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Brief introduction of glass

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）

The common form of silica is sand, but it also occurs in nature in a crystalline form known as quartz.

Pure silica can produce an excellent glass, but it is very high-melting (1,723<sup>o</sup>C, or 3,133<sup>o</sup>F), and the melt is so extremely viscous that it is difficult to handle. All common glasses contain other ingredients that make the silica easier to melt and the hot liquid easier to shape.

Natural Glass

Probably as early as 75,000 b.c.e., long before human beings had learned how to make glass, they had used natural glass to fashion knives, arrowheads, and other useful articles. The most common natural glass is obsidian, formed when the heat of volcanoes melts rocks such as granite, which then become glassy upon cooling. Other natural glasses are pumice, a glassy foam produced from lava; fulgurites, glass tubes formed by lightning striking sand or sandy soil; and tektites, lumps or beads of glass probably formed during meteoric impacts.

Manmade (Synthetic) Glass

When, where, or how human beings discovered how to make glass is not known. Very small dark-colored beads of glass have been dated back to 4000 b.c.e. These may well have been by-products of copper smelting or pottery glazing. By 2500 b.c.e. small pieces of true synthetic glass appeared in areas such as Mesopotamia, but an actual glass industry did not appear until about 1500 b.c.e. in Egypt By this time various small vases, cosmetic jars, and jewelry items made of glass had begun to appear.

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.

High silica glass (96.5–100% silica) remains difficult to make because of the very high melting point of pure silica. However, it is made for special purposes because of its outstanding durability, excellent resistance to thermal shock or chemical attack, and ability to transmit ultraviolet light (an ability that ordinary glass does not have). Spacecraft windows, made of 100 percent silica, can withstand temperatures as high as 1,200oC (2,192oF). Table 1 lists the five major types of glass along with properties and uses.

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.

To make glasses stronger and more durable, stabilizers are added. The most common stabilizer is lime (CaO), but others are magnesia (MgO), baria (BaO), and litharge (PbO). The most common glass, made in largest amounts by both ancient and modern glassmakers, is based on silica as the glass former, soda as the flux, and lime as the stabilizer. It is the glass used to make windows, bottles, jars, and lightbulbs.

Colored Glass. The natural glasses used by the ancients were all dark in color, usually ranging from olive green or brown to jet black. The color was