Glass

For other uses, see the material.

The amorphous structure of glassy Silica (SiO₂). No long range order is present, however local ordering with respect to the tetrahedral arrangement of Oxygen (O) atoms around the Silicon (Si) atoms.

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

Macro detail from a glass bowl blown by artisans at The World of Glass Museum (incorporating the Pilkington Glass Museum), St. Helens, UK. The white effect is created by rolling the molten glass in white glass powder before spinning the glass out to make the bowl.

Glass is a noncrystalline material that can maintain indefinitely, if left undisturbed, its overall form and amorphous microstructure at a temperature below its glass transition temperature. It is formed via a supercooled liquid and hence if cooled sufficiently rapidly from its molten state through its glass transition temperature, T_g, the supercooled disordered atomic configuration at T_g, is frozen into the solid state. The structure of glass therefore exists in a metastable state with respect to its crystalline form. By definition as an amorphous solid, the atomic structure of a glass lacks any long range translational periodicity. However, by virtue of the local chemical bonding constraints glasses do posses a high degree of short-range order with respect to local atomic polyhedra. While nonceramic materials such as metals and polymers may be produced as a glass by rapidly cooling the materials from a liquid state to a temperature below their glass transition temperature, ordinary usage of the term "glass" refers to an amorphous form of ceramics, e.g., silica-based material, used for household objects such as light bulbs and windows. Common glass formulations contain about 70–72 % by weight of silicon dioxide (Si O₂). The most common form of glass is soda-lime glass, which contains nearly 30 % sodium and calcium oxides or carbonates. Pyrex is borosilicate glass containing about 10 % boric oxide. Lead crystal is a form of lead glass that contains a minimum of 24 % lead oxide. Silica glass may be produced by using sand as a raw material (or "quartz sand") that contains almost 100 % crystalline silica in the form of quartz. Although it is almost pure quartz, it may still contain a small amount (less than 1 %) of iron oxides that would color the glass, so this sand is usually depleted before production to reduce the iron oxide amount to less than 0.05 %. Large natural single crystals of quartz are pure silicon dioxide, and upon crushing are used for high quality specialty glasses. Synthetic amorphous silica, an almost 100 % pure form of quartz, is the raw material for the most expensive specialty glasses. The most common method for glass production is using molten tin, where the molten glass floats on top of the tin, thus giving it the name "float glass". Glass may be formed into smooth and impervious surfaces. Under tension, glass is brittle and will break into sharp shards. Under compression, pure glass can withstand a great amount of force. The properties of glass can be modified or changed with the addition of other compounds or heat treatment.

Properties and uses

The types and uses of glass for scientific and technical purposes are myriad, and range from applications involving the smallest of devices such as DNA microarrays to football field sized enormously powerful neodymium doped glass (as shown above) lasers used for laser fusion applications.

The most obvious characteristic of ordinary glass is that it is transparent to visible light (not all glassy materials are). This 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 between 400 and 300 nm) and completely blocks UVC and UVB (wavelengths shorter than 300 nm) 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 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 flintknappers, who have used it to make extremely sharp knives since the stone age. Collecting obsidian from national parks and other locations may be prohibited by law in some countries, but the same toolmaking techniques can be applied to industrially-made glass.glass is gay your gay

Glass ingredients

Pure silica (SiO₂) has a melting point of about 2,000° C (3,632° F). While pure silica can be made into glass for special applications (see fused quartz), other substances are added to common glass to simplify processing. One is sodium carbonate (Na₂CO₃), which lowers the melting point to about 1,000° C (1,832° F); "soda" refers to the original source of sodium carbonate in the soda ash obtained from certain plants. However, the soda makes the glass water soluble, which is usually undesirable, so "lime" (calcium oxide (CaO), generally obtained from limestone), some magnesium oxide (MgO) and aluminum oxide are added to provide for a better chemical durability. The resulting glass contains about 70 to 72 percent silica by weight and is called a soda-lime glass. Soda-lime glasses account for about 90 percent 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.

In 2006 Italian scientists created a new type of glass using extreme pressure and carbon dioxide. The substance was named amorphous carbonia(a-CO₂) which has an atomic structure resembling that of ordinary window glass .

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.

Colors

Metallic additives in the glass mix can produce a variety of colors. Here cobalt has been added to produce a bluish colored decorative glass.

The inside of a blue glass cup.

Glass appears colorless to the naked eye when it is thin, though it can be seen to be green when it is thick, or with the aid of scientific instruments. However, metals and metal oxides can be added to glass during its manufacture to change its color.

• 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.
• Sulphur, together with carbon and iron salts, is used to form iron polysulphides and produce amber glass ranging from yellowish to almost black. In borosilicate glasses rich in boron, sulphur imparts a blue color. With calcium it yields a 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 color. Manganese is one of the oldest glass additives, and purple manganese glass was used since early Egyptian history.
• Magnanese dioxide, which is black, is used to remove the green color from the glass; in a very slow process this is converted to sodium permanganate, a dark purple compound. In New England some houses built more than 300 years ago have window glass which is lightly tinted violet because of this chemical change; and such glass panes are prized as antiques.
• Selenium, like manganese, can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish color, 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 color.
• Pure metallic copper produces a very dark red, opaque glass, which is sometimes used as a substitute for gold in the production of ruby-colored 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 sulphur 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-colored 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 color . 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 colors from orange-red to yellow. The way the glass is heated and cooled can significantly affect the colors produced by these compounds. The chemistry involved is complex and not well understood.

Glass as a liquid

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 research or theoretical analysis.

From a more commonsense point of view, glass should be considered a solid since it is rigid according to everyday experience

A myth does exist that glass rods and tubes can bend under their own weight over time. To test this, in the 1920s, Robert John Rayleigh, son of the Nobel Prize winner John William Rayleigh, conducted an experiment on a 1 metre (~39 in) long, 5 millimetre (~3/16 in) thick glass rod, which was supported horizontally on two pins with a 300 gram (~0.66 lb) weight in the middle. Apart from the initial bending of 28 millimetre (~1.1 in), the position of the weight did not change until the end of the experiment, which lasted for 7 years. At the same time, another man, a worker of General Electric named K. D. Spenser, conducted a similar experiment independently. Two months after Rayleigh, he published his own results which also disproved the myth. Spenser suggested that the myth was composed before the 1920s, when the tubes were made by hand, and naturally some of them were curved to begin with. Over time the straight tubes were taken away, and only the curved ones remained. Some people probably thought it was the glass flowing.

Some people believe glass is a liquid due to its lack of a first-order phase transition where certain thermodynamic variables such as volume, entropy and enthalpy are continuous through the glass transition temperature. However, the glass transition temperature may be described as analogous to a second-order phase transition where the intensive thermodynamic variables such as the thermal expansivity and heat capacity are discontinuous. Despite this, thermodynamic phase transition theory does not entirely hold for glass and hence the glass transition cannot be classed as a genuine thermodynamic phase transition.

Although glass has properties of a supercooled liquid, it is generally classed as solid at room temperature. There is also the problem that a supercooled liquid is still a liquid — moves and behaves like a liquid, not a solid — but is below the freezing point of the material and will crystalize almost instantly if a crystal is added as a core.

Behavior 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, which is a property of liquid. 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.

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. Modern glass intended for windows is produced as float glass and is very uniform in thickness.

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

• 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. Similarly, prehistoric obsidian blades should have lost their edge; this is not observed either.
• 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 (causing a loss of focus), but this is due to elastic deformation and not because of the glass flowing over time; 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 centimetres (40 in).
• The "cathedral glass" phenomenon that is often cited as a demonstration of flow generally refers to old leaded glass windows in churches. The windows often appear to have sagged at their bottoms. On closer examination, it is found that the individual pieces of glass have remained flat, and that the bending has occurred at the soft lead "cames" that join the pieces. This bending may be largely due to successive thermal expansions and contractions of the glass over time, combined with the constant weight of the glass above. The lead cames are essentially plastic; that is, they tend not to recover their original shape after being distorted. Thus, successive temperature fluctuations are able to create progressive deformations, and the illusion of flow.

History of glass

Phoenicia and Egypt

A piece of Obsidian

Naturally occurring glass, such as obsidian, has been used since the stone age. According to Pliny the Elder, the Phoenicians made the first glass:

The tradition is that a merchant ship laden with nitrum being moored at this place, the merchants were preparing their meal on the beach, and not having stones to prop up their pots, they used lumps of nitrum from the ship, which fused and mixed with the sands of the shore, and there flowed streams of a new translucent liquid, and thus was the origin of glass.

Glass used as a glaze for pottery is known as early as 3000 BC. However, there is archaeological evidence to support the claim that the first glass was made in Mesopotamia. Glass beads, seals, and architectural decorations date from around 2500 BC. Glass was also discovered by Native Americans during the same time period.

The color of natural glass is green to bluish green. This color 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 colored glass by adding metallic compounds and mineral oxides to produce brilliant hues of red, green, and blue; the colors of gemstones. When gem-cutters learned to cut glass, they found clear glass was an excellent lifter of light. The earliest known beads from Egypt were made during the New Kingdom around 1500 BC and were produced in a variety of colors. They were made by winding molten glass around a metal bar and were highly prized as a trading commodity, especially blue beads, which were believed to have magical powers.

Core-formed amphoriskos (17 cm / 6.7 in tall) 1st century BC, Cyprus

The Egyptians also made small jars and clothing using the core-formed method. Glass threads were wound around a bag of sand tied to a rod. The glass was continually reheated to fuse the threads together. The glass-covered sand bag was kept in motion until the required shape and thickness was achieved. The rod was allowed to cool, then finally the bag was punctured and the rod removed. The Egyptians also created the first colored glass rods which they used to create colorful beads and decorations. They also worked with cast glass, which was produced by pouring molten glass into a mold, much like iron and the more modern crucible steel. By the 5th century BC this technology had spread to Greece and beyond. In the first century BC there were many glass centres located around the Mediterranean. Around this time, at the eastern end of the Mediterranean, glass blowing, both free-blowing and mould-blowing, was discovered.

Romans

Roman Cage Cup from the 4th Century A.D.

Roman Glass

During the Roman Empire craftsmen working as non-citizens developed many new techniques for the creation of glass. Through conquest and trade, the use of glass objects and the techniques used for producing them were spread as far as Scandinavia, the British Isles and China. This spreading of technology resulted in glass artists congregating in areas such as Alexandria in Egypt where the famous Portland Vase was created, the Rhine Valley where Bohemian glass was developed and to Byzantium where glass designs became very ornate and where processes such as enamelling, staining and gilding were developed. At this time many glass objects, such as seals, windows, pipes, and vases were manufactured. Window glass was commonly used during the 1st century BC. Examples found in Karanis, Egypt were translucent and very thick. After the fall of the Empire, the Emperor Constantine moved to Byzantium where the use of glass continued, and spead to the Islamic world, the masters of glass-vessel making in the later Middle Ages. However, in Europe, the use of glass declined and many techniques were forgotten. The production of glass did not completely stop; it was used throughout the Anglo-Saxon period in Britain. But it did not become common again in the West until its resurgence in the 7th century.

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. Around 1000 AD, an important technical breakthrough was made in Northern Europe when soda glass, produced from white pebbles and burnt vegetation 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.

A 16th Century Stained Glass Window

The 11th century saw the emergence in Germany of new ways of making sheet glass by blowing spheres. The spheres were swung out to form cylinders and then cut while still hot, after which the sheets were flattened. This technique was perfected in 13th century Venice. The 11th century also saw the emergence of glass mirrors in Islamic Spain. Until the 12th century, stained glass, glass with metallic and other impurities for coloring, was not widely used.

The Crown glass process was used up to the mid-1800s. In this process, the glassblower would spin approximately 9 pounds (4 kg) of molten glass at the end of a rod until it flattened into a disk approximately 5 feet (1.5 m) in diameter. The disk would then be cut into panes. Venetian glass was highly prized between the 10th and 14th centuries.

Murano glassmaking

Main article: Murano glass

The center for glass making from the 14th century was the island of Murano, which developed many new techniques and became the center of a lucrative export trade in dinnerware, mirrors, and other luxury items. What made Venetian Murano 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 soda ash obtained from the Levant, for which the Venetians held the sole monopoly. The Venetian ability to produce this superior form of glass resulted in a trade advantage over other glass producing lands. Murano’s reputation as a center for glassmaking was born when the Venetian Republic, fearing fire might burn down the city’s mostly wood buildings, ordered glassmakers to move their foundries to Murano in 1291. Murano's glassmakers were soon the island’s most prominent citizens. Glassmakers weren't allowed to leave the Republic, however. Many craftsmen, however, took a risk and set up glass furnaces in surrounding cities and as far afield as England and the Netherlands.

Renaissance glassmaking

Around 1688, a process for casting glass was developed, which led to its becoming a much more commonly used material.

Industrial revolution glassmaking

The invention of the glass pressing machine in 1827 allowed the mass production of inexpensive glass products.

The cylinder method of creating flat glass was used in the United States of America for the first time 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 glass.

The International Studio Glass Movement

With the dominance of Modernism in the arts, there was a broadening of artistic media throughout the 20th century. Indeed, glass was part of the curriculum at art schools such as the Bauhaus. Frank Lloyd Wright's glass windows are masterpieces not only of design, but of painterly composition as well. But while glass was being made and used as part of the Modernist art movement, the Studio Glass Movement did not really blossom until the 1960s in the United States. Unquestionably, great glass was being designed and made in Italy, Sweden and many other places, but it had more to do with previous traditions than building anything new. In the context of postwar America, artists associated with the American Craft Movement of the late 1940s and 1950s wanted to employ glass to make art. Harvey Littleton (often referred to as the "Father of the Studio Glass Movement") along with Dominic Labino staged a now-famous glass workshop at the Toledo Museum of Art in 1962. From that time, the notion of glass as an artistic spread through the United States. Numerous teaching programs were set up across the country. Littleton became an extremely important teacher and his students included Dale Chihuly among other important names. Fritz Dreisbach founded the Glass Art Society and traveled the country seeking to spread the seed of glass art. Other important glass ambassadors include: Erwin Eisch, Marvin Lipofsky, Richard Marquis, Benjamin Moore, David Huchthausen, Dan Dailey and many, many others.

While many think of the Pilchuck Glass School as the wellspring for glass art in America, it opened in 1971 after over 100 glass programs had been started around the country. Pilchuck, founded by John and Anne Gould Hauberg with Dale Chihuly, over time became the gathering point for glass artists from around the world. With the goal of self-expression and artistic creativity, glass artists working at Pilchuck assimilated every the techniques and design goals from around the world. In turn, Pilchuck became an extremely attractive place to teach. By the late 1970s, masters from Murano such as Checco Ongaro and Lino Tagliapietra had come and began teaching advanced techniques to the international students. Bertil Vallien--artist and designer at Kosta Boda--became a regular presence at Pilchuck as well and taught sculptural glass sandcasting for 14 years. With the United States as the gathering point, and specifically Seattle, the International Studio Glass Movement took hold and has continued to grow in stature, scale and scope. Now there are glass artists, glass galleries, and museums of contemporary glass art all around the world.

While the International Studio Glass Movement became a reality through what happened in the United States, it effects echoed back to the traditional glass centers such as Sweden, Italy and the Czech Republic. Now these places are also major centers for glass art which sees itself as participating in the Studio Glass Movement. As well, many artists of note are working in Australia, Britain, New Zealand, Japan, Denmark and around the globe. The Glass Art Society (geared to serving the artists) meets every year at different sites around the globe. Different forums for fans, dealers and collectors occur as well: SOFA Chicago, SOFA New York, the Pilchuck Auction, the Urban Glass Auction, etc. Glass Weekend at Wheaton Village in New Jersey is a biennial glass art fair that hosts 20 galleries from around the United States and, in 2007, from other countries including Italy, Canada, Australia, and England.

At least one author has noted that the shift in public perception of glass as art (as opposed to not art) came about by seeing glass in art galleries rather than in shops. This was an important mission of Anne Gould Hauberg: she begged and pressured several galleries in the Northwest to begin showing glass along with paintings and sculpture in the 1970s. William Traver Gallery opened in 1977 and can arguably be described as the most pioneering glass art gallery in the world. "Annie told me I should show some of the glassblowers who were working at Pilchuck," notes Traver, "She said she would support me if I did. She immediately bought the first piece of glass from me that I sold." Other major glass galleries to help establish Seattle as the center of the Studio Glass Movement included the Foster/White Gallery and the Elliot/Brown Gallery, both of which began showing glass at about the same time as William Traver. Major art galleries featuring glass around the United States also include: Habatat Galleries, Thomas Riley Galleries, Hawk Gallery, R. Duane Reed, Imago, Sardella Fine Art, Holsten Galleries, Douglas Heller Gallery, Daniel Kany Gallery, Bullseye Gallery, Snyderman Galleries, Leo Kaplan, Barry Friedman, Chappell Gallery, Friesen Gallery, Marx-Saunders Gallery, and the Maurrine Littleton Gallery.

Glass artifacts

A vase being created at the Reijmyre glassworks, Sweden

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, cathode ray tubes, 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

Main article: Glass art

Glass sculpture by Dale Chihuly at a 2005 exhibition in Kew Gardens, London, England. The piece is 13 feet (4 m) high

Lampworked glass beads and pendants illustrate some of the myriad colors and shapes of glass art.

Swan made of colored glass.

Beginning in the late 20th century, glass started to become highly collectable as art. While earlier modern glass masters such as Rene Lalique, Louis Comfort Tiffany, Emile Gallee, Carlo Scarpa and Paul Venini were sought after for important glass collections, the scale and ambition of glass art scaled new heights. Some important contemporary glass artists in glass include Dale Chihuly, Lino Tagliapietra, William Morris, Stanislaw Libensky, Bertil Vallien, Livio Seguso, Harvey Littleton, Dante Marioni, Dan Dailey, Sonja Blomdahl, Tom Patti, Stephen Rolfe Powell, Richard Marquis, Therman Statom, Hiroshi Yamano, Ann Robinson, Paul Marioni, Nancy Callan to name just a few.

Works of art in glass can be seen in a variety of museums, including the Chrysler Museum, the Museum of Glass in Tacoma, the Metropolitan Museum of Art, the Toledo Museum of Art, and Corning Museum of Glass, in Corning, NY, which houses the world's largest collection of glass art and history, with more than 45,000 objects in its collection.

Several of the most common techniques for producing glass art include: blowing, kiln-casting, fusing, slumping, pate-de-verre, hot-sculpting, and cold-working.

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 colored 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 not only traditional objects such as vessels (bowls, vases, bottles, and other containers), paperweights, marbles, beads, smoking pipes, bongs, but an endless range of sculpture and installation art as well. Colored glass is often used, though sometimes the glass is painted; notable examples of painted glass include the work of contemporary artists Walter Lieberman and Cappy Thompson. Innumerable examples exist of the use of stained glass, such as those by Tiffany-affiliated artist John La Farge, and contemporary artists such Jim Gary and Dick Weiss.

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.

Glass in buildings

Main articles: Architectural Glass, Glazing in architecture, and Window

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

It is also possible to use glass as a structural material, for example, in beams and columns, as well as in the form of "fins" for wind reinforcement, which are visible in many glass frontages like large shop windows. Safe load capacity is, however, limited; although glass has a high theoretical yield stress, it is very susceptible to brittle (sudden) failure, and has a tendency to shatter upon localized impact. This particularly limits its use in columns, as there is a risk of vehicles or other heavy objects colliding with and shattering the structural element. One well-known example of a structure made entirely from glass is the northern entrance to Buchanan Street subway station in Glasgow.

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

As insulation, glass (e.g., fiberglass) is also used. In the form of long, fluffy-looking sheets, it is commonly found in homes. Fiberglass insulation is used particularly in attics, and is given an R-rating, denoting the insulating ability.

The glass used for building facades usually has a green hue because of small amounts of iron oxide added to its mixture. The iron oxide prevents sunlight from discoloring clothes or other objects behind the glass.

Glass in vehicles

Main article: Car glass

Calculation of glass properties

Glass properties can be calculated through statistical analysis of glass databases such as SciGlass and Interglad. If the desired glass property is not related to crystallization (e.g., liquidus temperature) or phase separation, linear regression can be applied using common polynomial functions up to the third degree. Below is an example equation of the second degree. The C-values are the glass component concentrations like Na₂O or CaO in percent or other fractions, the b-values are coefficients, and n is the total number of glass components. The glass main component silica (SiO₂) is excluded in the equation below because of over-parametrization due to the constraint that all components sum up to 100%. Many terms in the equation below can be neglected based on correlation and significance analysis. Further details and examples are available at Glassproperties.com.

${\mbox{Glass Property}}=b_{0}+\sum _{i=1}^{n}\left(b_{i}C_{i}+\sum _{k=i}^{n}b_{ik}C_{i}C_{k}\right)$

The liquidus temperature has been modeled using neural networks regression in the following article: C. Dreyfus, G. Dreyfus: "A machine learning approach to the estimation of the liquidus temperature of glass-forming oxide blends"; J. Non-Cryst. Solids, vol. 318, 2003, p 63–78.

It is often required to optimize several glass properties simultaneously, including production costs. This can be performed in a spreadsheet as follows:

1. Listing of the desired properties;
2. Entering of models for the reliable calculation of properties based on the glass composition, including a formula for estimating the production costs;
3. Calculation of the squares of the differences (errors) between desired and calculated properties;
4. Reduction of the sum of square errors using the Solver option in Microsoft Excel with the glass components as variables. Other software (e.g. Microcal Origin) can also be used to perform these optimisations.

It is possible to weight the desired properties differently. Basic information about the principle can be found in the article: N. T. Huff, A. D. Call: "Computerized Prediction of Glass Compositions from Properties"; J. Am. Ceram. Soc., vol. 56, 1973, p 55–57.

Market structure

The global market for flat glass in 2005 was approximately 41 million tonnes. At the level of primary manufacture this represents a value of around $19 billion.[1]

See also

A decorative glass store in Rome

• Aluminium oxynitride
• Acrylic
• Art glass
• Aerogel
• Beveled glass
• Blenko Glass Company
• Blown plate
• Borosilicate glass
• Broad sheet
• Bulletproof glass
• Calumite
• Crystal glass
• Cylinder blown sheet
• Edinburgh crystal
• Favrile iridescent glass - Tiffany's technique to make stained glass art
• Fiberglass
• Float glass
• Fire polishing
• Glassblowing
• Glass container industry
• Glass-reinforced plastic
• Glass recycling
• Lexan
• Machine drawn cylinder sheet
• Magnifying glass
• Marvering
• Metallic glass
• Millefiori
• Murano glass
• Obsidian (volcanic glass)
• Opaline glass
• Polished plate
• Prince Rupert's Drops
• Pyrex
• Recycling glass
• Sea glass
• Smart glass
• Stained glass
• Supercooling
• Toughened glass
• Trinitite
• Tyrone Crystal
• Venetian Glass
• Waterford Crystal
• Water glass

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 1-84217-104-6

External links

• Corning Museum of Glass
• A comprehensive guide to art glass and crystal around the world
• Working Description Furnace & Moleria - Murano Glass
• Informative website about the glass industry
• Substances used in the Making of Colored Glass
• Almost 400 articles and images about glass (mostly art glass)

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