Glass is a state of matter. It is a solid produced by cooling molten material so that the internal arrangement of atoms, or molecules, remains in a random or disordered state, similar to the arrangement in a liquid. Such a solid is said to be amorphous or glassy. Ordinary solids, by contrast, have regular crystalline structures.
Many materials can be made to exist as glasses. Hard candies, for example, consist primarily of sugar in the glassy state. What the term "glass" means to most people, however, is a product made from silica (SiO2）
All the ancient glasses were based on silica (sand), modified with considerable amounts of various metal oxides, mainly soda (Na2O) and lime (CaO). This is still the most common glass being used today. It is known as soda lime glass. However, the ancient glass was usually colored and opaque due to the presence of various impurities, whereas most modern glass has the useful property of transparency.
Hundreds of thousands of different glass compositions have been devised, and they have been used in different ways. Much has been learned about which combination of chemicals will make the best glass for a particular purpose. For example, in 1664 an Englishman named Ravenscroft found that adding lead oxide (PbO) to a glass melt produced a brilliant glass that was much easier to melt and to shape. Since that time lead glass has been used to make fine crystal bowls and goblets and many kinds of art glass
An important kind of glass was developed in the early 1900s to solve a serious problem—the inability of glass to withstand temperature shock. This failure resulted in tragic accidents in the early days of the railroads. Glass lanterns used as signals would get very hot, and then, if it started to rain, the rapid cooling would sometimes cause the glass to break and the signal to fail. The problem was solved by replacing much of the soda in the glass with boron oxide (B2O3). The resulting glass, called borosilicate, contains about 12 percent boron oxide and can withstand a temperature variation of 200oC (392oF). It also has greater chemical durability than soda lime glass. Today borosilicate glass is used in most laboratory glassware (beakers, flasks, test tubes, etc.) and in glass kitchen bakeware.
For even greater heat shock resistance and chemical durability, alumina (Al2O3) can be used instead of boron oxide. The resultant aluminosilicate glass has such resistance to heat shock that it can be used directly on the heating element of the kitchen stovetop. It is also used to make the special bottles used for liquid pharmaceutical prescriptions, and to produce the glass thread that is woven into fiberglass fabric.
Glass Composition. The making of glass involves three basic types of ingredients: formers, fluxes, and stabilizers. The glass former is the key component in the structure of a glassy material. The former used in most glasses is silica (SiO2). Pure silica is difficult to melt because of its extremely high melting point (1,723oC, or 3,133oF), but fluxes can be added to lower the melting temperature. Other glass formers with much lower melting points (400oC–600oC, or 752–1,112oF) are boric oxide (B2O3) and phosphorus pentoxide (P2O5). These are easily melted, but because their glass products dissolve in water, they have limited usefulness.
Most silica glasses contain an added flux, so that the silica can be melted at a much lower temperature (800oC–900oC, or 1,472–1,652oF). Standard fluxes include soda (Na2O), potash (K2O), and lithia (Li2O). Frequently the flux is added as a carbonate substance (e.g., Na2CO3), the CO2 being driven off during heating. Glasses containing only silica and a flux, however, have poor durability and are often water-soluble.