   #copyright

Glass

2007 Schools Wikipedia Selection. Related subjects: Materials science

   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.
   Enlarge
   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.

   Glass is a uniform material of arguable phase, usually produced when
   the viscous molten material cools very rapidly to below its glass
   transition temperature, without sufficient time for a regular crystal
   lattice to form. The most familiar form of glass is the Silica-based
   material used for windows.

   Glass is a biologically inactive material that can be formed into
   smooth and impervious surfaces. Glass is 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 70-72 weight % of silicon dioxide (SiO[2]).
   The major raw material is sand (or "quartz sand") that contains almost
   100% of crystalline silica in the form of quartz. Although it is an
   almost pure quartz, it may still contain a small amount (< 1%) of iron
   oxides that would colour the glass, so this sand is usually enriched in
   the factory to reduce the iron oxide amount to < 0.05%. Large natural
   single crystals of quartz are purer silicon dioxide, and upon crushing
   are used for high quality specialty glasses. Synthetic amorphous silica
   (practically 100% pure) is the raw material for the most expensive
   specialty glasses.

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.
   Enlarge
   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 SiO[2] 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 SiO[2]/GeO[2] 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 SiO[2] 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 ingredients

   Pure silica (SiO[2]) has a melting point of about 2,000 ° C (3,632 °
   F). It can be made into glass for special applications (see fused
   quartz), and other substances are added to common glass to simplify
   processing. One is soda ( sodium carbonate Na[2]CO[3]), which lowers
   the melting point to about 1,000° C (1,832° F). However, the soda makes
   the glass water soluble, which is usually undesirable, so lime (
   calcium oxide, CaO), some MgO and aluminium 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-CO[2]) 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.
   Enlarge
   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.
   Enlarge
   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 colour.
     * Iron(II) oxide results in bluish-green glass, frequently used for
       beer bottles. Together with chromium it gives a richer green
       colour, 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 imparts
       a blue colour. With calcium it yields a deep yellow colour.
     * 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 colour 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-colored glass.
     * Nickel, depending on the concentration, produces blue, or violet,
       or even black glass. Lead crystal with added nickel acquires
       purplish colour. 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 colour. 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 colour, 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 colour 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
       colour.
     * 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.wow

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 polynomial
   functions up to the third degree. The image below shows 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_iC_i +
          \sum_{k=i}^n b_{ik}C_iC_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
   Microsoft Excel 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
   between desired and calculated properties; 4) Reduction of the sum of
   squares using the Solver option in Microsoft Excel with the glass
   components as variables. 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.

History of glass

Phoenicia and Egypt

   A piece of Obsidian
   Enlarge
   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. Pliny wrote: "The tradition is that a merchant ship laden with
   nitrum (soda and potash) 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." That the
   Phoenicians used glass as a glaze for pottery was 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 B.C.

   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 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 refractor of light.
   The earliest known beads from Egypt were made during the New Kingdom,
   about 1500 BC and came 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 ones because they were reported to
   have magical powers.
   Core-formed amphoriskos (17 cm / 6.7 in tall) 1st century BC, Cyprus
   Enlarge
   Core-formed amphoriskos (17 cm / 6.7 in tall) 1st century BC, Cyprus

   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 colored glass rods which they used to create colorful beads and
   decorations, they also worked with cast glass. . By the 5th century BC
   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

   Roman Glass Beaker from the 4th Century A.D.
   Enlarge
   Roman Glass Beaker from the 4th Century A.D.

   The advent of the Roman Empire saw the development of many new
   techniques and as the Empire spread so did the popularity of glass.
   Through conquest and trade the use of glass objects and the techniques
   used for making glass were spread as far north as Scandinavia, the
   British Isles and China. This spreading of technology resulted in glass
   artists congregating in areas such as
   Roman Glass
   Enlarge
   Roman Glass

   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 processes such as enamelling,
   staining and gilding were developed. Window glass was quite 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
   Constatine moved to Byzantium where the use of glass continued.
   However, in the rest of the Empire the use of glass declined and many
   previously known techniques disappeared. Glass didn't completely go out
   of use, but it didn't become popular 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. 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.
   A 16th Century Stained Glass Window
   Enlarge
   A 16th Century Stained Glass Window

   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.

   The 11th century also saw the emergence of glass mirrors in Islamic
   Spain.

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

   The centre for glass making from the 14th century was Venice in the
   island of Murano, 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 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 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.

   History of Murano Glassmaking

   Murano’s reputation as a centre for glassmaking was born when the
   Venetian Republic, fearing fire and destruction to the city’s mostly
   wood buildings, ordered glassmakers to move their foundries to Murano
   in 1291. Murano glass is still interwoven with Venetian glass. Murano's
   glassmakers were soon the island’s most prominent citizens. By the 14th
   century, glass makers were allowed to wear swords, enjoyed immunity
   from prosecution by the Venetian state and found their daughters
   married into Venice’s most affluent families. Of course there was a
   catch: Glassmakers weren't allowed to leave the Republic. However, many
   craftsmen took this risk and set up glass furnaces in surrounding
   cities and as far afield as England and the Netherlands. Murano’s
   glassmakers held a monopoly on quality glassmaking for centuries,
   developing or refining many technologies including crystalline glass,
   enameled glass (smalto), glass with threads of gold (aventurine),
   multicolored glass (millefiori), milk glass (lattimo), and imitation
   gemstones made of glass. Today, the artisans of Murano are still
   employing these century-old techniques, crafting everything from
   contemporary art glass and glass jewelry to murano glass chandeliers
   and wine stoppers. The Promovetro Consortium was set up in 1985 to
   safeguard, promote and defend original glass production on the island
   of Murano, always a synonym for unique quality and style. The
   introduction of the Vetro Artistico® Murano mark represents a
   fundamental milestone for the Consortium which has been its sole
   administrator in Italy and abroad since 2001. The Consortium is the
   only body representing the Murano glassworks who can affix the origin
   mark to their products to make them recognisable on the market in an
   effort to oppose the numerous attempts at speculation and imitation
   which damage this Made in Italy symbol. The Consortium, authentic
   custodian of the Murano art, is committed to defending the glass making
   tradition of Murano by providing information to the consumer and
   through initiatives aimed at heightening public awareness of this
   unique, precious and inimitable product.

   English glass was still sufficiently poor that anything a witness saw
   through a closed window was not admissible as evidence in court until
   the late 1700s. 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 pound (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. 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.

Glass artifacts

   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

   Glass sculpture by Dale Chihuly at a 2005 exhibition sponsored by
   GlaxoSmithKline in Kew Gardens, London, England. The piece is 13 feet
   (4 m) high
   Enlarge
   Glass sculpture by Dale Chihuly at a 2005 exhibition sponsored by
   GlaxoSmithKline in Kew Gardens, London, England. The piece is 13 feet
   (4 m) high
   Hand-blown glass beads and pendants illustrate some of the myriad
   colors and shapes of glass art. The Canadian Nickel is for scale.
   Enlarge
   Hand-blown glass beads and pendants illustrate some of the myriad
   colors and shapes of glass art. The Canadian Nickel is for scale.

   Even with the availability of common glassware, hand blown or
   lampworked glassware remains popular for its artistry. Some artists in
   glass include Dale Chihuly, Lino Tagliapietra, Kenji Ito, Hans Godo
   Frabel, Rene Lalique, 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 colorless 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 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 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 in vehicles

   See also: sunroof, greenhouse, windshield.

Glass as a liquid

   One arguably justifiable belief is that glass is a liquid of
   practically infinite viscosity at room temperature and as such flows,
   though very slowly, similar to pitch. 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.

   A myth does exist that glass rods and tubes can bend under their own
   weight over time. To check it, 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.

   There is no clear answer to the question "Is glass solid or liquid?".
   In terms of molecular dynamics and thermodynamics it is possible to
   justify various different views that it is a highly viscous liquid, an
   amorphous solid, or simply that glass is another state of matter which
   is neither liquid nor solid.

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.

   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 centre 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 GeO[2] at room
       temperature is 10^32 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 (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).

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.

   Retrieved from " http://en.wikipedia.org/wiki/Glass"
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