Glass (Forensic Science)
When glass breaks, fragments of the material shower other objects and people in the vicinity, and often tiny pieces of glass become embedded in clothing, shoes, and other objects. When glass fragments are found at a crime scene, forensic scientists can collect them and compare them with the various sources of glass at the scene. Such analysis consists mainly of the comparison of density and refractive index. Although these comparisons can link the fragments to a particular kind of source, they are not sufficient to link the fragments to one source to the exclusion of all other sources.
For density determinations, two thin glass columns are filled with a liquid mixture. The glass fragment of interest is placed in one column, and a similarly sized piece of possible source glass is placed in the second. Each piece of glass will float at the level where the density of the liquid is equal to the density of the glass. Therefore, the two fragments will float at the same level if they have the same density. If the fragments cannot be distinguished by density, the refractive index can be determined.
For refractive index determinations, the glass fragment is immersed in an oil of known refractive index and viewed through a microscope. A bright halo, known as the Becke line, is observed around the fragment. As the microscope stage is lowered, the Becke line will move into the medium (oil or glass) that has the higher refractive index. This...
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Further Reading (Forensic Science)
Caddy, Brian, ed. Forensic Examination of Glass and Paint: Analysis and Interpretation. New York: Taylor & Francis, 2001.
Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006.
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 9th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2007.
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Background (Encyclopedia of Global Resources)
Glass, although it has been a commonplace material for centuries, is an exceptional substance: It is a solid that is technically considered a liquid. All other familiar solids are crystalline in structure. That is, they possess a definite, orderly internal geometric form that is a reflection of the arrangement of their constituent atoms. Their atoms are packed in repetitive forms called crystal networks or lattices. Liquids, in contrast, are termed amorphous in structure. They lack the rigid, repeating internal structure of solids. Glasses can be considered a borderline case between classic solids and liquids, and they have been called “amorphous solids.”
Glasses are considered to be “supercooled” liquids—liquids chilled so rapidly that they never undergo the crystallization process of true solids. When a solid’s molecules cool down from a molten state, the material undergoes a series of internal dynamic changes in response to the loss of heat. Molecules move in a more rigid fashion until reaching a point at which their patterns of movement and their interatomic bonds reach a state of discontinuity. This point of discontinuity is commonly called the freezing point of the solid; at this point it begins rapidly to lock into the pattern of crystallinity. Liquids, such as glasses, never actually reach this point of discontinuity and are considered to be in a “metastable” state. Glasses, besides possessing liquid...
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Ingredients and Manufacture (Encyclopedia of Global Resources)
Silica—in the form of sand that is processed and cleaned before use—is the primary ingredient in almost all glass. In addition, the common glass that is generally used in such items as bottles, drinking glasses, lightbulbs, and window glass (sheet glass) contains soda (Na2O), which makes the glass easier to work with in manufacturing, and lime (CaO), which overcomes weaknesses introduced by the soda. A wide range of other materials may be used in small amounts, among them aluminum oxide and magnesium oxide. The three most common types of glass are soda-lime glass, borosilicate glass, and lead glass. Lead glass, used in optics and “crystal” tableware, is soda-lime glass to which lead oxide is added to provide exceptional clarity and refractivity. Boron oxide is added in the production of borosilicate glass, used in kitchenware (such as Pyrex) and laboratory ware because it resists breakage during rapid temperature changes.
Both window glass (sheet glass) and plate glass are soda-lime glass, but their manufacturing processes are different. Window glass, for example, is cooled, flattened into shape by rollers, then finished and cut into standard sizes. The manufacture of plate glass is more complex; the glass is strengthened by annealing, then ground smooth and polished. Plate glass is stronger and has less distortion than window glass. Safety glass, or laminated glass, as used in automobile windshields,...
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History (Encyclopedia of Global Resources)
The production of synthetic glass has a long history. In fact, aside from metallurgy, glassmaking can be considered the oldest of industrial arts practiced by early civilizations. The use of natural high-silica minerals having glasslike properties, such as obsidian (produced by volcanic action and sometimes called volcanic glass), is even older. It can be traced many tens of thousands of years into prehistory back to the early Paleolithic era (the Old stone Age). Early humans and even protohominids made tools and weapons by “flintknapping”: shaping obsidian and obsidian-like rocks and minerals by percussion and pressure flaking. These materials were artfully manipulated; prehistoric artisans took advantage of the natural tendency of glasses to be brittle and to break at the surface into chonchoidal fractures (arcuate shapes). Blades, chisels, awls, gouges, and other implements could be produced in this way.
The earliest artificial glass was produced at least three thousand years ago in Egypt for decorative purposes. Colored glazes were fired onto pottery or stone beads and other objects, originally in imitation of the surface colors and lusters of precious and semiprecious stones. Eventually, experimentation led to the development of freestanding, three-dimensional glass objects such as vials and bottles. This development is believed to have occurred in Egypt around 1500 b.c.e. during the New Kingdom period. Eventually, much...
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Uses of Glass (Encyclopedia of Global Resources)
The earliest use of synthetic glass seems to have been in the form of decorative or artistic objects, including jewelry. Glass is still considered an artistic medium and an attractive material for decoration; it is used in sculpture, stained glass windows, vases, vials, jewelry, and mirrors. Particularly beginning with the Industrial Revolution, however, glass has been much more extensively used in the form of utilitarian objects and devices. Plate glass, sheet glass, and wired glass are found in virtually every modern building and vehicle, whether automobile, boat, or aircraft. Countless glass bottles and jars are used in every country to store and transport liquids of all sorts. Lighting fixtures in the form of incandescent and fluorescent lightbulbs and tubes are one of the most familiar of modern uses of glass, and they number in the billions. Hundreds of millions of glass cathode-ray tubes (CRTs) are found worldwide in the form of television sets and video display terminals (VDTs) for personal computers. Military and civilian applications of optical-quality glass elements in the form of magnifying lenses for microscopes, telescopes, binoculars, periscopes, prisms, and other eyepieces also number in the millions and are in use on land, at sea, and in the air. Structural insulation in the form of glass fiber mats is a common manufactured good produced from fine, woollike glass fibers.
Chemistry and physics laboratories use...
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Further Reading (Encyclopedia of Global Resources)
Doremus, Robert H. Glass Science. 2d ed. New York: Wiley, 1994.
Frank, Susan. Glass and Archaeology. New York: Academic Press, 1982.
Macfarlane, Alan, and Gerry Martin. Glass: A World History. Chicago: University of Chicago Press, 2002.
Shackelford, James F., and Robert H. Doremus, eds. Ceramic and Glass Materials: Structure, Properties, and Processing. New York: Springer, 2008.
Shelby, James E. Introduction to Glass Science and Technology. 2d ed. Cambridge, England: Royal Society of Chemistry, 2005.
Sinton, Christopher W. Raw Materials for Industrial Glass and Ceramics: Sources, Processes, and Quality Control. Hoboken, N.J.: Wiley, 2006.
Zerwick, Chloe. A Short History of Glass. Redesigned and updated 2d ed. New York: H. N. Abrams in association with the Corning Museum of Glass, 1990.
Corning Museum of Glass. A Resource on Glass. http://www.cmog.org/dynamic.aspx?id=264
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Glass (Encyclopedia of Science)
Glass is a hard, brittle substance that is usually transparent or translucent. It is made by melting together sand (silicon dioxide), soda (sodium carbonate), limestone (calcium carbonate), and other ingredients. The simplest form of glass (containing only sand, soda, and lime) is known today as plate or window glass.
Scholars believe that the first humans to make glass may have been Phoenician sailors living around 5000 B.C. Examples of glass used for
weapons, ornaments, and money from Egypt and Mesopotamiaating to about 1550 B.C.till survive.
Humans may well have learned about glass-making by witnessing the natural formation of glass by lightning bolts. When lightning strikes areas where sand, soda, and limestone occur naturally, it can fuse these materials to produce a natural form of glass known as obsidian.
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Glass (World of Earth Science)
Although a glass is a substance that is non-crystalline, it is almost completely undeformable and therefore brittle. A glass exists in a state of matter termed a vitreous state. Vitreous substances, when heated, will transform slowly through stages of decreasing viscosity. As a sample of glass is heated, it becomes increasingly deformable, eventually reaching a point where it resembles a very viscous liquid. Ice, on the other hand, does not go through these changes as it is heated.
Excepting sublimation (direct solid to gas transformations) most substances change directly from a solid to a liquid. Ice, therefore, is not a vitreous substance. Glasses are only very slightly deformable. Glasses tend to bend and elongate under their own weight, especially when formed into rods, plates, or sheets. Glasses can be either organic or inorganic materials.
Because solidification is the act of crystallization, the depiction of glass as a non-crystalline solid may not be entirely correct. However, true crystallization occurs when the molecules of a substance arrange themselves in a systematic, periodic fashion. The atoms or molecules of glass do not exhibit this periodicity; this is consistent with the depiction of glass as an extremely viscous, or "supercooled"' liquid.
Glass is often referred to as an amorphous solid. An amorphous solid has a definite shape without the geometric regularity of crystalline solids. Glass can be molded into any shape. If glass is shattered, the resulting pieces are irregularly shaped. A crystalline solid would exhibit regular geometrical shapes when shattered. Amorphous solids tend to hold their shape, but they also tend to flow very slowly. If left undisturbed for a long period of time, a glass will very slowly crystallize. Once it crystallizes, it is no longer considered to be glass. At this point, it has devitrified. This crystallization process is extremely slow and in many cases may never occur.
The chemical make-up of standard window glass, which will be described in greater detail below, is quite similar to the mineral quartz. An x-ray crystallographic picture of quartz would show atoms arranged in an orderly, periodic sequence. X-ray crystallography studies of glass show no such arrangement. The atoms in glass are disordered and show no periodic structure. This irregular arrangement of atoms not only defines a substance as glass, but also determines several of its properties.
The bonds between the molecules or ions in a glass are of varying length, which is why they show no symmetry or periodic structure. Because the bonds are not symmetrical, glass is isotropic and has no definite melting point. The melting of glass instead takes place over a wide temperature range. Changing the state of a substance with asymmetric bonds requires more energy than a crystalline structure would. The tendency of glass to devitrify is a result of the atoms moving from a higher to a lower energy state.
The most common glasses are silicon based. Most glasses are 75% silicate. These glasses are based on the SiO2molecule. This molecule creates an asymmetric, aperiodic structure. Some of the oxygen atoms are not bridged together, creating ions that need to be neutralized by metal cations. These metal cations are randomly scattered throughout the glass structure, adding to the asymmetry. The oxides of elements other than silicon can also form glasses. These other oxides include Al2O3, B2O3, P2O5, and As2O5.
The production of glasses is a complicated process. In general, certain molten materials are cooled in a specific manner so that no crystallization occurs, i.e., they remain amorphous. There are four basic materials that are used in glass production. These materials are the glass-forming substances, fluxes, stabilizers, and secondary components.
A glass-forming substance is any mineral that remains vitreous when cooled. Glass-forming substances are usually silica, boric oxide, phosphorous pentoxide, or feldspars. Sometimes aluminum oxide (Al2O3) is used. Silica, as the most commonly used material to make glass, is usually obtained from sand, which is 99.1-99.7% SiO2. Occasionally, natural silica deposits are discovered that are pure enough to use in glass manufacturing, but these deposits are rare and the silica found in them is usually expensive to obtain. Even the lowest quality sands can be purified rather economically. Impurities in the natural silica are important because they can dramatically alter the quality of the glass produced. The most common impurities found in natural silica are iron sesquioxide (Fe2O3), alumina (Al2O3), and calcium compounds. Ferric oxide is sometimes found as an impurity. Even if the amount of ferric oxide in a natural silica sample is only 0.1% of the sample, the glass produced would have a deep yellow-green color and the impurity would have detrimental effects on the thermal and mechanical properties of the glass.
Occasionally impurities are added to the glass-forming substances to give the glass certain qualities such as transparency, fusibility, or stability. Stabilizers also are used to give the finished product particular characteristics. For example, calcium carbonate can be added as a stabilizer that will make the glass produced insoluble in water. Lead oxide added as a stabilizer gives the glass extreme transparency, brightness, and a high refractive index. Lead oxide also makes glass easier to cut. Zinc oxide can be added to glass to make it more resistant to changes in temperature as well as to increase its refractive index (a measure of the ability to bend light). Aluminum oxide can also be added as a stabilizer to increase the physical strength of the glass. Secondary components are added to determine some of the final properties of the glass and to correct any defects in the glass. The secondary components can be classified as decolorants, opacifiers, colorants, or refiners.
The production of glass includes many steps that can be generalized as follows. First, the fluxes, glass-forming substances, and stabilizers are crushed and milled, then blended and mixed together. They are then re-milled and granulated. At this point, the secondary components are added, if needed. The granules are then fused, refined, homogenized, and corrected, using more secondary components if necessary. Finally, the glass is formed and finished.
The final product is one of many hundreds of different types of glass. One popular type of glass, especially in laboratory settings or for use in the kitchen, is borosilicate glass. Some well-known borosilicate glasses are Jena, Pyrex, Durax, and Thermoglass. These glasses contain 12% or more B2O3. The addition of the boron oxide increases the softening temperature of the glass, making it more resistant to high temperatures such as those experienced while cooking or while performing laboratory experiments. Borosilicate glasses are also used in the production of thermometers, television tubes, and other objects that need to have constant dimensions or a high softening point.
See also Chemical bonds and physical properties
Glass (World of Forensic Science)
Glass is a product of inorganic materials that solidified, but did not crystallize. Glass is mainly composed of silicon dioxide (SiO2), and is extremely prevalent in everyday life. Often, windows are the most fragile elements of a building or a vehicle, and are thus broken by thieves or criminals in order to penetrate the premises or the vehicle. When glass breaks at the scene of a crime, small particles of glass are projected not only forward, but also backward, onto the perpetrator and into the immediate environment. These particles can later be retrieved and used to establish a link between a suspect and a crime scene.
Glass can be classified either by chemical composition or by use. There are four main chemical compositions of glass: soda-lime, lead, borosilicate, and special glass. While glass is mainly composed of silicon dioxide (SiO2), it also contains modifiers that are used to vary the quality and properties of the glass. Soda-lime glass is obtained by adding a certain amount of soda (Na2CO3) and lime (CaO). It is this glass that constitutes most windows and bottles. Borosilicate glass is made by the addition of boron oxide and is much more resistant to heat. Different colors of glass are achieved by introducing small amounts of additives. For example, chromium (Cr) is used to give a green tint, cobalt (Co) for a blue tint.
Almost all types of glass are commercially available. Window glass is probably the most common type of glass, and is usually found as a flat, transparent piece composed of soda-lime glass. This type of glass does not resist high temperatures, quick temperature changes, or corrosive substances. Most of flat glass is now prepared using the floating process. This consists of laying the molten glass onto a bath of molten tin in an inert atmosphere in order to achieve a perfectly flat surface. Tempered glass is another type of glass that is much stronger than regular glass. This particular strength is achieved by introducing extra forces on both sides of the glass through rapid cooling and heating during the manufacturing process. This glass will shatter in very small pieces when it breaks. It is used on side and rear windows of cars. Laminated glass is a glass composed of multiple sheets of glass bonded together with a plastic film such as polyvinyl butyral.
When a criminal breaks glass during a criminal act, some small particles are projected onto his/her clothing, hair, or shoes. If the suspect is apprehended within a relatively short time span after the crime, these small particles of glass can be found on the hair, clothing, shoes, or inside pockets. At the crime scene, the crime scene investigator usually collects some of the broken glass as evidence for further comparison with any glass fragments found on a suspect. The comparison process might lead to the exclusion of a common origin between the glass from the suspect and the glass from the crime scene. Conversely, it might also show that the characteristics are similar and the two samples cannot be differentiated, thus supporting the hypothesis that the two samples of glass come from the same origin. It is important to apprehend the suspect shortly after the glass was broken, because the number of glass fragments on the clothing or shoes of the suspects diminishes very quickly after the activity. About 90% of glass fragments are shed from clothing within 24 hours.
Glass is characterized according to its physical and chemical characteristics. When investigating glass, the first examination is visual. The investigator observes its color, its thickness (if the fragments are big enough), its patterns, and its fluorescing (light-emitting) properties. Pieces of the glass can often be reassembled, revealing patterns that can be compared to crime scene samples. Demonstration of origin by assembly is the only way the common origin between two fragments of glass can be clearly established. The refractive index of the glass fragments is then measured. This is typically achieved by immersion of the fragment in oil and observing the lines of refraction at different temperatures. Finally, elemental composition of the glass is determined.
The interpretation of glass is complicated by the fact that the characteristics exhibited by a large piece of glass (such as a window) might vary from one end to the other. Thus, the analyst needs to determine the extent of the intravariability (variations of characteristics within a same sample) before it can be compared to a different sample. If the variation exhibited between the two samples is greater than the variation exhibited within one sample, then the two samples can be excluded as having a common origin. On the contrary, if the two samples cannot be differentiated, then this supports the hypothesis that they have a common source. However, it does not indicate that they have the exact same common source. Again, the characteristics exhibited by the samples might be very common and found in many other pieces of glass. Thus, the analyst usually expresses his/her findings using statistics.
SEE ALSO Criminalistics; Minerals; Monochromatic light.