by interferometry) in a matter of days - but this also not observed.
Pitch, on the other hand, is a highly viscous liquid which appears solid, and, unlike glass, does flow at room temperature, very very slowly. See pitch drop experiment for more details.
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