Glass

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

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 household objects such as light bulbs and windows. Glass is a biologically inactive material that can be formed into smooth and impervious surfaces. When in tension, glass is brittle and will break into sharp shards. When in 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. Common glass contains about 70–72 % by weight of silicon dioxide (SiO₂). The major raw material is sand (or "quartz sand") that contains almost 100 % of 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, which is both flat and relatively light for a metal. As a result, the molten glass floats on top of the tin, thus giving it the name "float glass".

History of glass

Phoenicia and Egypt

Naturally occurring glass, such as obsidian, has been used since the stone age. According to Pliny the Elder, the Phoenicians made the first glass:^[1]

That the Phoenicians used glass as a glaze for pottery was known as early as 3000 BCE (Before Common Era). 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 BCE. 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 refractor 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.

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. 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.^[2] 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

The Roman Empire 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.^[3] 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. However, in the rest of the Empire, the use of glass declined and many techniques were forgotten. The production of glass did not completely stop, 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.^[4]

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. 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 products.

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.

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 this risk and set up glass furnaces in surrounding cities and as far afield as England and the Netherlands.

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

Main article: Glass art

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. Works of art in glass can be seen in a variety of museums, including the 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. 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, bottles, 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

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 as a 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 as although glass has a high theoretical yield stress, it is very susceptible to brittle (sudden) failure, and has a tendency to shatter due to 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 an 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, 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.

As insulation, glass (e.g. fiberglass) is also used. Coming in long, fluffy-looking sheet, it is commonly found in homes. fiberglass insulation is used particularly in attics - this is given an R-rating, denoting the insulating ability.

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.

<math>\mbox{Glass Property} = b_0 + \sum_{i=1}^n \left( b_iC_i + \sum_{k=i}^n b_{ik}C_iC_k \right)</math>

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.

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.

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.^[5] However, the notion that glass flows to an appreciable extent over extended periods of time is not supported by empirical research or theoretical analysis.

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.

One of the main reasons people believe glass to be a liquid is its apparent lack of a melting point. There is no temperature at which it simply melts, the viscosity just decreases as temperature rises.^[6]

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.^{[citation needed]}

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.^{[citation needed]} 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,^[7] 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.^{[citation needed]}
• 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.

Comparison with pitch

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

See also

• Aluminium oxynitride
• 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
• Glass container industry
• Glass-reinforced plastic
• Glass recycling
• Lexan
• Machine drawn cylinder sheet
• Magnifying glass
• Marvering
• Metallic glass
• Murano glass
• Obsidian (volcanic glass)
• Opaline glass
• Polished plate
• Prince Rupert's Drops
• Pyrex
• Recycling glass
• Sea glass
• Stained glass
• Toughened glass
• Trinitite
• Tyrone Crystal
• Venetian Glass
• Waterford Crystal
• Water glass

References

Bibliography

• Noel C. Stokes; The Glass and Glazing Handbook; Standards Australia; SAA HB125–1998
• Brugmann, Birte. Glass Beads from Anglo-Saxon Graves: A Study on the Provenance and Chronology of Glass Beads from Anglo-Saxon Graves, Based on Visual Examination. Oxbow Books, 2004. ISBN 1-84217-104-6

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