The manmade （Synthetic） glass

[raymond723]

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.

[Frank EU]

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