Glass [3]
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Wikipedia-Article "Glass [3]"
This article refers to the material. For other uses, see Glass (disambiguation).
The materials definition of a 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. The resulting solid is amorphous, not crystalline 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.
The remainder of this article will be concerned with a specific type of glass—the silica-based glasses in common use as a building, container or decorative material.
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 is about 70% amorphous silicon dioxide (SiO2), which is the same chemical compound found in quartz, or in its polycrystalline form, sand.
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Properties and Uses
One of the most obvious characteristics 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 to the fact that such glass is homogeneous on all length scales greater than about a wavelength of visible light (inhomogeneities cause light to be scattered, breaking up any coherent image transmission). Ordinary glass does not allow light at a wavelength of lower than 400 nm, also known as ultraviolet light or UV, to pass. 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 hundreds of kilometres of glass are transparent at infrared wavelengths in fibre optic cables. Individual fibers are given an equally transparent cladding of SiO2/GeO2 glass, which has only slightly different optical properties (the germanium contributing to a lower index of refraction). Undersea cables have sections doped with erbium, which amplify transmitted signals by laser emission from within the glass itself.
Amorphous SiO2 is also used as a dielectric material in integrated circuits, due to the smooth and electrically neutral interface it forms with silicon.
Glasses used for making optical devices are commonly categorized using a six-digit glass code, or alternatively a letter-number code from the Schott Glass catalog. For example, BK7 is a low-dispersion borosilicate crown glass, and SF10 is a high-dispersion dense flint glass. The glasses are arranged by composition, refractive index, and Abbe number.
Glass is sometimes created naturally from volcanic magma. This glass is called obsidian, and is usually black with impurities. Obsidian is a raw material for flint knappers, who have used it to make extremely sharp knives since the stone age. Obsidian collection is prohibited by law in some places (including the United States), but the same toolmaking techniques can be applied to industrially-made glass.
Glass Ingredients
Pure silica (SiO2) has a melting point of about 2000 °C (3600 °F), and while it can be made into glass for special applications (see fused quartz), two other substances are always added to common glass to simplify processing. One is soda (sodium carbonate Na2CO3), or potash, the equivalent potassium compound, which lowers the melting point to about 1000 °C (1800 °F). However, the soda makes the glass water-soluble, which is obviously undesirable, so lime (calcium oxide, CaO) is the third component, added to restore insolubility. The resulting glass contains about 70% silica and is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass.
As well as soda and lime, most common glass has other ingredients added to change its properties. Lead glass, such as lead crystal or flint glass, is more 'brilliant' because the increased refractive index causes noticeably more 'sparkles', while boron may be added to change the thermal and electrical properties, as in Pyrex. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion, and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern glasses. Large amounts of iron are used in glass that absorbs infrared energy, such as heat absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs UV wavelengths (biologically damaging ionizing radiation).
Glass as a polymer
An innovative way for making glass involves preperation by polymerization. Putting in additives that modify the properties of glass is problematic, because the high temperature of preperation destroys most of them. By polymerizing glass it is possible to embed active molecules, such as enzymes, to add a new level functionality to the glass vessels.
Colors
Metals and metal oxides are added to glass during its manufacture to change its color. Manganese can be added in small amounts to remove the green tint lent by iron, or in higher concentrations to give glass an amethyst color. Like manganese, selenium can be used in small concentrations to decolorize glass, or in higher concentrations to impart a reddish color. 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 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. Adding titanium produces yellowish-brown glass. Metallic gold, in very small concentrations (around 0.001%), produces a rich ruby-colored glass, while lower concentrations produces a less intense red, often marketed as "cranberry". Uranium (0.1 to 2%) can be added to give glass a fluorescent yellow or green 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. 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. New colored glasses are frequently discovered.
History of glass
Naturally occurring glass, such as obsidian, has been used since the stone age. The first documented instructions for glass making is in Egypt around 1500 BC, when glass was used as a glaze for pottery and other items. In the first century BC the technique of blowing glass was developed and what had once been an extremely rare and valuable item became much more common. During the Roman Empire many forms of glass were created, usually for vases and bottles. Glass was made from sand, plant ash and lime. The earliest use of glass was as a colored, opaque, or transparent glaze applied to ceramics before they were fired. Small pieces of colored glass were considered valuable and often rivaled precious gems as jewelry items. As time passed, it was discovered (most likely by a potter) that if glass is heated until it becomes semi-liquid, it can be shaped and left to cool in a new, solid, independently standing shape. In the first century BC, somewhere at the eastern end of the mediterranean, a new invention caused a true revolution in the glass industry. This was the discovery of glassblowing, both free-blowing and mold-blowing. The color of "natural glass" is green to bluish green. This color is caused by the varying amounts of naturally occurring iron impurities in the sand. Common glass today usually has a slight green or blue tint, arising from these same impurities. Glassmakers learned to make colored glass by adding metallic compounds and mineral oxides to produce brilliant hues of red, green, and blue - the colors of gemstones. When gemcutters learned to cut glass, they found clear glass was an excellent refractor of light, the popularity of cut clear glass soared, that of colored glass diminished.
Glass objects from the 7th and 8th centuries have been found on the island of Torcello near Venice. These form an important link between Roman times and the later importance of that city in the production of the material. About 1000 AD, an important technical breakthrough was made in Northern Europe when soda glass was replaced by glass made from a much more readily available material: potash obtained from wood ashes. From this point on, northern glass differed significantly from that made in the Mediterranean area, where soda remained in common use.
The 11th century saw the emergence, in Germany, of new ways of making sheet glass by blowing spheres, swinging these out to form cylinders, cutting these while still hot, and then flattening the sheets. This technique was perfected in 13th century Venice.
Until the 12th century, stained glass (i.e., glass with some coloring impurities, usually metals) was not widely used.
The centre for glass making from the 14th century was Venice, which developed many new techniques and became the center of a lucrative export trade in dinner ware, mirrors, and other luxury items. Eventually some of the Venetian glass workers moved to other areas of northern Europe and glass making spread with them.
The Crown glass process was used up to the mid-1800s. In this process, the glassblower would spin around 9 lb (4 kg) of molten glass at the end of a rod until it flattened into a disk approximately 5 ft (1.5 m) in diameter. The disk would then be cut into panes. Venetian glass was highly prized between the 10th and 14th centuries as they managed to keep the process secret. Around 1688, a process for casting glass was developed, which led to its becoming a much more commonly used material. The invention of the glass pressing machine in 1827 allowed the mass production of inexpensive glass articles.
The Cylinder method was invented by William J. Blenko in the early 1900s.
Art is sometimes etched into glass via acid or other caustic substance (causing the image to be eaten into the glass). Traditionally this was done by a trained artisan after the glass was blown or cast. In the 1920s a new mold-etch process was invented, in which art was etched directly into the mold, so that each cast piece emerged from the mold 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 popular glassware in the 1930s, which later became known as Depression glass.
Glass tools
Since glass is strong and unreactive, it is a very useful material. Many household objects are made of glass. Drinking glasses, bowls, and bottles are often made of glass, as are light bulbs, mirrors, the picture tubes of computer monitors and televisions, and windows. In laboratories doing research in chemistry, biology, physics and many other fields, flasks, test tubes, lenses and other laboratory equipment are often made of glass. For these applications, borosilicate glass (such as Pyrex) is usually used for its strength and low coefficient of thermal expansion, which gives greater resistance to thermal shock and allows for greater accuracy in laboratory measurements when heating and cooling experiments. For the most demanding applications, quartz glass is used, although it is very difficult to work. Most such glass is mass-produced using various industrial processes, but most large laboratories need so much custom glassware that they keep a glassblower on staff. Volcanic glasses, such as obsidian, have long been used to make stone tools, and flint knapping techniques can easily be adapted to mass-produced glass.
Glass art
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Even with the availability of common glassware, hand blown or lampworked glassware remains popular for its artistry. Some artists in glass include Lino Tagliapietra, Sidney Waugh, Rene Lalique, Dale Chihuly, and Louis Comfort Tiffany, who were responsible for extraordinary glass objects. The term "crystal glass", derived from rock crystal, has come to denote high-grade colorless glass, often containing lead, and is sometimes applied to any fine hand-blown glass.
There are many techniques for creating fine glass art; each is suitable for certain kinds of object and unsuitable for others. Someone who works with hot glass is called a glassblower or lampworker, and these techniques are how most fine glassware is created. Glass that is manipulated in a kiln is called warm glass, and traditional stanined glass work is commonly called cold glass work. Glass can also be cut with a diamond saw, and polished to give gleaming facets.
Objects made out of glass include vessels (bowls, vases, and other containers), paperweights, marbles, beads, smoking pipes, bongs, and sculptures. Colored glass is often used, and sometimes the glass is painted, although many glassblowers consider this crude. A significant exception is the collection of pieces by the Blaschkas.
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 stand as 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.
Architectural glass
Float (annealed) glass
90% of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is annealed by cooling in a temperatured controlled oven called a "lehr". The finished product has near-perfect parallel surfaces.
A very small amount of the tin is imbedded in the glass on the side it touched. The tin side is easier to make into a mirror. This "feature" quickened the switch from plate to float glass.
Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly thicker glass is pushed back and not permitted to expand as it cools on the tin.
Annealed glass is considered a hazard in architectural applications as it breaks in large, jagged shards that can cause serious injury. Building codes across the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in door panels, fire exits and at low heights in schools.
Sheet glass
Before Pilkington's invention, flat glass panels were generally made as plate glass or sheet glass. Sheet glass (sometimes called window glass or drawn glass) was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat. This film or ribbon was pulled up continiously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. You may still see this glass in older houses. Float glass replaced this process.
Plate glass
The plate glass process starts with extruded or rolled glass that is rather rough. The rough panes are ground flat then pollished clear. This is a fairly expensive process. Before the float process, mirrors were plate glass as sheet glass had distortions that would be too objectionable if made into mirrors.
Cylinder glass
Glass is blown into a cylindrical iron mold. The ends are cut off and a cut is made down the side of the cylinder. The cut cylinder is then placed in an oven where the cylinder bends flat into a glass sheet. Before the introduction of the Pilkington method this was a popular method for glass manufacture. William J. Blenko used this method in the early 1900s to make stained glass. These imperfect panes have led to the misconception that glass is actually a high-viscosity liquid at room temperature, which is not the case. (See below.)
Insulated glazing
Main article: insulated glazing.
Insulated glazing, or double glazing is a piece of glazing consisting of two or more layers of glazing separated by a spacer along the edge and sealed to create a dead air space between the layers.
Toughened glass
Toughened glass (or tempered glass) a type of safety glass that has increased strength and a tendency to shatter in small, square pieces when broken. It is typically used in unframed assemblies such as frameless doors and in structurally loaded applications. Using toughened glass could pose a security risk in some situations due to the tendency the glass has to shatter utterly upon edge impact.
Toughened glass is typically assumed to be six times the strength of annealed glass. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin.
However, this strength comes with a penalty. Due to the balanced stresses in the glass, any damage to the glass edges will result in the glass shattering into thumbnail sized pieces. Because of this, the glass must be cut to size before toughening and cannot be re-worked once toughened. Also, ironically, the toughened glass surface is not as hard as annealed glass and is more susceptible to scratching.
Toughened glass is made from annealed glass via a thermal tempering process. The glass is cut to the required size and any required processing (such as polishing the edges or drilling holes in the glass) is carried out before the toughening process starts. Toughened glass can also be made by a chemical process where typically some of the sodium ions at the surface are replaced with potassium ions. This was used on some fighter aircraft canopies.
The glass is placed onto a roller table, taking it through a furnace which heats it to above its annealing point of 600 °C. The glass is then rapidly cooled with forced draughts of air. This rapidly cools the glass surface below its annealing point, causing it to harden and contract, while the inner portion of the glass remains free to flow for a short time. The final contraction of the inner layer induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. The pattern of cooling can be revealed by observing the glass with polarized light.
Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In the 1640s, Prince Rupert of Bavaria (1619–1682), who was grandson of James I of England, and nephew of Charles I, brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of the King. These are remarkable teardrop shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. The very rapid cooling produces tremendously high tensile stress in the glass giving it unusual qualities such as the ability to withstand a blow from a hammer on the bulbous end without breaking. However, even the smallest scratch on the "tail" of the drop will allow the large amount of potential energy contained in the internal stresses of the glass to be released, causing it to explosively shatter so thoroughly that it is converted to a fine powder.
The drops were often used as a practical joke, as the King would tell a subject to hold the bulb end in the palm of their hand while he broke the tip, producing a small explosion in the surprised person's hand. A video of the technique can be seen here [1].
Laminated glass
Laminated glass is a type of safety glass that holds together when shattered. In the event of breakage, it is held in place by an interlayer of PVB between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic "spiderweb" cracking pattern when the impact is not enough to completely pierce the glass.
Laminated glass is normally used when there is a possibility of human impact or where the glass could fall if shattered. Shopfront glazing and windshields are typically laminated glasses. The PVB interlayer also gives the glass a much higher sound insulation rating, due to the damping effect, and also blocks 99% of transmitted UV light. Using toughened glass on windshields would be a problem when a small stone hits the windshield at speed, if it were toughened and the stone hit with enough force the whole windshield would shatter into the small squares making visiblilty difficult and likely the wind would blow the small squares into the driver and passengers.
Laminated glass was invented in 1903 by the French chemist Edouard Benedictus, inspired by a laboratory accident. A glass flask had become coated with the plastic cellulose nitrate and when dropped shattered but did not break into pieces. Benedictus fabricated a glass-plastic composite to reduce injuries in car accidents. However, it was not immediately adopted by automobile manufacturers, and the first widespread use of laminated glass was in the eyepieces of gas masks during World War I.
Today, laminated glass is produced by bonding two or more layers of ordinary annealed glass together with a plastic interlayer, usually polyvinyl butyral (PVB). The PVB is sandwiched by the glass which is passed through rollers to expel any air pockets and form the initial bond then heated to around 70 °C in a pressuredized oil bath. The tint at the top of some car windshields is in the PVB.
A typical laminated makeup would be 3 mm glass / 0.38 mm interlayer / 3 mm glass. This gives a final product that would be referred to as 6.38 laminated glass.
Multiple laminates and thicker glass increases the strength. Bulletproof glass panels, made up of thick glass often toughened and several interlayers often thicker than that in windshields, can be as thick as 50 mm. A similar glass is often used in airliners on the front windows, often three sheets of 6mm toughened with thick PVB between them.
Low-emissivity glass
Metallic-based coatings applied to one or more surfaces of insulated glass can greatly decrease the glass unit's ability to transfer thermal energy, resulting in more efficient windows.
Self-cleaning glass
A recent innovation is so-called self-cleaning glass, aimed at building, automotive and other technical applications. A 50 nanometre coating of titanium dioxide on the outer surface of glass introduces two mechanisms which lead to the self-cleaning property. The first is a photocatalytic effect, in which ultra-violet rays catalyse the breakdown of organic compounds on the window surface; the second is a hydrophilic effect in which water is attracted to the surface of the glass, forming a thin sheet which washes away the broken-down organic compounds.
Evacuated glazing
Another recent innovation for insulated glazing is evacuated glass, which as yet is produced commercially only in Japan. The extreme thinness of evacuated glazing offers many new architectural possibilities, particularly in building conservation and historicist architecture, where evacuated glazing can replace traditional (much less energy-efficient) single glazing.
An evacuated glazing unit is made by sealing the edges of two glass sheets, typically by using a solder glass, and evacuating the space inside with a vacuum pump. The evacuated space between the two sheets can be very shallow and yet be a good insulator, yielding insulative window glass with nominal thicknesses as low as 6mm overall. The reasons for this low thickness are deceptively complex, but the potential insulation is good essentially because there can be no convection or gaseous conduction in a vacuum.
Unfortunately, evacuated glazing does have some disadvantages; its manufacture is complicated and difficult. For example, a necessary stage in the manufacture of evacuated glazing is outgassing; that is, heating it liberates any gasses adsorbed on the inner surfaces, which could otherwise later escape and destroy the vacuum. This heating process currently means that evacuated glazing cannot be toughened or heat-strengthened. If an evacuated safety glass is required, the glass must be laminated. The high temperatures necessary for offgassing also tend to destroy the highly effective "soft" low-emissivity coatings that are often applied to one or both of the internal surfaces (i.e. the ones facing the air gap) of other forms of modern insulative glazing, in order to prevent loss of heat through infra-red radiation. Slightly less effective "hard" coatings are still suitable for evacuated glazing, however.
Furthermore, because of the atmospheric pressure present on the outside of an evacuated glazing unit, its two glass sheets must somehow be held apart in order to prevent them flexing together and touching each other, which would defeat the object of evacuating the unit. The task of holding the panes apart is performed by a grid of spacers, which typically consist of small stainless steel discs that are placed around 20mm apart. The spacers are small enough that they are visible only at very close distances, typically up to 1m. However, the fact that the spacers will conduct some heat often leads in cold weather to the formation of temporary, grid-shaped patterns on the surface of an evacuated window, consisting either of small circles of interior condensation centered around the spacers, where the glass is slightly colder then average, or, when there is dew outside, small circles on the exterior face of the glass, in which the dew is absent because the spacers make the glass near them slightly warmer.
The conduction of heat between the panes, caused by the spacers, tends to limit evacuated glazing's overall insulative effectiveness. Nevertheless, evacuated glazing is still as insulative as much thicker conventional double glazing and tends to be stronger, since the two constituent glass sheets are pressed together by the atmosphere, and hence react practically as one thick sheet to bending forces. Evacuated glazing also offers very good sound insulation in comparison with other popular types of window glazing.
Glass as a liquid
One common belief is that glass is a super-cooled liquid of practically infinite viscosity when at room temperature. Supporting evidence for this position is that old windows are often thicker at the bottom than at the top. It is then assumed that the glass was once uniform, but has flowed to its new shape.
One possible source of this belief is that when panes of glass were commonly made by glassblowers, the technique that was used was to spin molten glass so as to create a round, mostly flat and even plate (the Crown glass process, described above). This plate was then cut to fit a window. The pieces were not, however, absolutely flat; the edges of the disk would be thicker because of centripetal forces. When actually installed in a window frame, the glass would be placed thicker side down for the sake of stability. Also, the sparkle is greater and the visual effect stronger when the thicker side is down. There is anecdotal evidence that occasionally such glass has been found thinner side down, as would be caused by carelessness at the time of installation.
Writing in the American Journal of Physics, physicist Edgar D. Zanotto states "...the predicted relaxation time for GeO2 at room temperature is 1032 years. Hence, the relaxation period (characteristic flow time) of cathedral glasses would be even longer" (Am. J. Phys, 66(5):392-5, May 1998). In layperson's terms, he wrote that glass at room temperature is very strongly on the solid side of the spectrum from solids to liquids.
Evidence against glass flow
- 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.
- Likewise, precision optical lenses and mirrors used in microscopes and telescopes should gradually deform and lose focus. This is also not observed.
See also
- Aluminum Oxynitride
- Art glass
- Beveled glass
- Bulletproof glass
- Fiberglass
- Magnifying glass
- Prince Rupert's Drops
- Pyrex
- Stained glass
- Glass-reinforced plastic
References
- "Do Cathedral Glasses Flow?" Am. J. Phys., 66 (May 1998), pp 392–396
- Noel C. Stokes; The Glass and Glazing Handbook; Standards Australia; SAA HB125-1998
- Glass Beads from Anglo-Saxon Graves: A Study on the Provenance and Chronology of Glass Beads from Anglo-Saxon Graves, Based on Visual Examination by Birte Brugmann
External links
- Corning Museum of Glass, especially Research, Teach, and Learn section.
- Is glass liquid or solid? by Philip Gibbs on the spr USENET physics FAQ
- Antique windowpanes and the flow of supercooled liquids
- article on the non-flowness of glass
- Page devoted to the AFU glass flow controversy, with links to citations
- Page stating that glass does not flow
- Substances used in the Making of Colored Glass
- Self-cleaning glass - Product information from Pilkington.
- The Straight Dope article on glass, article discusses why glass is a liquid treated as a solid
- Recycling Glass - Waste Management Issues