Common glass is mostly amorphous silicon dioxode (SiO2), which is the same material as sand and quartz. Most common glass has other ingredients added to change the properties. Leaded glass is more brillant, while boron may be added to change the thermal and electrical properties. Adding barium will increase the refractive index, and cerium is used in glass that adsorbs 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 other colors.
Glass is usually produced when molten material cools very rapidly, not giving enough time for a regular crystal lattice to form. It is sometimes created naturally from volcanic flows in the form of obsidian.
History of Glass
Naturally occuring obsidian glass 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 1100's stained glass (which is glass with some metals added for color) glass was not widely used. 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.
Even with the availablity of comman glassware, there remains place for hand blown glassware. Some artists in glass include Sidney Waugh, René Lalique, 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 and is sometimes applied to any fine hand-blown glass.
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.
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.
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.
- Page devoted to the AFU glass flow controversy, with links to citations
- Page stating that glass does not flow
- Florin Neumann's glass does not flow page
- Page from the sci.physics newsgroup FAQ