In its pure form glass is a transparent, strong, hard-wearing, essentially inert, and biologically inactive material that can be formed with very smooth and impervious surfaces. Glass is, however, brittle and will break into sharp shards. These properties can be modified or changed with the addition of other compounds or heat treatment.

Common glass contains about 70% amorphous silicon dioxide (SiO₂), which is the same chemical compound found in quartz and in its polycrystalline form, sand.

Properties and uses

Pure SiO₂ glass (also called fused quartz) does not absorb UV light and is used for applications that require transparency in this region, although it is more expensive. This type of glass can be made so pure that, when made into fibre optic cables, hundreds of kilometres of glass are transparent at infrared wavelengths. Individual fibres are given an equally transparent core of SiO₂/O₂ glass, which has only slightly different optical properties (the germanium contributing to a higher index of refraction). Undersea cables have sections doped with erbium, which amplify transmitted signals by laser emission from within the glass itself. Amorphous SiO₂ is also used as a dielectric material in integrated circuits, due to the smooth and electrically neutral interface it forms with silicon.

Glasses used for making optical devices are categorized using a six-digit glass code, or alternatively a letter-number code from the Schott Glass catalogue. For example, BK7 is a low-dispersion borosilicate crown glass, and SF10 is a high-dispersion dense flint glass. The glasses are arranged by composition, refractive index, and Abbe number.

Glass is sometimes created naturally from volcanic magma. This glass is called obsidian, and is usually black with impurities. Obsidian is a raw material for flint knappers, who have used it to make extremely sharp knives since the stone age. Collecting obsidian from national parks and some places may be prohibited by law in some countries, but the same toolmaking techniques can be applied to industrially-made glass.

Glass ingredients

Pure silica (SiO₂) has a melting point of about 2000 °C (3600 °F), and while it can be made into glass for special applications (see fused quartz), two other substances are always added to common glass to simplify processing. One is soda (sodium carbonate Na₂CO₃), or potash, the equivalent potassium compound, which lowers the melting point to about 1000 °C (1800 °F). However, the soda makes the glass water-soluble, which is usually undesirable, so lime (calcium oxide, CaO) is the third component, added to restore insolubility. The resulting glass contains about 70% silica and is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass.

As well as soda and lime, most common glass has other ingredients added to change its properties. Lead glass, such as lead crystal or flint glass, is more 'brilliant' because the increased refractive index causes noticeably more "sparkles", while boron may be added to change the thermal and electrical properties, as in Pyrex®. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion, and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern glasses. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs UV wavelengths (biologically damaging ionizing radiation).

Glasses that do not include silica as a major constituent are sometimes used for fibre optics and other specialized technical applications. These include fluorozirconate, fluoroaluminate, and chalcogenide glasses.

Glass as a polymer

An innovative way of making glass involves preparation by polymerization. Putting in additives that modify the properties of glass is problematic, because the high temperature of preparation destroys most of them. By polymerizing glass it is possible to embed active molecules, such as enzymes, to add a new level of functionality to the glass vessels. Sol gel is a very good example of glass prepared in this way.

Colours

Metals and metal oxides are added to glass during its manufacture to change its colour.

• Iron(II) oxide results in bluish-green glass, frequently used for beer bottles. Together with chromium it gives a richer green color, used for wine bottles.
• Sulfur, together with carbon and iron salts, is used to form iron polysulfides and produce amber glass ranging from yellowish to almost black. In borosilicate glasses rich in boron, sulfur impairs blue color. With calcium it yields deep yellow color. ^*
• Manganese can be added in small amounts to remove the green tint given by iron, or in higher concentrations to give glass an amethyst colour. Manganese is one of the oldest glass additives, and purple manganese glass was used since early Egyptian history.
• Selenium, like manganese, can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish colour, caused by selenium atoms dispersed in glass. It is a very important agent to make pink and red glass. When used together with cadmium sulfide, it yields a brilliant red color known as "Selenium Ruby".
• Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. The best results are achieved when using glass containing potash. Very small amounts can be used for decolorizing.
• Tin oxide with antimony and arsenic oxides produce an opaque white glass, first used in Venice to produce an imitation porcelain.
• 2 to 3% of copper oxide produces a turquoise colour.
• Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production of ruby-coloured glass.
• Nickel, depending on the concentration, produces blue, or violet, or even black glass. Lead crystal with added nickel acquires purplish color. Nickel together with small amount of cobalt was used for decolorizing of lead glass.
• Chromium is a very powerful colorizing agent, yielding dark green or in higher concentrations even black color. Together with tin oxide and arsenic it yields emerald green glass. Chromium aventurine, in which aventurescence was achieved by growth of large parallel chromium(III) oxide plates, was also made from glass with added chromium.
• Cadmium together with sulfur results in deep yellow color, often used in glazes. However cadmium is toxic.
• Adding titanium produces yellowish-brown glass. Titanium is rarely used on its own, is more often employed to intensify and brighten other colorizing additives.
• Metallic gold, in very small concentrations (around 0.001%), produces a rich ruby-coloured glass ("Ruby Gold"), while lower concentrations produces a less intense red, often marketed as "cranberry". The color is caused by the size and dispersion of gold particles. Ruby gold glass is usually made of lead glass with added tin.
• Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green colour. Uranium glass is typically not radioactive enough to be dangerous, but if ground into a powder, such as by polishing with sandpaper, and inhaled, it can be carcinogenic. When used with lead glass with very high proportion of lead, produces a deep red color.
• Silver compounds (notably silver nitrate) can produce a range of colours from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colours produced by these compounds. The chemistry involved is complex and not well understood.

History of glass

Phoenicia and Egypt

The color of "natural glass" is green to bluish green. This colour is caused by naturally occurring iron impurities in the sand. Common glass today usually has a slight green or blue tint, arising from these same impurities. Glassmakers learned to make coloured glass by adding metallic compounds and mineral oxides to produce brilliant hues of red, green, and blue - the colours of gemstones. When gem-cutters learned to cut glass, they found clear glass was an excellent refractor of light. The earliest known beads from Egypt were made during the New Kingdom, about 1500 BC and came in a variety of colours. They were made by winding molten glass around a metal bar and were highly prized as a trading commodity, especially blue ones because they were reported to have magical powers.

The Egyptians also made small jars and bottles using the core-formed method. Glass threads were wound around a bag of sand tied to a rod and the glass was continually reheated to fuse the threads together. The glass had to be kept in motion until the required shape and thickness was achieved. The final step was to allow the rod to cool then to puncture the bag and remove the rod. The Egyptians also formed the first coloured glass rods which they used to create colourful beads and decorations, they also worked with cast glass. Glassmaking in Antiquity Hampton, Susan. Retrieved 24 May 2006. By the 5th century BCE this technology had spread to at least Greece. In the first century BC there were many glass centres located around the Mediterranean and at the eastern end of the Mediterranean glass blowing, both free-blowing and mould-blowing, was discovered.

Romans

Europe

Glass objects from the 7th and 8th centuries have been found on the island of Torcello near Venice. These form an important link between Roman times and the later importance of that city in the production of the material. About 1000 AD, an important technical breakthrough was made in Northern Europe when soda glass was replaced by glass made from a much more readily available material: potash obtained from wood ashes. From this point on, northern glass differed significantly from that made in the Mediterranean area, where soda remained in common use. The 11th century saw the emergence, in Germany, of new ways of making sheet glass by blowing spheres, swinging these out to form cylinders, cutting these while still hot, and then flattening the sheets. This technique was perfected in 13th century Venice.

Until the 12th century, stained glass (i.e., glass with some colouring impurities, usually metals) was not widely used.

The centre for glass making from the 14th century was Venice, which developed many new techniques and became the centre of a lucrative export trade in dinner ware, mirrors, and other luxury items. What made Venetian glass significantly different was that the local quartz pebbles were almost pure silica and were ground into a fine clear sand that was combined with another locally occurring product called "Levant soda ash", for which the Venetians held the sole monopoly. This resulted in the Venetians producing a superior form of glass which resulted in them having a trade advantage over other glass producing lands. 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 this process, 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. Around 1688, a process for casting glass was developed, which led to its becoming a much more commonly used material. The invention of the glass pressing machine in 1827 allowed the mass production of inexpensive glass articles.

The cylinder method of creating flat glass was first used in the United States of America in the 1820s. It was used to commercially produce windows. This and other types of hand-blown sheet glass was replaced in the 20th century by rolled plate.

See also: Broad sheet, Blown plate, Polished plate, Cylinder blown sheet, Machine drawn cylinder sheet

Glass tools

Since glass is strong and non-reactive, it is a very useful material. Many household objects are made of glass. Drinking glasses, bowls, and bottles are often made of glass, as are light bulbs, mirrors, the picture tubes of computer monitors and televisions, and windows. In laboratories doing research in chemistry, biology, physics and many other fields, flasks, test tubes, lenses and other laboratory equipment are often made of glass. For these applications, borosilicate glass (such as Pyrex) is usually used for its strength and low coefficient of thermal expansion, which gives greater resistance to thermal shock and allows for greater accuracy in laboratory measurements when heating and cooling experiments. For the most demanding applications, quartz glass is used, although it is very difficult to work. Most such glass is mass-produced using various industrial processes, but most large laboratories need so much custom glassware that they keep a glassblower on staff. Volcanic glasses, such as obsidian, have long been used to make stone tools, and flint knapping techniques can easily be adapted to mass-produced glass.

Glass art

Even with the availability of common glassware, hand blown or lampworked glassware remains popular for its artistry. Some artists in glass include Lino Tagliapietra, Rene Lalique, Dale Chihuly, and Louis Comfort Tiffany, who were responsible for extraordinary glass objects. The term "crystal glass", derived from rock crystal, has come to denote high-grade colourless glass, often containing lead, and is sometimes applied to any fine hand-blown glass such as Edinburgh Crystal and other brands.

Someone who works with hot glass is called a glassblower or lampworker, and these techniques are how most fine glassware is created. Warm glass refers to the technique of manipulating glass in a kiln .

Cold work includes traditional stained glass work as well as other methods of shaping glass at room temperature. Glass can also be cut with a diamond saw, or copper wheels embedded with abrasives, and polished to give gleaming facets; the technique used in creating waterford crystal. Art is sometimes etched into glass via the use of acid, caustic, or abrasive substances. Traditionally this was done after the glass was blown or cast. In the 1920s a new mould-etch process was invented, in which art was etched directly into the mould, so that each cast piece emerged from the mould with the image already on the surface of the glass. This reduced manufacturing costs and, combined with a wider use of coloured glass, led to cheap glassware in the 1930s, which later became known as Depression glass. As the types of acids used in this process are extremely hazardous, abrasive methods have gained popularity.

Objects made out of glass include vessels (bowls, vases, and other containers), paperweights, marbles, beads, smoking pipes, bongs, and sculptures. Colored glass is often used, though sometimes the glass is painted; notable examples of painted glass include the work of contemporary artists Judith Schaechter and Walter Lieberman. Innumerable examples exist of the use of stained glass, such as those by John La Farge in Boston's Trinity Church, or the life-sized sculptures among the fine art of Jim Gary.

The Harvard Museum of Natural History has a collection of extremely detailed models of flowers made of painted glass. These were lampworked by Leopold Blaschka and his son Rudolph, who never revealed the method he used to make them. The Blaschka Glass Flowers are still 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.

Glass in buildings

Glass has been used in buildings since the 11th century. Uses for glass in buildings include as a transparent material for windows, as internal glazed partitions and as architectural features.

Glass in buildings can be of a safety type, including wired, toughened and laminated glasses. Glass fibre insulation is common in roofs and walls. Foamed glass, made from waste glass, can be used as lightweight, closed-cell insulation.

Glass as a liquid

One common misconception is that glass is a super-cooled liquid of practically infinite viscosity at room temperature and as such flows, though very slowly. Glass is generally treated as an amorphous solid rather than a liquid, though different views can be justified since characterizing glass as either 'solid' or 'liquid' is not an entirely straightforward matter ^*. However, the notion that glass flows to an appreciable extent over extended periods of time is not supported by empirical evidence or theoretical analysis.

Behaviour of antique glass

The observation that old windows are often thicker at the bottom than at the top is often offered as supporting evidence for the view that glass flows over a matter of centuries. It is then assumed that the glass was once uniform, but has flowed to its new shape.

The likely source of this belief is that when panes of glass were commonly made by glassblowers, the technique 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 because of centrifugal forces. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability and visual sparkle. Occasionally such glass has been found thinner side down, as would be caused by carelessness at the time of installation.

According to the Corning Museum of Glass, mass production of glass window panes in the early twentieth century caused a similar effect. In glass factories, molten glass was poured onto a large cooling table and allowed to spread. The resulting glass is thicker at the location of the pour, located at the center of the large sheet. These sheets were cut into smaller window panes with nonuniform thickness.

Several other points indicate that the 'cathedral glass' theory is misconceived:

• Writing in the American Journal of Physics* "Do Cathedral Glasses Flow?" Am. J. Phys., 66 (May 1998), pp 392–396, physicist Edgar D. Zanotto states "...the predicted relaxation time for GeO₂ at room temperature is 10³² years. Hence, the relaxation period (characteristic flow time) of cathedral glasses would be even longer" (Am. J. Phys, 66(5):392-5, May 1998). In layman's terms, he wrote that glass at room temperature is very strongly on the solid side of the spectrum from solids to liquids.
• 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 is not observed. Similarly, it should not be possible to see Newton's rings between decade-old fragments of window glass—but this can in fact be quite easily done.
• Glass in refracting telescopes, with objective lenses of large diameter, are observed to sag under their own weight. This sag happens because the lens is only supported around its edge. The result is a loss of focus, and occurs not because the glass is flowing over time, but because it is not infinitely rigid. This (along with chromatic aberration and other effects) limits the size of refracting telescopes, with the largest refractor in the world being the Yerkes Observatory telescope with a diameter of 102 cm.

Comparison with pitch

Note that pitch, another seemingly-solid material, is in fact a highly viscous liquid, 100 billion times as viscous as water. This property can be seen in the University of Queensland's pitch drop experiment, where each drop has taken approximately 10 years to fall into the beaker.

See also

• Aluminium Oxynitride
• Art glass
• Beveled glass
• Bulletproof glass
• Edinburgh crystal
• Fibreglass
• Glass-reinforced plastic
• Glass Container Industry
• Crystal
• Lexan
• Magnifying glass
• Opaline glass
• Prince Rupert's Drops
• Pyrex
• Recycling glass
• Stained glass
• Favrile Iridescent Glass - Tiffany's technique to make stained glass art
• Sea glass

References

Bibliography

• Noel C. Stokes; The Glass and Glazing Handbook; Standards Australia; SAA HB125-1998
• Brugmann, Birte. Glass Beads from Anglo-Saxon Graves: A Study on the Provenance and Chronology of Glass Beads from Anglo-Saxon Graves, Based on Visual Examination. Oxbow Books, 2004. ISBN 1842171046

External links

• Corning Museum of Glass, especially Research, Teach, and Learn section.
• Is glass liquid or solid? by Philip Gibbs on the spr USENET physics FAQ
• Antique windowpanes and the flow of supercooled liquids
• article on the non-flowness of glass
• Page devoted to the AFU glass flow controversy, with links to citations
• Page stating that glass does not flow
• Substances used in the Making of Colored Glass
• Self-cleaning glass - Product information from Pilkington.
• The Straight Dope article on glass, article discusses why glass is a liquid treated as a solid
• Recycling Glass - Waste Management Issues
• The Physics of Glass Just Got Blurry

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