Glass is a material (see below) and a drinking vessel made of this material. See also glasses (spectacles).
In its pure form, Glass is a transparent, relatively strong, hard-wearing, essentially inert, and biologically inactive material which can be formed with very smooth and impervious surfaces. These desirable properties lead to the very many uses of glass. These properties can also be slightly modified, or entirely changed, with the addition of other compounds.
Typically, glass is brittle and will break into sharp shards.
Glasses are uniform amorphous solid materials, usually produced when a suitably viscous molten material cools very rapidly, thereby not giving enough time for a regular crystal lattice to form. Glasses can be made from many materials, although only a few varieties are in common use.
Common glass is mostly amorphous silicon dioxide (SiO2), which is the same chemical compound as quartz, or in its polycrystalline form, sand. Pure silica has a melting point of about 2000 Celsius (3632 Fahrenheit), so two other substances are always added to the sand in the glass-making process. One is soda (sodium carbonate Na2CO3), or potash, the equivalent potassium compound, which lowers the melting point to about 1000 Celsius. However, the soda makes the glass soluble, which is obviously unhelpful, so lime (calcium oxide, CaO) is the third component, added to restore insolubility.
Glass can be made extremely pure so that hundreds of kilometers of glass are transparent at infrared wavelengths in fibre optic cables.
Most common glass has other ingredients added to change its properties. Leaded glass is more brilliant, because of its increased refractive index, while boron may be added to change the thermal and electrical properties, as in Pyrex. Adding barium will also increase the refractive index. Thorium oxide gives glass a very high refractive index, and is used in producing high-quality lenses. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium is used in glass that absorbs UV wavelengths harmful to the eyes. Additional soda or potash is sometimes added to further lower the melting point.
Metals and metal oxides are added to glass during its manufacture to change its color. Manganese can be added in small amounts to remove the green tint lent by iron, or in higher concentrations to give glass an amethyst color. Like manganese, selenium can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish color. Small concentrations of cobalt (0.025%-0.1%) yield blue glass. 2-3% of copper oxide produces a turquoise color. Tin oxide with antimony and arsenic oxides produce an opaque white glass, first used in Venice to produce an imitation porcelain. Metallic copper produces a very dark red, opaque glass. Nickel, depending on the concentration, produces blue, violet, through black glass. Adding titanium metal produces yellowish-brown glass. Metallic gold, in very small concentrations (around 0.001%), produces a ruby-colored glass. Silver compounds (notably silver nitrate) can produce a range of colors from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colors produced by these compounds. The precise chemistry involved is complex and not well understood. New colored glasses are being discovered every year.
Glass is sometimes created naturally from volcanic flows in the form of obsidian. Obsidian can be used to make extremely sharp knives using primitive tools. In many countries, including the United States, obsidian is protected by law - tourists may not take it with them if found.
Naturally occurring glass, such as obsidian, has been used since the stone age. The first documented glass making is in Egypt around 2000 BC, when glass was first used as a glaze for pottery and other items. In the first century BC the technique of blowing glass was developed and what had once been an extremely rare and valuable item became much more common. During the Roman Empire many forms of glass were created mostly for use in vases and bottles. Early glass is green, from iron impurities in the sand used in its production. Common glass today usually has a slight green tint, arising from these same impurities.
The centre for glass making from the 14th century was Venice which developed many new techniques and became the center of a lucrative export trade in dinner ware, mirrors, and other luxury items. Eventually some of the Venetian glass workers moved to other areas of northern Europe and glass making spread with them.
The Crown glass process was used up to the mid-1800s, in which the glassblower would spin around 9 lb (4 kg) of molten glass at the end of a rod until it flattened into a disk approximately 5 ft (1.5 m) in diameter. The disk would then be cut into panes. Venetian glass was highly prized between the 10th and 14th 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.
The word glass,Latinglacis (ice) GermanGlas, 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 transparency as glass, to shine (glare) and its ability to melt.
There are many techniques for creating fine glass art; each is suitable for certain kinds of object and unsuitable for others. Someone who works with hot glass is called a glassblower, and this is how most fine glassware is created. Glass can also be cut with a diamond saw, and polished to give gleaming facets.
Objects made out of glass include vessels (bowls, vases, and other containers), marbles, beads, smoking pipes, bongs, and sculptures. Colored glass is often used, and sometimes the glass is painted, although many glassblowers consider this crude. A significant exception is the collection of pieces by the Blaschkas.
The Harvard Museum of Natural History has a collection of extremely detailed models of flowers made of painted glass. Hand-blown by Leopold Blaschka and his son Rudolph, they carried the secret of how these were made to their grave. The Blaschka glass flowers stand as an inspiration to glassblowers today. See the Harvard Museum of Natural History's page on the exhibit for further information.
Stained glass is an art form with a long history; many churches have beautiful stained-glass windows.
90% of the world's flat glass is produced by the float glass process invented by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to either side. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then fire-polished. The finished product has near-perfect parallel surfaces.
Glass is produced in the standard metric sizes of 2mm, 3mm, 4mm, 5mm, 6mm, 8mm, 10mm, 12mm, 15mm, 19mm and 22mm. Standard sheet sizes are in multiples of 1220mm in each direction (for example 3660 x 2440mm).
Annealed glass is considered a hazard in architectural applications as it breaks in large, jagged shards that can cause serious injury. Building legislation across the world restricts the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in doors panels, and at low heights in schools.
However, this strength comes with a penalty. Due to the balanced stresses in the glass, any damage to the glass edges will result in the glass shattering into thumbnail sized pieces. This is why the glass must be cut to size before toughening and cannot be re-worked once toughened. Also, ironically, the toughened glass surface is not as tough as annealed glass and is more susceptible to scratching.
Toughened glass is typically used in unframed assemblies such as frameless doors and in structurally loaded applications.
Toughened glass is considered a safety glass due to its increased strength and its tendency to shatter in small, rounded pieces.
It is sometimes claimed that glass may show some of the properties of liquids that flow at room temperature, albeit very slowly. This has led to controversial statements such as the claim that "glass is a supercooled liquid". 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.