Glass

This article refers to the material. For other uses, see Glass (disambiguation).

The material definition of glass is a uniform amorphous solid material, usually produced when a suitably viscous molten material cools very rapidly to below its glass transition temperature, thereby not giving enough time for a regular crystal lattice to form. By far the most familiar form of glass is the silica-based material used for windows, containers and decorative objects.

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 a great many uses of glass. Glass is, however, brittle and will break into sharp shards. These properties can be modified, or even changed entirely, with the addition of other compounds or heat treatment.

Common glass contains about 70% amorphous silicon dioxide (Si O₂), which is the same chemical compound found in quartz, and its polycrystalline form, sand.

Properties and uses

Glass can be made transparent and flat, or into other shapes and colours as shown in this ball from the Verrerie of Brehat in Brittany.

The obvious characteristic of ordinary glass is that it is transparent to visible light (not all glassy materials are). The transparency is due to an absence of electronic transition states in the range of visible light, and because ordinary glass is homogeneous on all length scales greater than about a wavelength of visible light. (Heterogeneities cause light to be scattered, breaking up any coherent image transmission). Ordinary glass partially blocks UVA (wavelength 400 and 300 nm) and it totally blocks UVC and UVB (wavelength lower than 300 nm). This is due to the addition of compounds such as soda ash (sodium carbonate).

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/Template:GermaniumO₂ 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 commonly 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).

Glass as a polymer

An innovative way for 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 good example of glass prepared in this way.

Colours

Metals and metal oxides are added to glass during its manufacture to change its colour. Manganese can be added in small amounts to remove the green tint lent by iron, or in higher concentrations to give glass an amethyst colour. Like manganese, selenium can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish colour. Small concentrations of cobalt (0.025 to 0.1%) yield blue glass. 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. Adding titanium produces yellowish-brown glass. Metallic gold, in very small concentrations (around 0.001%), produces a rich ruby-coloured glass, while lower concentrations produces a less intense red, often marketed as "cranberry". 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. 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. New coloured glasses are frequently discovered.

History of glass

Naturally occurring glass, such as obsidian, has been used since the stone age. Glass making instructions were first documented in Egypt around 1500 BC, when glass was 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, usually for vases and bottles. Glass was made from sand, plant ash and lime. The earliest use of glass was as a coloured, opaque, or transparent glaze applied to ceramics before they were fired. Small pieces of coloured glass were considered valuable and often rivalled precious gems as jewellery items. As time passed, it was discovered (most likely by a potter) that if glass is heated until it becomes semi-liquid, it can be shaped and left to cool in a new, solid, independently standing shape.

In the first century BC, somewhere at the eastern end of the Mediterranean, a new invention caused a true revolution in the glass industry. This was the discovery of glassblowing, both free-blowing and mould-blowing.

The colour of "natural glass" is green to bluish green. This colour is caused by the varying amounts of 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 popularity of cut clear glass soared, that of coloured glass diminished.

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. 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 as they managed to keep the process secret. 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 by William J. Blenko in the 1920s.

Art is sometimes etched into glass via acid or other caustic substance (causing the image to be eaten into the glass). Traditionally this was done by a trained artisan 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 popular glassware in the 1930s, which later became known as Depression glass.

Glass tools

Since glass is strong and unreactive, 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

Glass sculpture by Dale Chihuly at an exhibition in Kew Gardens, London, England. The piece is 13 feet (4 metres) high

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.

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 or lampworker, and these techniques are how most fine glassware is created. Glass that is manipulated in a kiln is called warm glass, and traditional stained glass work is commonly called cold glass work. 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), paperweights, marbles, beads, smoking pipes, bongs, dildos, 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

Main articles: Architectural Glass and Glazing

Glass has been used in buildings since the 11th century. Typical uses for glass in buildings include as a transparent material for windows in the building envelope, 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.

Several methods of producing glass for applications have been developed, including:

These glass types can be further utilized by the following processes:

See also Window.

Glass as a liquid

One common misconception is that glass is a super-cooled liquid of practically infinite viscosity when at room temperature (see amorphous solid). Supporting evidence that is often offered is that old windows are often thicker at the bottom than at the top. 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 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 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.

Writing in the American Journal of Physics, 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, is observed to sag under its 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.

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.

References

"Do Cathedral Glasses Flow?" Am. J. Phys., 66 (May 1998), pp 392–396
Noel C. Stokes; The Glass and Glazing Handbook; Standards Australia; SAA HB125-1998
Glass Beads from Anglo-Saxon Graves: A Study on the Provenance and Chronology of Glass Beads from Anglo-Saxon Graves, Based on Visual Examination by Birte Brugmann

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