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. A simple example is when table sugar is melted and cooled rapidly by dumping the liquid sugar onto a cold surface. It has lost its crystalline structure and is now an amorphous, not crystalline entity, like the sugar was originally, which can be seen in its conchoidal fracture.
The word 'glass' comes from Latin glacies (ice) and corresponds to German Glas, M.E. glas, A.S. glaes. Germanic tribes used the word glaes to describe amber, recorded by Roman historians as glaesum. Anglo-Saxons used the word glaer for amber.
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 (SiO2), which is the same chemical compound found in quartz, and its polycrystalline form, sand.
Properties and uses
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 kilometers of glass are transparent at infrared wavelengths. Individual fibres are given an equally transparent core of SiO2/GeO2 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.
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.
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. 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 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).
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. 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. 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.
Stained glass
Stained glass can be used for decorative purposes, such as this tabletop
The term stained glass today generally refers to glass that has been colored by adding metallic salts during its manufacture. For example, copper can be used to produce green or blue glass. The molten glass is then annealed slowly in a furnace to produce sheets of colored glass. Early stained glass artists were limited to a very few primary colors, but today almost any color can be produced.
If fine details such as shadows or outlines are required, the artist paints them on the cold glass with special paint made from metal oxides. The piece is then fired in a kiln. This process is, in itself, an art. The oxides permanently fuse with the glass to produce the painting.
Glass tools
These colored glasses are available in many different textures—smooth, wavy, rippled, hammered, pebbled, or very rough. These different textures cause the glass to have light and color transmission characteristics that, even for the same color, can provide surprising results. Stained glass is sold by weight and by square foot in sheets, usually about 3' x 4'.
Stained glass windows involve the art of cutting colored glass into different shapes, then assembling the pieces using channeled lead-came strips, or copper-foil. Once assembled, the pieces are then soldered together and installed in a frame to create a window.
History
Begun in Eastern Asia and among Muslim designers, the art of stained glass reached its height in the Middle Ages, particularly 1150-1250. As the solid Romanesque wall was eliminated, the use of glass dramatically expanded. Integrated with the lofty verticals of Gothic cathedrals, large windows afforded greater illumination that was regarded as symbolic of divine grace.
In the nineteenth century, Romanticism and the Gothic revival caused renewed interest in stained glass. Important contributions to the art were made by William Morris (English, 1834-1898), Edward Burne-Jones (English, 1833-1898), John La Farge (American, 1835-1910) and Louis Comfort Tiffany (American, 1848-1933).
Historians have long argued about whether La Farge or Tiffany invented the copper-foil alternative to lead came, but Tiffany is universally credited with developing an opalescent colored glass and with making extensive use of copper-foil in windows, lamps, and other decorations.
Stained glass is an art and a craft that requires the artistic skill to conceive of the design and the engineering skills necessary to assemble the piece so that it is capable of supporting its own weight and (for a window) surviving the elements.
After centuries of repetition and little innovation, stained glass underwent a major renaissance of form. The impetus for this new modern glass was the restoration of thousands of church windows throughout Europe, destroyed by World War II. German artists led the way, notable artists include Ervin Bossanyi, Ludwig Schaffrath, Johannes Shreiter and many others who transformed an ancient art form into a contemporary art form.
Today there are few academic establishments that teach the traditional skillset. One of those establishments is Florida State University's Master Craftsman Program who recently completed the world's largest secular stained-glass windows installed in Bobby Bowden Field at Doak Campbell Stadium.
Art glass
Art glass normally means the modern art glass movement in which individual artists working alone or with a few assistants to create works from molten glass in relatively small furnaces of a few hundred pounds of glass. It began in the early 1960s and showed continued growth through the end of the century. The glass objects created are not primarily utilitarian but are intended to make a sculptural or decorative statement. On the market, their prices may range from a few hundred to tens of thousands of dollars (US).
Prior to the early 1960s, art glass would have referred to glass made for decorative use, usually by teams of factory workers, taking glass from furnaces with a thousand or more pounds of glass. In fact, the turn of the 19th Century was the height of the old art glass movement while the factory glass blowers were being replaced by mechanical bottle blowing and continuous window glass. In the factory, every member of the team does the same job repeatedly turning out dozens or hundreds of the same item in a days work.