GLASS MANUFACTURE
Glass is manufactured by melting sand and the other desired ingredients and then allowing them to cool without crystallizing. The cooling process may take place in a mold or it may be injected into a particular shape or it may be cooled in such a way that it is perfectly flat. For example, molten glass can be cooled on top of a bath of molten tin. This allows the glass to be very flat so it can be used in windows and other similar applications. This type of glass is called float glass. A diagram of how float glass is manufactured is shown in Figure 1. Basic silicon glass is seldom used in a pure form. Instead, while it is in the melted state it is “doped” with measured amounts of various impurities that alter its proper ties in a predictable way. For example, when soda (sodium carbonate, Na2CO3) is added, the glass will melt at a lower temperature and flow more easily, making it easier to work with. Other materials such as lime (calcium oxide, CaO) can be added to stabilize the glass and make it less soluble in solvents. When both calcium oxide and sodium carbonate are added, the product is called soda lime glass. If boron oxide (B2O3) is added to glass, it becomes more heat resistant. It is then called borosilicate glass (Pyrex). This type of glass is used in cook ware, thermometers, and automobile headlights because it can take fast, extreme changes in temperature without cracking or shattering. Glass may be strengthened by tempering. This is a process whereby the glass is heated and cooled rapidly producing deliberate stress in the surface. When this glass breaks, it shatters into small solid pieces rather than sharp shards. This type of glass is used in car windows. In the US, tempered glass is not used in front windshields. Instead, a laminate consisting of two layers of glass with a layer of plastic between is used. If the windshield is broken, the laminate helps keep shards of glass from flying around the car and injuring the passengers. Three major types of glass are encountered as evidence in forensic cases: sheet or flat glass, container glass, and glass fibers. Flat glass is used to make windows and windshields and can also be shaped to form light bulbs, headlights, and other materials. Container glass is used to make bottles and drinking glasses. Glass fibers are used to make fiberglass and fiber optic cables as well as glass–plastic composite materials. Less frequently, optical glass used to make eyeglass lenses and similar materials may be encountered in forensic cases.
FIGURE 1 Method for manufacturing float glass. The molten glass is floated on a bed of liquid tin which has an extremely flat surface so that the glass surface in contact with the tin is very flat.
