9-12 COMMERCIAL GLASSES

9-12 COMMERCIAL GLASSES

Commercial glasses can be classified as soda lime or lime glasses, lead glasses, borosilicate glasses, and high-silica glasses. Their typical chemical compositions are given in Table 9-3.

Soda lime glasses have compositions approximating the formula Na 2 O.CaO.6SiO 2 . Additional small amounts of alumina and magnesium oxide are introduced to improve the chemical resistance and durability of glass. To mask the colors developed by contained iron compounds, minute amounts of coloring agents can be added. Soda lime glasses are produced in largest quantity because they are low in cost, resistant to devitrification, and relatively resistant to water. They are easily hot-worked and are widely used as window glass, electric bulbs, bottles, and cheaper tableware, where high- temperature resistance and chemical stability are not required.

Lead glasses, also called “flint” glasses, usually contain from 15% to 30% lead oxide. They are used for high-quality tableware, optical purposes, neon sign tubing, and in art objects because of their high luster. The glasses of high lead content, up to 80%, are used for extra dense optical glasses and for windows and shields to protect personnel from X- ray radiation. Lead glasses have a relatively low melting point, but they exhibit good hot work ability, high electrical resistivity, and high refractive indices. Borosilicate glasses contain virtually only silica and boron with a small amount of alumina and still less alkaline oxide. The substitution for alkali and basic alkali oxides of the lime glasses by boron and aluminum results in a glass of low thermal coefficient of expansion and high chemical resistance. The glass is known under the trade name Pyrex. Because of their high chemical stability, high thermal shock resistance, and excellent electrical resistivity, borosilicate glasses are extensively used in industry as piping, gauge glasses, laboratory ware, electrical insulation, and for some domestic purposes. Aluminosilicate glasses Lead glasses, also called “flint” glasses, usually contain from 15% to 30% lead oxide. They are used for high-quality tableware, optical purposes, neon sign tubing, and in art objects because of their high luster. The glasses of high lead content, up to 80%, are used for extra dense optical glasses and for windows and shields to protect personnel from X- ray radiation. Lead glasses have a relatively low melting point, but they exhibit good hot work ability, high electrical resistivity, and high refractive indices. Borosilicate glasses contain virtually only silica and boron with a small amount of alumina and still less alkaline oxide. The substitution for alkali and basic alkali oxides of the lime glasses by boron and aluminum results in a glass of low thermal coefficient of expansion and high chemical resistance. The glass is known under the trade name Pyrex. Because of their high chemical stability, high thermal shock resistance, and excellent electrical resistivity, borosilicate glasses are extensively used in industry as piping, gauge glasses, laboratory ware, electrical insulation, and for some domestic purposes. Aluminosilicate glasses

Ninety-six percent silica glasses are made by chemically removing the alkalies from a borosilicate glass. After it has been melted and shaped to the desired oversized dimensions, the borosilicate glass, is heat treated. This causes a separation into two layers: one high in alkalies and boron oxide, the other high in silica. The alkali layer is dissolved in immersing the article in hot acid, leaving a porous. high-silica layer. By reheating the article at about 1200°C (2192°F), the glass becomes perfectly clear and vacuum tight. Ninety-six percent silica glasses are much more expensive than other types of glasses. They are used mainly where extreme thermal shock resistance and high temperature resistance up to 900°C (1652°F) are required.

Glasses possess high chemical resistance to most corrosive agents. Commercial silicate glasses are corroded only by hydrofluoric acid, hot concentrated phosphoric acid, and concentrated alkaline solutions. Borosilicate and high-silica glasses have much higher chemical resistance, and fused silica has even higher resistance. These glasses are actually used in the construction of chemical plants.

Fused, also called vitreous, silica is almost pure silica (99.6% to 99.9% SiO 2 ) made by fusing pure quartz crystals or glass sand in an electric arc or a high-frequency furnace or in the oxyhydrogen flame. Since there are no fluxing constituents present. the fusion temperature is about 1750°C (3182°F), even though the molten glass is so viscous that it is very difficult to obtain complete homogeneity and freedom from bubbles.

Fused silica is available in a translucent and transparent variety. Transparent silica, also called fused quartz, is highly transparent to ultraviolet, visible, and infrared radiation and is much stronger mechanically, more resistant to devitrification, and less permeable to gases than the translucent form. Transparent silica is mostly used for optical instruments and other instruments for which high transparency to a wide range of radiation is required. It is very expensive material. The translucent form, or vitreous silica, owing to its lower price, is used mainly for wares for chemical plants, for chemical laboratory wares, and for electrical insulating materials in electrical heaters, furnaces. and the like.

Fused silica has a very low and regular coefficient of thermal expansion, which makes it highly resistant to thermal shock. Its high fusion point gives it stability over a wide range of temperature. The useful temperature range is limited, however, to about 1100°C (2012°F) because of flow and a tendency to devitrification. Various specialty glasses, such as optical glass, photosensitive glass, opal glass, radiation-absorbing glass, and metal-coated glasses, are also available.