Glass in Materials Science

Glass is an inorganic product of fusion that has cooled to a rigid condition without crystallizing. Glass is typically hard and brittle, and has a conchoidal fracture. A glass may be colorless or colored. It is usually transparent, but may be made translucent or opaque (such as in white, opal glass). Objects made of glass are loosely and popularly referred to as glass; such as glass for a tumbler, a barometer, a window, a magnifier, or a mirror. The subject of studying glass in materials science is an important part.

1. Glass in Materials Science By Prof. A. Balasubramanian Centre for Advanced Studies in Earth Science University of Mysore, India

2. Types of Ceramics Whitewares Refractories Glasses Abrasives Cements

3. Amorphous Ceramics (Glasses) Main ingredient is Silica (S i O 2 ) If cooled very slowly will form crystalline structure. If cooled more quickly will form amorphous structure consisting of disordered and linked chains of Silicon and Oxygen atoms. This accounts for its transparency as it is the crystal boundaries that scatter the light, causing reflection. Glass can be tempered to increase its toughness and resistance to cracking.

4. Ancient findings

5. There are innumerable examples of glass, both in public and private collections, which are unquestionably of ancient origin, yet because of the lack of proper inscriptions it is impossible to classify them in chronological order. The paintings of the Theban glassmakers reproduced herein were discovered on the tombs of Beni Hassan .

6. About 2000 B. C, Figure 1 represents an ancient Theban taking molten glass from the foot of a furnace.

7. Figure 2 shows two others seated on the ground, holding pipes similar to those used at the present time. The glass on the end of the pipes, which are pointed toward the fire, is ready to be blown.

8. Figure 3 illustrates the blowing of a large glass vase by two men

9. With the encouragement of Constantine, and also of Theodosius III, who reigned from 408 A. D. to 450 A. D., the manufacture of glass became an important industry. This monopoly of the East was not overcome by the West until the fourteenth century , when Venice became a factor. For several centuries the Venetian Republic maintained its leadership as the principal producer of glass . The Germans , in spite of the attempted monopoly of the Venetians , began at this period to manufacture glass in their own country.

10. Ancient use of glass Bottles , drinking glasses, vases and toilet articles , many of which bear a striking resemblance to those of the present day, were produced by these early Roman glassmakers. It bears the name of Maximianu Herculius , a Roman emperor (250 A. D . — 310 A. D.).

11. Bohemian glass About 1609 Gaspar Lehmann, a Bohemian , invented a new method of decoration—that of engraving on glass. This new decoration revolutionized the industry, and while the Bohemian glass of this time was clear and light in weight, it unfortunately lacked brilliancy.

12. Molds used in glass making

13. Spanish glass

14. Venetian and German glass

15. French glass

16. When the Roman Emperor, Caesar Augustus, conquered Egypt (26 B. C.) he quickly recognized the commercial value of glass and ordered that it should form part of the tribute which he imposed upon the conquered country . According to Pliny, the Romans began the manufacture of glass in their own country'. With characteristic intelligence and industry, they assimilated the knowledge of the Egyptians , and within a comparatively short time Roman glass rivaled that of Egyptian origin.

17. USA In 1607 the first glass furnace was erected about a mile distant from Jamestown. The product was confined to bottles. The second plant was erected in 1620 to manufacture glass beads , which were used extensively at that time in trading with the Indians. Both works were destroyed in the great massacre of 1622. The next attempt to make glass in America was at Salem , Mass., where a plant was built in 1639 to produce bottles and other articles.

18. It is generally conceded that the two most powerful influences giving the greatest impetus to the manufacture of glass during the last fifty years were (1) the substitution of gas for coal and (2) the invention of the glass-blowing machine.

19. 1869, at the foot of Gist Street, in that part of Pittsburgh known as the Bluff, Thos . Evans established a glass works which marked the beginning of the Macbeth-Evans Glass Company . The plant was operated under the name of Reddick & Company. In 1872, three years later, Geo. A. Macbeth, with several associates, purchased the Keystone Flint Glass Works , Second and Try Streets, Pittsburgh, known as the "Dolly Varden ." The company operating the plant was called Muzzy & Company.

20. 1886

21. Semi-automatic/ blow mold

22. Mega event marking the manufacture

23. Today

24. Glass in Buildings

25. Leaded Glass

26. Glass Containers

27. Glass images

28. Glass blowing

29. The Glassy State Glass is an inorganic product of fusion that has cooled to a rigid condition without crystallizing. Glass is typically hard and brittle, and has a conchoidal fracture. A glass may be colorless or colored. It is usually transparent, but may be made translucent or opaque (such as in white, opal glass). Objects made of glass are loosely and popularly referred to as glass; such as glass for a tumbler, a barometer, a window, a magnifier, or a mirror.

30. The glassy state Glass is a unique material that presents many possibilities to designers and engineers, as well as to artists. Many of the unique possibilities that come with this material often are coupled with a large set of limitations. The glassy state is a state of matter that is amorphous or disordered like a liquid in structure, hence capable of continuous composition variation and lacking a true melting point, but softening gradually with increasing temperature.

31. Good control on glass Part of truly being able to control this material, for both artists and engineers, is to understand, the often complex processes involved in the heating, forming, and cooling of this material.

32. Properties of Glass Strain Point Annealing Point Softening Point Working Range Melting Point Physical Properties Involved in Lampworking Gravity and Surface Tension Physical properties involved in the forming of molten glass. Proper Spinning Technique Necessary to evenly distribute the effects of gravity on the piece. Heat Base

33. Strain Point The most crucial part of the annealing process. The strain point is right below the annealing point, when the molecules can no longer re-arrange. This is when the majority of stresses due to improper cooling will be reintroduced to the glass. With thicker glass this usually requires long soak times to make sure that all parts of the glass are cooling at the same rate.

34. Annealing Point Relieves stresses induced by the shaping/forming processes. There is inevitable stress due to the rapid and often uneven heating and cooling processes when forming glass at the torch. This, coupled with mechanical stresses due to sharp angles in your form or inconsistent thicknesses, will be enough to crack your piece if it is not properly annealed.

35. Softening Point When the glass has a viscosity high enough to prevent deformation under its own weight (very dull red).

36. Working Range The variety of viscosities ranging from very stiff to very fluid. Minute differences in viscosity caused by slight temperature changes give the artist a broad range of working properties (dull red – bright yellow).

37. Melting Point This is the point at which the glass is very fluid and moving almost uncontrollably fast. The closer you are to the melting point, glass becomes more fluid and has a higher surface tension (bright yellow – white).

38. Other properties Gravity and Surface Tension Physical properties involved in the forming of molten glass. Proper Spinning Technique Necessary to evenly distribute the effects of gravity on the piece.

39. Heat Base An even distribution of heat. Heat base allows for symmetrical blowing and forming. If glass is allowed to cool slightly before forming, the heat will start to even out, allowing for more balanced forms.

40. Importance of Glass Glass has countless uses. Food is preserved in glass jars. People drink from glass containers called glasses. Windows in homes, schools, and office buildings are glass. Motor vehicles have glass windshields and windows. People with vision problems wear eyeglasses. Scientists use glass test tubes, and microscopes and telescopes with glass lenses. Glass optical fibers carry data all over the world at the speed of light over the Internet, the worldwide network of computers.

41. Glass can take many different forms It can be spun finer than a spider web. Or it can be molded into a disk for a telescope lens or mirror weighing many tons. Glass can be stronger than steel, or more fragile than paper. Most glass is transparent, but glass can also be colored to any desired shade.

42. Glass industries Most countries of the world have glass industries. For many years, Germany was the major world source for optical glass, laboratory glassware, and glass Christmas tree ornaments. Today, glass manufacturers in many countries produce such objects on a large scale. Beautiful art glassware is made in many countries, including the Czech Republic, France, Ireland, Italy, and Sweden.

43. Composition of Glass Glass can be generally divided into two groups: oxide glass and non-oxide glass. The ingredients of oxide glasses include  oxides  (chemical compounds that include oxygen). Non-oxide glasses are made from compounds that contain no oxides, and which often instead contain sulfides or metals. Oxide glasses are much more widely used commercially. The common types of glass discussed below are all oxide glasses.

44. Soda-lime glass Soda-lime glass is the kind of glass used for flat glass, most containers and electric light bulbs, and many other industrial and art objects. More than 90 percent of all glass is soda-lime glass. It has been made of almost the same materials for hundreds of years. The composition is about 72 percent silica (from sand), about 13 percent sodium oxide (from soda ash), about 11 percent calcium oxide (from limestone), and about 4 percent minor ingredients. Soda-lime glass is inexpensive, easy to melt and shape, and reasonably strong.

45. All glass containers All glass container manufacturers use the same basic soda-lime composition, making the containers easy to recycle. Manufacturers sort the glass by color and then later reuse it in the production of new containers.

46. Soda-lead glass Soda-lead glass, commonly called  crystal  or  lead glass,  is made by substituting lead oxide for calcium oxide and often for part of the silica used in soda-lime glass. Soda-lead glass is easy to melt. It is much more expensive than soda-lime glass. Soda-lead glass has such beautiful optical properties that it is widely used for the finest tableware and art objects. In addition, lead oxide improves the electrical properties of glass.

47. Borosilicate glass Borosilicate glass is heat-shock resistant and better known by such trade names as Pyrex and Kimax . It contains about 80 percent silica, 4 percent sodium oxide, 2 percent alumina, and 13 percent boric oxide. Such glass is about three times as heat-shock resistant as soda-lime glass and is excellent for chemical and electrical uses. This glass makes possible such products as ovenware and beakers, test tubes, and other laboratory equipment.

48. Fused silica glass This glass is a highly heat-shock resistant glass that consists entirely of silica. It can be heated to extremely high temperatures and then plunged into ice-cold water without cracking. Fused silica is expensive because exceptionally high temperatures must be maintained during production. It is used in laboratory glassware and optical fibers .

49. Glass  resists heat 96 percent silica glass resists heat almost as well as fused silica, but it is less expensive to produce. It consists of a special borosilicate composition that has been made porous by chemical treatment. The pores shrink when the glass is heated, leaving a smooth, transparent surface. The glass is sold under the trade name Vycor .

50. Colored glass Colored glass gets its coloring from certain oxides that are added to the glass. For example, 1 part of nickel oxide in 50,000 produces a tint that may range from yellow to purple, depending on the base glass. One part of cobalt oxide in 10,000 gives an intense blue. Red glasses are made with gold, copper, or selenium oxides. Other colors can be produced in glass with other chemicals.

51. Glass Material Properties Hard Glasses Your typical silica based glasses with the coefficient of thermal expansions lower than 6  ppm /C. They are most commonly used to form matched seals with the likes of  Molybdenum, and Tungsten. Typical hard glasses are Alkali Barium Borosilicate and Alkali Borosilicate.

52. Soft Glasses These are the glasses with the coefficient of thermal expansions above 6  ppm /C. The most common use for these glasses is for forming hermetic seals between Alloy 52 pins and Stainless Steel housings. Soft glasses are also used for many other compression sealing applications. Typical soft glasses are Soda Lime glass, Alkai Barium glass.

53. Specialty Glasses glasses for specific uses such as lithium battery sealing glasses, titanium sealing glasses & aluminum sealing glasses.

54. Glass Manufacture Glassmaking involves two main steps: (1) heating and mixing raw materials to produce molten glass, and (2) forming the molten glass into the desired shape. Most glass then receives further treatment to produce the final product.

55. Making Molten Glass Glass manufacture begins with the careful selection and measurement of raw materials. The most important raw material is sand. Other raw materials used depend on the type of glass being made. Broken glass, called cullet, is usually added to the raw materials. It promotes the melting of the raw materials as they are heated. Most cullet is waste from glass-forming operations; some is obtained from recycled glass products.

56. Melting Melting is done by batches or as a continuous process. With the continuous-tank furnace, the most common type of furnace used for melting, raw materials are fed into one end and molten glass is withdrawn continuously from the other end.

57. Raw materials and cullet are heated The raw materials and cullet are heated until they have melted into a spongy mass full of bubbles. The temperature of the melt is then increased to make it more fluid, allowing the bubbles to rise to the surface and escape. The glass at this stage is clear and homogeneous. Soda-lime glass, the most common type of glass, is initially heated to about 2,550 F. (1,400 C.) and then rapidly heated to about 2,800 F. (1,540 C.). Most other kinds of glass must be heated to higher temperatures.

58. Forming When the glass is withdrawn from the furnace its viscosity is too low for it to hold any form. As the glass is cooled, it flows less easily and can be formed.

59. Any of several processes can be used: Blowing. In machine blowing, blasts of compressed air are used to force gobs or ribbons of glass into molds . Bottles are made by machine blowing. In hand blowing, a mass of molten glass is gathered on the end of a four- to five-foot (120- to 150-cm) pipe called a blowpipe or blowing iron. The glassmaker blows through the pipe, giving the material a hollow, balloon-shaped form. The glass is further shaped with various metal tools. It is usually reheated several times to keep it pliable while it is being shaped.

60. Drawing In producing flat drawn glass, a horizontal wire called a bait is lowered into the molten glass and then raised. Glass adheres to the wire and is drawn upward as a continuous sheet. Once hardened, the glass is cut into sheets for use in windows and inexpensive mirrors. Flat drawn glass has a fire-finished surface that forms naturally as the molten glass cools. Drawn glass tubing is made by forcing molten glass along a ceramic cone, called a mandrel, that forms the glass into tubing.

61. Floating Molten glass is allowed to flow onto a bath of molten tin and is then allowed to cool. The tin has a low melting point and remains liquid at temperatures at which the glass hardens. The surface of the metal leaves the glass with a very smooth surface. Float glass is used in windows and other flat glass products.

62. Pressing A measured amount of molten glass is placed, by machine, into a mold , and a metal plunger presses the glass outward to fill the mold . When the glass has cooled and is firm enough to hold its shape, the mold is removed. Pressing techniques are well-suited for rapid mass production. Many of the glass objects made for everyday use, such as drinking glasses and heat-resistant glassware, are produced by pressing.

63. Rolling As the molten glass leaves the furnace, it flows between sets of large, heavy rollers. This method is typically used for flat glass that does not require a fine finish, such as glass made with a figured pattern on its surface.

64. Further Processing After forming, glass is usually heat-treated, either by annealing or by tempering. It may also be decorated.

65. Annealing Stresses build up in glass objects as they are cooled from forming temperatures to room temperature. These stresses greatly weaken the objects and may cause them to fracture spontaneously. In annealing, glass products are reheated in ovens called lehrs , and then allowed to cool slowly under controlled conditions, so that the stresses do not recur.

66. Tempering Tempered glass is tougher (more resistant to breakage) than ordinary glass. In tempering, controlled, uniform stresses are deliberately set up in the surface of a glass object. The glass is heated to just below the temperature at which it would begin to soften and is then quenched (rapidly chilled) to stress its entire surface uniformly. Tempered glass cannot be cut or drilled. If a piece of tempered glass is pierced in any way, the uniformity of the stress is destroyed and the whole piece shatters into small pieces.

67. Decorating Glass objects can be decorated in many ways. Some glass objects are painted or glazed. Others are etched with hydrofluoric acid, either to produce an artistic design or to frost the entire surface. Designs can also be cut or ground into the surface.

68. Reversible transition Glass is an amorphous solid (non-crystalline) material that exhibits a glass transition, which is the reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle state into a molten or plastic state. Glasses are typically brittle and can be optically transparent.

69. Soda -lime The most familiar type of glass is soda-lime glass, which is composed of about 75% silicon dioxide (SiO2), sodium oxide (Na2O) from sodium carbonate (Na2CO3), lime ( CaO ), and several minor additives.

70. Silicate glass Silicate glass generally has the property of being transparent. Because of this, it has many applications. One of its primary uses is as a light-transmitting building material, traditionally as small panes set into window openings in walls, but in the 20th-century often as the major cladding material of many large buildings.

71. Make optical lenses Glass is both reflective and refractive of light, and these qualities can be enhanced by cutting and polishing to make optical lenses, prisms, fine glassware, and optical fibers for high speed data transmission by light. Glass can be colored by adding metallic salts, and can also be painted. These qualities have led to the extensive use of glass in the manufacturing ofart objects and in particular, stained glass windows.

72. Glass is extremely durable Although brittle, glass is extremely durable, and many examples of glass fragments exist from early glass-making cultures. Because glass can be formed or molded into any shape, and also because it is a sterile product, it has been traditionally used for vessels: bowls, vases,bottles , jars and drinking glasses. In its most solid forms it has also been used for paperweights, marbles, and beads. When extruded asglass fiber and matted as glass wool in a way to trap air, it becomes a thermal insulating material, and when these glass fibers are embedded into an organic polymer plastic, they are a key structural reinforcement part of the composite material fiberglass .

73. Fused quartz Fused quartz is relatively chemically-pure silica, or SiO2, which is not in crystalline form but rather in vitrified or glass form, with no true melting point. It can be used for some special glass applications. However, this is not very common due to silica's high glass transition temperature of over 1200 °C (2192 °F). Normally, other substances are added to simplify processing. One is sodium carbonate (Na2CO3, "soda"), which lowers the glass transition temperature. However, the soda makes the glass water soluble, which is usually undesirable, so lime (calcium oxide [ CaO ], generally obtained from limestone), some magnesium oxide ( MgO ) and aluminium oxide (Al2O3) are added to provide for a better chemical durability. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass.  Soda-lime glasses account for about 90% of manufactured glass.

74. Most common glass contains other ingredients to change its properties.  Lead glass or flint glass is more 'brilliant' because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. 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 eyeglasses. Iron can be incorporated into glass to absorb infrared energy, for example in heat absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs  Uv wavelengths .

75. Lead-oxide glass, crystal glass Silica 59% + lead oxide ( PbO ) 25% + potassium oxide (K 2 O) 12% + soda (Na 2 O) 2.0% + zinc oxide ( ZnO ) 1.5% + alumina 0.4%. Because of its high density (resulting in a high electron density) it has a high refractive index, making the look of glassware more brilliant (called "crystal", though of course it is a glass and not a crystal). It also has a high elasticity, making glassware 'ring'. It is also more workable in the factory, but cannot stand heating very well.

76. Aluminosilicate glass : Silica 57% + alumina 16% + lime 10% + magnesia 7.0% + barium oxide ( BaO ) 6.0% + boric oxide (B 2 O 3 ) 4.0%. Extensively used for fiberglass , used for making glass-reinforced plastics (boats, fishing rods, etc.) and for halogen bulb glass.

77. Oxide glass : Alumina 90% + germanium oxide (GeO 2 ) 10%. Extremely clear glass, used for fiber -optic waveguides in communication networks. Light loses only 5% of its intensity through 1 km of glass fiber .. However , most optical fiber  is based on silica, as are all the glasses above.

78. Another common glass ingredient is crushed alkali glass or "cullet" ready for recycled glass. The recycled glass saves on raw materials and energy. Impurities in the cullet can lead to product and equipment failure. Fining agents such as sodium sulfate, sodium chloride, or antimony oxide may be added to reduce the number of air bubbles in the glass mixture. Glass batch calculation is the method by which the correct raw material mixture is determined to achieve the desired glass composition.

79. Physical properties Optical properties Glass is in widespread use largely due to the production of glass compositions that are transparent to visible light. In contrast, polycrystalline materials do not generally transmit visible light. The individual crystallites may be transparent, but their facets (grain boundaries) reflect or scatter light resulting in diffuse reflection.

80. Glass does not contain the internal subdivisions associated with grain boundaries in polycrystals and hence does not scatter light in the same manner as a polycrystalline material. The surface of a glass is often smooth since during glass formation the molecules of the supercooled liquid are not forced to dispose in rigid crystal geometries and can follow surface tension, which imposes a microscopically smooth surface. These properties, which give glass its clearness, can be retained even if glass is partially light-absorbing—i.e., colored .

81. Glass has the ability to refract, reflect, and transmit light Glass has the ability to refract, reflect, and transmit light following geometrical optics, without scattering it. It is used in the manufacture of lenses and windows. Common glass has a refraction index around 1.5. This may be modified by adding low-density materials such as boron, which lowers the index of refraction (see crown glass), or increased (to as much as 1.8) with high-density materials such as (classically) lead oxide (see flint glass and lead glass), or in modern uses, less toxic oxides of zirconium, titanium, or barium.

82. These high-index glasses (inaccurately known as "crystal" when used in glass vessels) cause more chromatic dispersion of light, and are prized for their diamond-like optical properties.

83. According to Fresnel equations, the reflectivity of a sheet of glass is about 4% per surface (at normal incidence in air), and the  transmissivity  of one element (two surfaces) is about 90%. Glass with high germanium oxide content also finds application in optoelectronics—e.g., for light-transmitting optical fibers .

84. Other properties In the process of manufacture, silicate glass can be poured, formed, extruded and molded into forms ranging from flat sheets to highly intricate shapes. The finished product is brittle and will fracture, unless laminated or specially treated, but is extremely durable under most conditions. It erodes very slowly and can withstand the action of water. It is resilient to chemical attack and is an ideal material for the manufacture of containers for foodstuffs and most chemicals.

85. Contemporary production Following the glass batch preparation and mixing, the raw materials are transported to the furnace.  Soda-lime glass for mass production is melted in gas fired units. Smaller scale furnaces for specialty glasses include electric melters , pot furnaces, and day tanks. After melting, homogenization and refining (removal of bubbles), the glass is formed. 

86. Flat glass for windows and similar applications is formed by the float glass process, developed between 1953 and 1957 by Sir Alastair Pilkington and Kenneth Bickerstaff of the UK's Pilkington Brothers, who created a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity. The top surface of the glass is subjected to nitrogen under pressure to obtain a polished finish. Container glass for common bottles and jars is formed by blowing and pressing methods. This glass is often slightly modified chemically (with more alumina and calcium oxide) for greater water resistance.

87. Once the desired form is obtained, glass is usually annealed for the removal of stresses . Surface treatments, coatings or lamination may follow to improve the chemical durability (glass container coatings, glass container internal treatment), strength (toughened glass, bulletproof glass, windshields), or optical properties (insulated glazing, anti-reflective coating).

88. Network glasses A  CD-RW (CD).  Chalcogenide glasses form the basis of rewritable CD and DVD solid-state memory technology. Some glasses that do not include silica as a major constituent may have physico -chemical properties useful for their application in  fiber optics and other specialized technical applications. These include fluoride glasses ,  aluminosilicates ,  phosphate glasses,  borate glasses, and   chalcogenide glasses.

89. There are three classes of components for oxide glasses: network formers, intermediates , and modifiers . The network formers (silicon, boron, germanium) form a highly cross-linked network of chemical bonds. The intermediates (titanium, aluminium, zirconium, beryllium, magnesium, zinc) can act as both network formers and modifiers, according to the glass composition.

90. The modifiers (calcium, lead, lithium, sodium, potassium) alter the network structure; they are usually present as ions, compensated by nearby non-bridging oxygen atoms, bound by one covalent bond to the glass network and holding one negative charge to compensate for the positive ion nearby. Some elements can play multiple roles; e.g. lead can act both as a network former (Pb 4+  replacing Si 4+ ), or as a modifier.

91. Glass Types- Three common types of glass : Soda-lime glass - 95% of all glass, windows containers etc. Lead glass - contains lead oxide to improve refractive index Borosilicate - contains Boron oxide, known as Pyrex.

92. Glasses Flat glass (windows) Container glass (bottles) Pressed and blown glass (dinnerware) Glass fibres (home insulation) Advanced/specialty glass (optical fibres)

93. Importance of Glass Glass  is one of the most useful materials in the world. Few manufactured substances add as much to modern living as does glass. Yet few products are made of such inexpensive raw materials. Glass is made chiefly from  silica sand  (silica, also called silicon dioxide),  soda ash  (sodium carbonate), and  limestone  (calcium carbonate ).

94. Glass has countless uses. Food is preserved in glass jars. People drink from glass containers called glasses. Windows in homes, schools, and office buildings are glass. Motor vehicles have glass windshields and windows. People with vision problems wear eyeglasses. Scientists use glass test tubes, and microscopes and telescopes with glass lenses. Glass optical fibers carry data all over the world at the speed of light over the Internet, the worldwide network of computers.

95. Glass can take many different forms. It can be spun finer than a spider web. Or it can be molded into a disk for a telescope lens or mirror weighing many tons. Glass can be stronger than steel, or more fragile than paper. Most glass is transparent, but glass can also be colored to any desired shade.

96. Most countries of the world have glass industries. For many years, Germany was the major world source for optical glass, laboratory glassware, and glass Christmas tree ornaments. Today, glass manufacturers in many countries produce such objects on a large scale. Beautiful art glassware is made in many countries, including the Czech Republic, France, Ireland, Italy, and Sweden.

97. Major glass companies spend millions of dollars each year on research to discover ways to make better glass and to develop new uses for glass. Many of the revolutionary developments in glass during the 1900's have come from the laboratories of glass manufacturers.

98. Types of Glass When people speak of glass, they ordinarily mean a transparent, shiny substance that breaks rather easily. They may think of the glass in windows and the glass used in eyeglasses as being the same material. Actually, they are not. There are many kinds of glass. Several important kinds of glass are :

99. Flat glass   , Glass containers   Optical glass, Fiberglass   , Laminated safety glass   , Bullet-resisting glass   Tempered safety glass, Colored structural glass  , Opal glass  , Foam glass, Glass building blocks  , Heat-resistant glass

100. Flat glass  is used chiefly in windows. It is also used in mirrors, room dividers, and some kinds of furniture. All flat glass is made in the form of flat sheets. But some of it, such as that used in automobile windshields, is reheated and  sagged  (curved) over molds .

101. Glass containers  are used for packaging food, beverages, medicines, chemicals, and cosmetics. Glass jars and bottles are made in a wide variety of shapes, sizes, and colors . Many are for common uses, such as soft-drink bottles or jars for home canning. Others are made from special glass formulas to make sure there will be no contamination or deterioration of blood plasma, serums, and chemicals stored in them.

102. Optical glass  is used in eyeglasses, microscopes, telescopes, camera lenses, and many instruments for factories and laboratories. The raw materials must be pure so that the glass can be made almost flawless. The care required for producing optical glass makes it expensive compared with other kinds of glass.

103. Fiberglass   consists of fine but solid rods of glass, each of which may be less than one-twentieth the width of a human hair. These tiny glass fibers can be loosely packed together in a woollike mass that can serve as heat insulation. They also can be used like wool or cotton fibers to make glass yarn, tape, cloth, and mats. Fiberglass has many other uses. It is used for electrical insulation, chemical filtration, and firefighters' suits. Combined with plastics, fiberglass can be used for airplane wings and bodies, automobile bodies, and boat hulls. Fiberglass is a popular curtain material because it is fire-resistant and washable.

104. Laminated safety glass  is a “sandwich” made by combining alternate layers of flat glass and plastics. The outside layer of glass may break when struck by an object, but the plastic layer is elastic and so it stretches. The plastic holds the broken pieces of glass together and keeps them from flying in all directions. Laminated glass is used where broken glass might cause serious injuries, as in automobile windshields.

105. Bullet-resisting glass  is thick, multilayer laminated glass. This glass can stop even heavy- caliber bullets at close range. Bullet-resisting glass is heavy enough to absorb the energy of the bullet, and the several plastic layers hold the shattered fragments together. Such glass is used in bank teller windows and in windshields for military tanks, aircraft, and special automobiles .

106. Tempered safety glass Tempered safety glass,  unlike laminated glass, is a single piece that has been given a special heat treatment. It looks, feels, and weighs the same as ordinary glass. But it can be several times stronger. Tempered glass is used widely for all-glass doors in stores, side and rear windows of automobiles, and basketball backboards, and for other special purposes. It is hard to break even when hit with a hammer. When it does break, the whole piece of glass collapses into small, dull-edged fragments.

107. Colored structural glass Colored structural glass  is a heavy plate glass, available in many colors . It is used in buildings as an exterior facing, and for interior walls, partitions, and tabletops.

108. Opal glass Opal glass  has small particles in the body of the glass that disperse the light passing through it, making the glass appear milky. The ingredients necessary to produce opal glass include fluorides  (chemical compounds containing fluorine). This glass is widely used in lighting fixtures and for tableware .

109. Foam glass Foam glass,  when it is cut, looks like a black honeycomb. It is filled with many tiny cells of gas. Each cell is surrounded and sealed off from the others by thin walls of glass. Foam glass is so light that it floats on water. It is widely used as a heat insulator in buildings, on steam pipes, and on chemical equipment. Foam glass can be cut into various shapes with a saw.

110. Glass building blocks Glass building blocks  are made from two hollow half-sections sealed together at a high temperature. Glass building blocks are good insulators against heat or cold because of the dead-air space inside. The blocks are laid like bricks to make walls and other structures.

111. Heat-resistant glass Heat-resistant glass  is high in silica and usually contains boric oxide. It expands little when heated, so it can withstand great temperature changes without cracking. This quality is necessary in cookware and other household equipment, and in many types of industrial gear.

112. Laboratory glass Laboratory glassware  includes beakers, flasks, test tubes, and special chemical apparatus. It is made from heat-resistant glass to withstand severe  heat shock  (rapid change in temperature). This glass is also much more resistant to chemical attack than ordinary glass.

113. Glass for electrical uses Glass has properties that make it useful in electrical applications: ability to resist heat, resistance to the flow of electric current, and ability to seal tightly to metals without cracking. Because of these properties, glass is used in electric light bulbs and for picture tubes in television sets.

114. Glass optical fibers Glass optical fibers   are glass fibers used to transmit information as pulses of light. Thin, extremely pure optical fibers are used to carry telephone and television signals and  digital  (numeric) data over long distances. Glass optical fibers are also used in control board displays and in medical instruments.

115. Glass tubing Glass tubing  is used to make fluorescent lights, neon signs, glass piping, and chemical apparatus. Glass tubing is made from many kinds of glass and in many sizes.

116. Glass-ceramics Glass-ceramics  are strong materials made by heating glass to rearrange some of its atoms into regular patterns. These partially crystalline materials can withstand high temperatures, sudden changes in temperature, and chemical attacks better than ordinary glass can. They are used in a variety of products, including heat-resistant cookware, turbine engines, electronic equipment, and nose cones of guided missiles. Glass-ceramics have such trade names as Pyroceram , Cervit , and Hercuvit .

117. Radiation-absorbing and radiation-transmitting glass Radiation-absorbing and radiation-transmitting glass  can transmit, modify, or block heat, light, X rays, and other types of radiant energy. For example, ultraviolet glass absorbs the ultraviolet rays of the sun but transmits visible light. Other glass transmits heat rays freely but passes little visible light. Polarized glass cuts out the glare of brilliant light. One-way glass is specially coated so that a person can look through a window without being seen from the other side.

118. Laser glass Laser glass  is an optical glass containing small amounts of substances that enable the glass to generate laser beams efficiently. Such glass is used as the active medium in  solid-state lasers,  a type of laser that sends light out through crystals or glass (One substance commonly used in laser glass is the element neodymium. Researchers are using glass lasers in an attempt to harness nuclear fusion  (the joining of atomic nuclei) as a source of commercially useful amounts of energy. In their experiments, powerful glass lasers heat hydrogen atoms until hydrogen nuclei fuse, releasing large amounts of energy.

119. Invisible glass "Invisible glass"  is used principally for coated camera lenses and eyeglasses. The coating is a chemical film that decreases the normal loss of light by reflection. This allows more light to pass through the glass.

120. Photochromic glass Photochromic glass  darkens when exposed to ultraviolet rays and clears up when the rays are removed. Photochromic glass is used for windows, sunglasses, and instrument controls.

121. Photosensitive glass Photosensitive glass  can be exposed to ultraviolet light and to heat so that any pattern or photograph can be reproduced within the body of the glass itself. Because the photographic print then becomes an actual part of the glass, it will last as long as the glass itself.

122. Photochemical glass Photochemical glass  is a special composition of photosensitive glass that can be cut by acid. Any design can be reproduced on the glass from a photographic film. Then when the glass is dipped in acid, the exposed areas are eaten away, leaving the design in the glass in three dimensions. By this means, lacelike glass patterns can be made.

123. Heavy metal fluoride glass Heavy metal fluoride glass  is an extremely transparent glass being developed for use in optical fibers that transmit infrared rays. Infrared rays are much like light waves but are invisible to the human eye. In optical fibers , infrared light transmits better over distance than visible light does .

124. Chalcogenide glass Chalcogenide glass  is made up of elements from the chalcogen group, including selenium, sulfur , and tellurium. The glass is transparent to infrared light and is useful as a semiconductor in some electronic devices. Chalcogenide glass fibers are a component of devices used to perform laser surgery .

125. Sol-Gel glass Sol-Gel glass  can be used as a protective coating on certain solar collectors or as an insulating material. It is also used to make short, thick tubes that are drawn into optical fibers . To make Sol-Gel glass, workers dissolve the ingredients in a liquid. They then heat the liquid. The liquid evaporates, leaving behind small particles of glass. Heating these particles  fuses  (joins) them to form a solid piece of glass. The temperatures involved in Sol-Gel processes are often lower than those needed to make ordinary glass.

126. Plate Glass Drawing Processes

127. Pressed Glass Processing Softened Gob

128. Blow Molding Softened glass

129. Armoured Glass Many have tried to gain access with golf clubs and baseball bats but obviously the glass remains intact ! From time to time a local TV station intends to show videos of those trying to get at the cash!!

130. Tempered Glass The strength of glass can be enhanced by inducing compressive residual stresses at the surface. The surface stays in compression - closing small scratches and cracks. Small Scratches

131. Hardening Processes Tempering: Glass heated above T g but below the softening point Cooled to room temp in air or oil Surface cools to below T g before interior when interior cools and contracts it draws the exterior into compression. Chemical Hardening: Cations with large ionic radius are diffused into the surface This strains the “lattice” inducing compressive strains and stresses.

132. Commercial glasses Commercial glasses are mainly complex silicates (Si02) in chemical composition with numerous other oxide substances (e.g., Na2O, CaO , Al2O3, B2O3). Glasses are made by melting the source materials together, forming in various ways while fluid, and allowing to cool.

133. Crystalline solids Crystalline solids have a very ordered structure; they crystallize into specific forms such as cubes. When liquids crystallize, there is a contraction in volume due to the transition from an open, random structure to an ordered structure.

134. Most materials when cooled from the liquid state will crystallize into a rigid solid. Consider melting lead or solder to form a liquid, when it cools it becomes a solid. Liquids do not have an ordered structure; they have a random, open network.

135. Viscosity When glass cools from a liquid state at high temperatures it does not crystallize. First it becomes what we call a supercooled liquid. As temperatures below the supercooled phase, the glass becomes very viscous (thick, stiff) and becomes a rigid solid.

136. Since glasses do not have distinct melting or fusion points like solids, we use viscosity to describe the various states of glass. Viscosity is a property related to liquid flow. The opposite of viscosity is fluidity. A liquid that pours easily has a low viscosity (high fluidity); a liquid that is thick and hard to pour has a high viscosity (low fluidity).

137. Glass melting Glass melting is conducted at high temperatures (1500oC, 2700oF) where the viscosity is about 50 - 150 Poise. For comparison, heavy machine oil and castor oil have viscosity values of 10 to 20 Poise at room temperature. Corn syrup has a viscosity of 200 Poise at room temperature which is close to that of glass at melting temperatures.

138. Glass melting Glass is normally formed or "worked" at temperatures from 800 to 1100oC (1470 - 2000 oF ) where viscosities range from 3,000 to 10,000 Poise. Glass at room temperature has a very high viscosity of about 10 followed by 18 zeros. Thus we normally use the log of viscosity since it is easier to write. We would then write the viscosity at room temperature as 1018 Poise.

139. Colored Glass There are three mechanisms of colored glasses: Ionic/Electronic transfer - transition metal ions Sulfides, selenides , and tellurides Metallic (colloidal)  Colored glass can be made by adding small amounts of certain chemicals such as cobalt (Co), manganese ( Mn ), copper (Cu), and chromium (Cr). These materials cause specific wavelengths of light to be absorbed, creating a colored glass.

140. Color Formation in Glasses Color Coloring Agents Purple Manganese(III), Neodymium , Nickel in K 2 O glasses Blue Cobalt, Copper(II), Sulfur in B 2 O 3 glasses Green Chromium , Copper with Ti, Cr, Fe Iron with chromium-uranium-vanadium Molybdenum in P 2 O 5 glasses Yellow Uranium , Cadmium sulfide *, Cerium , titanium-silver Orange Cadmium sulfide plus cadmium selenide* Amber Iron and sulfur* - manganese and sulfur* Brown Manganese and iron-titanium and iron-nickel in Na 2 O glasses; iron and selenium-manganese and titanium-manganese and cerium Red Cadmium sulfide plus selenium* - gold-copper-uranium in PbO glasses Black Combinations of cobalt, manganese, nickel, iron, copper, and chromium-iron sulfide* - manganese and cobalt in PbO glasses

141. Some typical colors are: Ion Color Comments Co2+ Blue Ni2+ Violet in glasses containing K2O Yellow in glasses containing Na2O Nd3+ Violet Pr3+ Yellow-Green Ce4+ Yellow Ce3+ is colorless Cr3+ Green Mn3+ Violet Mn2+ colors faintly yellow Cu2+ Blue Cu1+ is colorless Fe3+ Yellow-Brown Fe2+ + Fe3+ Blue-Green total reduction to Fe2+ not possible (UO4)2- Yellow-Green

142. With our borax glass we will make a copper blue glass. When we melt the borax glass on the copper wire loop, some of the copper metal will go into the glass.

143. BACKGROUND Glass is the rigid metastable solid produced by quenching a liquid form rapidly enough to prevent crystallization. In most cases glass can be cooled very slowly without worry of crystallization (except metallic glasses). Due to its extreme viscous nature at high temperature, atomic mobility is low preventing orderly arrangement of atoms upon cooling. Glass is characterized by an arrangement of amorphous semi-periodic regions, and therefore lacks any long-term crystalline order. Pure silicate glasses are transparent, and coloring is determined by impurities. Therefore, silicate glasses are hard, brittle and electrically insulating. In contrast, metallic glasses are opaque, corrosion resistant, electrically conducting, very strong, flexible, and some have useful magnetic properties.

144. When glass is in its highly viscous molten state, it is considered a supercooled liquid. In its solid form, glass is referred to as an amorphous or vitreous solid. The temperature at which this transition from a supercooled liquid state to a vitreous state is referred to as the glass transition temperature, Tg (fig. 7-1).

145. Transition paths for liquid to glass vs. liquid to crystal in terms of specific volume

146. The glass transition temperature The glass transition temperature is not fixed for a particular glass composition. Rather, the glass transition temperature varies slightly and is dependent on the cooling rate. Faster cooling rates promote glass transitions at higher temperatures and slower cooling rates promote glass transitions at lower temperatures.

147. Cooling rate dependence of the glass transition temperature and specific volume

148. Fast cooling Fast cooling yields a glass with a relatively open structure, compared to a slow cooled glass whose specific is lower (density higher). For this reason, among others, it is important to anneal glass artifacts in the range of the glass transition temperature to eliminate the effects (residual internal stresses) of differential cooling rates, which may result from glass processing.

149. Glass Compositions Soda-lime-silica glass is a very common glass which is used in windows, bottles, canning jars, drinking glasses, and other household glass objects. Soda-lime-silica glass is also called "flint" glass since its primary component is sand (SiO2) and the rock called "flint" is 100% SiO2.

150. The basic composition of flint glass is as follows.   SiO2: 75 wt % NaO2: 15 wt % CaO : 10 wt %

151. We only need to use three raw materials to produce flint glass: Common Name Chemical Name Chemical Formula Glass Component Wt % Sand Silica or Silicon Dioxide SiO2 SiO2 100 Soda Ash Sodium Carbonate Na2CO3 Na2O 58.5 Limestone Calcium Carbonate CaCO3 CaO 56

152. Let's calculate how much raw materials (batch) would be required to make 100 pounds (lb.) of flint glass Raw Material Wt % Oxide Glass Calculation Pounds Oxide Batch Calculation Pounds Batch Sand 100% SiO2 0.75 x 100 75 lb. 75 / 1.00 75.0 lb. Soda Ash 58.5% Na2O 0.15 x 100 15 lb. 15 / 0.585 25.6 lb. Limestone 56% CaO 0.10 x 100 10 lb. 10 / 0.56 17.9 lb. Total 100 lb. 118.5 lb.

153. As you can see, we require 118.5 pounds of batch to produce 100 pounds of glass. What happens to the extra 18.5 pounds? It goes out the furnace chimney as carbon dioxide (CO2) gas. Soda ash and limestone are called carbonates. Upon heating or decomposition, they release CO2.   Na2CO3 (s) Na2O (s) + CO2 (g)   CaCO3 (s) CaO (s) + CO2 (g)

154. Another common commercial glass is the borosilicate glass often referred to as "Pyrex" or " Kimax " glass. This glass contains boron oxide (B2O3) which makes it more resistant to thermal shock (sudden heating or cooling) than soda-lime-silica glass.

155. Borosilicate glass is the type of glass you find used in the beakers and test tubes used in your chemistry laboratory. It is also used at home in the kitchen for ovenware. The major components is borosilicate glass include:  SiO2 : 81% Al2O3 : 2% B2O3 : 13% Na2O : 4%

156. You can make a very simple glass using just a Bunsen burner or propane torch. For this glass we will use a common household ingredient, Borax, that you can buy at the grocery store. Borax has the following chemical composition:   Na2B4O7 - 10 H2O  When we heat the borax, the water (H2O) is released. We end up with a final glass composition of 69.2 wt% B2O3 and 30.8 wt% Na2O. Ten grams (10 g) of borax would produce 5.3 g of glass. Unfortunately, borax glass can easily react with water and decomposes after awhile in water atmospheres. Thus we can only use it to demonstrate the formation of glass.

157. Glasses are amorphous ceramic materials. The amorphous (or glassy) state of matter occurs when a substance has not been given sufficient time to crystallize. Glasses are most commonly made by rapidly quenching a melt. This means that the atoms making up the glass material are unable to move into positions which allow them to form the crystalline regularity. This may be attributed to the fact that each atom is strongly bonded to adjacent atoms while in the liquid state, and that the crystalline structures are very complex.

158. The end result of all these factors is that the glass structure is disordered and therefore amorphous. One of the most notable characteristics of glasses is the way they change between solid and liquid states. Unlike crystals, which transform abruptly at a precise temperature (i.e., their melting point) glasses undergo a gradual transition. Between the melting temperature (T m ) of a substance and the so-called glass transition temperature ( T g ), the substance is considered a supercooled liquid.

159. When glass is worked between T g and T m , one can achieve virtually any shape. The glass blowing technique is a fascinating demonstration of the incredible ability of glass to deform.

160. Advantage Of The Glass Forming A chief advantage of the glass forming process is that the item remains one single piece with continuous molecular structure and without internal surfaces. That is why optical fibers are drawn from glass. No scattering of light at grain boundaries occurs. Certain glasses have non-linear optical properties that can be used for optical switches making the development of optical computers more likely.

161. Properly doped with polyvalent transition metals, glasses become semiconducting . But , their semiconducting properties can be altered by electrical fields, making these glasses suitable for information storage devices. Glasses of this kind are used for the coatings on the printing drums in laser printers or Xerox copiers.

162. Some glasses exhibit very high ionic conductivity, which makes them useful as electrolytes in batteries or sensors. One commercial example can be found in every chemistry laboratory, the pH meter. While crystalline ceramics, for the most part, have well defined chemical compositions, the compositions of glasses can be widely varied. Glass is made out of silica which has a very high melting point.

163. In the attempt to lower the melting temperature, soda ash (a mixture of Na 2 O, sodium oxide, and Na 2 CO 3 , sodium carbonate), and limestone (CaCO 3 ) are added as fluxes. Other glass fluxes might include lead oxides or lead carbonates (leaded glass or flint glass) or borax/borax oxides (borosilicate glass).

164. Borax is a naturally occurring mineral that is chemically hydrated sodium borate or Na 2 B 4 O 7 · 10 H 2 O. The material is a white powder that is sold in super markets as a laundry aid. Borax is also used as a flux in working some metals because it coats and cleans the metal and allows soldering to take place. When Borax is heated, the water of hydration is driven off and the sodium, boron and oxygen form a non-crystalline glass. This glass is clear but will take a color from the various metal oxides such as cobalt or nickel. Thus the Borax beads can be used to identify some metal ions as well as demonstrate materials used to make colored glass. Borax Glass is also unstable in that it tends to absorb moisture from the air and revert back to a cloudy hydrated material.

165. Silicate glass Ingredients Silica (the chemical compound SiO 2 ) is a common fundamental constituent of glass. In nature, vitrification of quartz occurs when lightning strikes sand, forming hollow, branching rootlike structures called fulgurite.

166. More common types of silicate glasses, and their ingredients, properties, and applications: Fused quartz, also called  fused silica glass, vitreous silica glass , is silica (SiO 2 ) in vitreous or glass form (i.e., its molecules are disordered and random, without crystalline structure). It has very low thermal expansion, is very hard, and resists high temperatures (1000–1500 °C). It is also the most resistant against weathering (caused in other glasses by alkali ions leaching out of the glass, while staining it). Fused quartz is used for high temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc.

167. Soda-lime-silica glass, window glass : silica 72% + sodium oxide (Na 2 O) 14.2% + lime ( CaO ) 10.0% + magnesia ( MgO ) 2.5% + alumina (Al 2 O 3 ) 0.6%. Is transparent, easily formed and most suitable for window glass (see flat glass). It has a high thermal expansion and poor resistance to heat (500–600 °C). It is used for windows, some low temperature incandescent light bulbs, and tableware.  Container glass is a soda-lime glass that is a slight variation on flat glass, which uses more alumina and calcium, and less sodium and magnesium which are more water-soluble. This makes it less susceptible to water erosion.

168. Sodium borosilicate glass, Pyrex : silica 81% + boric oxide (B 2 O 3 ) 12% + soda (Na 2 O) 4.5% + alumina (Al 2 O 3 ) 2.0%. Stands heat expansion much better than window glass. Used for chemical glassware, cooking glass, car head lamps, etc. Borosilicate glasses (e.g. Pyrex) have as main constituents silica and boron oxide. They have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25×10 –6 /° C  as compared to about 9×10 −6 /°C for a typical soda-lime glass), making them more dimensionally stable. The lower CTE also makes them less subject to stress caused by thermal expansion, thus less vulnerable to cracking from thermal shock. They are commonly used for reagent bottles, optical components and household cookware.

169. Thank you