What do toffee, volcanic lava, enameled baths, and Perspex have in common? They are all examples of what has been called the fourth state of matter―they are GLASSES, just as much as a bottle or windowpane. A glass is a material with the frozen-in structure of a liquid, rather than that of a crystalline solid. A glass is a supercooled liquid. The atoms in a glass are arranged in a random, disordered way like the atoms in a liquid. In a crystal, the atoms are all arranged in a regular, ordered pattern. The diagram seen herewith shows the difference between glass and crystals. Diamond, sugar, and salt are examples of crystals, as is quartz, the raw material from which glass is made. Man's earliest weapons and cutting tools were made from volcanic glass or obsidian. It breaks to give sharp edges, which are useful for cutting. Australian aborigines still make arrowheads from pieces of glass bottles. As R. W. Douglas and S. Frank stated in their book, 'A History of Glassmaking' in 1972, "Life without glass is difficult to imagine; glass windows allow light from the sun to brighten our buildings, but they also help keep our rooms warm in winter; glass is essential for the provision of convenient artificial lights and as a component in the cameras, cine-projectors, and television sets which we use during our leisure time. It provides us with fine drinking glasses and robust cooking utensils, and with containers for foods and liquids of all kinds. This hard, inert, transparent material is made by heating together a mixture of materials such as sand, limestone, and soda. At a sufficiently high temperature a white heat of about 1400-1500° C just below the melting point of iron, these materials react to form a liquid. When this liquid is taken from the furnace it gets stiffer and stiffer as it cools, until at about 500° C it has become as solid as the glass we are familiar with in our windows or on our tables". Some Uses of Glass 1. Windows 2. Containers, bottles, and jars. 3. Laboratory glass wart 4. Cooking utensils 5. Light bulbs 6. Optical fiber 7. TV tubes 8. Optical instruments 9. Glass fiber 10. Many others Lava is molten rock from a volcano, made mainly of silica, which cooled rapidly and formed glass, instead of a crystal. Most rocks are ordered and crystalline and their main constituent is silica (silicon dioxide, SiO2), also known in its pure form as sand. Sand can be melted to give silica glass, but this requires a temperature of over 1700° C. Early man must have discovered that when sand as heated with plant ashes (containing alkalis) it turned into a liquid which cooled to give clear, sometimes colored, objects. The earliest glass object found dates from around 4500 B.C., in the Middle East. Truly glass comes from the sands of time! Lime is also needed to make a hard insoluble glass. This type of glass is called soda-glass or soda lime glass, because the raw materials are soda ash (sodium carbonate) and limestone (calcium carbonate). Glass is thus made from some simple and common raw materials. Different Types of Glass There are several different types of glass in use, each with a different chemical composition. Common glass contains 70% SiO2, 15% sodium oxide (Na2O), 10% calcium oxide (CaO), and 5% of other oxides. It is known as soda-lime glass. It is used for windows and glass bottles. Laboratory glassware and ovenware for cooking is made of a different type of glass which is more resistant to chemicals, scratches, and heat. It is known as borosilicate glass, and it contains 80% SiO2, 13% boric oxide (B2O3), and 7% other oxides. Pyrex is a trade name for one brand of borosilicate glass. Lead crystal glass, as made by Waterford Crystal, contains a large amount of lead oxide (PbO) which gives the glass its brilliance. By putting in small amount of metal oxides we can give glass a range of colors. The brown glass used for beer bottles results from iron in the sand used to make the glass. To get colorless glass, specially pure sand must be used and a decolorizer like manganese dioxide (MnO2) is added to oxidize any iron present and remove its color. Glass can be made into glass fiber used for insulation and making boats, etc.―this glass contains aluminum oxide, Al2O3―or into special optical fibers which are now being used for communication, using light instead of electrons to carry the message. Glass may be an old material, but it still has many surprises to offer and it is still indispensable in our modern world. Important Dates in the History of Glass ~4500 B.C. to 3000 B.C. Glass first used in Egypt and Mesopotamia ~1500 B.C. First glass vessels made in Egypt ~200 B.C. Glass blowing invented in the near East ~600 A.D. Stained glass windows introduced in Constantinople 1674 A.D. Lead crystal invented by George Ravenscroft in England 1688 A.D. Cast plate glass in France 1690 A.D. First glassworks in Ireland 1827 A.D. Glass pressed into molds in U.S.A. 1851 A.D. Crystal Palace built in London―first prefabricated building of iron and glass 1874 A.D. Glass toughened by oil quenching―France 1886 A.D. First semi-automated bottle maker―England 1903 A.D. First automatic bottle making machine invented by Michael Owens, an Irishman, in the U.S.A. 1903 A.D. Chance discovery of laminated glass by Benedictus in France 1915 A.D. Invention of borosilicate glass inthe U.S.A. 1937 A.D. Photosensitive glass invented by Dalton in the U.S.A. 1957 A.D. Glass ceramic made by accident by Stookey in the U.S.A. 1959 A.D. Float glass process for plate glass introduced by Pilkingtons in the U.K.