What Is Glass Made From?
Glass is made naturally from a fusion of silica (sand), soda and lime. This fusion can be achieved merely by lightning striking in a place where the right ingredients happen to be adjacent to each other, forming hollow, branching rootlike structures called fulgurite. When glass is made by man, other ingredients are added, such as potash, lead oxide and boric oxide. Some of these ingredients are used to make glass clear, some to color it, and others to give it a frosted effect.
Glass was made by potters in Egypt for glazing stone beads as early as 12,000 B.C. As Egyptian culture progressed, craftsmen used glass for the manufacture of personal ornaments and bottles. A tremendous step forward in the use of glass was made by the Phoenicians in about 300 to 200 B.C. by the invention of the blow pipe. The blowpipe is a hollow iron tube with a mouthpiece at one end and a knob-shape at the other.
The knob-shaped end is dipped into hot, viscous glass. A “gather” of molten glass remains on the end when the pipe is withdrawn. This hot glass can be blown by the worker into a hollow ball. The more he blows, the larger the ball. During the roman civilization the art of glass-making reached near-perfection.
In the 3rd century, the Romans cast glass on flat stones and produced the first window panes. The break-up of the Roman Empire and the ensuing Dark Ages brought an end to such cultural developments. The glazing of windows did not become wide spread over the whole of Europe until the 15th and 16th centuries.
Fused quartz is a glass made from chemically-pure SiO2 (silica). It has excellent resistance to thermal shock, being able to survive immersion in water while red hot. However, its high melting temperature (1723 °C) and viscosity make it difficult to work with. Normally, other substances are added to simplify processing. One is sodium carbonate (Na2CO3, “soda”), which lowers the glass-transition temperature.
The soda makes the glass water-soluble, which is usually undesirable, so lime (CaO, calcium oxide, generally obtained from limestone), some magnesium oxide (MgO) and aluminium oxide (Al2O3) are added to provide for a better chemical durability. The resulting glass contains about 70 to 74% silica by weight and is called a soda-lime glass. Soda-lime glasses account for about 90% of manufactured glass.
Most common glass contains other ingredients to change its properties. Lead glass or flint glass is more “brilliant” because the increased refractive index causes noticeably more specular reflection and increased optical dispersion. Adding barium also increases the refractive index. Thorium oxide gives glass a high refractive index and low dispersion and was formerly used in producing high-quality lenses, but due to its radioactivity has been replaced by lanthanum oxide in modern eyeglasses. Iron can be incorporated into glass to absorb infrared radiation, for example in heat-absorbing filters for movie projectors, while cerium(IV) oxide can be used for glass that absorbs UV wavelengths.
The following is a list of the more common types of silicate glasses and their ingredients, properties, and applications:
- Fused quartz, also called fused-silica glass, vitreous-silica glass: silica (SiO2) in vitreous, or glass, form (i.e., its molecules are disordered and random, without crystalline structure). It has very low thermal expansion, is very hard, and resists high temperatures (1000–1500 °C). It is also the most resistant against weathering (caused in other glasses by alkali ions leaching out of the glass, while staining it). Fused quartz is used for high-temperature applications such as furnace tubes, lighting tubes, melting crucibles, etc.
- Soda-lime-silica glass, window glass: silica + sodium oxide (Na2O) + lime (CaO) + magnesia (MgO) + alumina (Al2O3). Is transparent, easily formed and most suitable for window glass (see flat glass). It has a high thermal expansion and poor resistance to heat (500–600 °C). It is used for windows, some low-temperature incandescent light bulbs, and tableware. Container glass is a soda-lime glass that is a slight variation on flat glass, which uses more alumina and calcium, and less sodium and magnesium, which are more water-soluble. This makes it less susceptible to water erosion.
- Sodium borosilicate glass, Pyrex: silica + boron trioxide (B2O3) + soda (Na2O) + alumina (Al2O3). Stands heat expansion much better than window glass. Used for chemical glassware, cooking glass, car head lamps, etc. Borosilicate glasses (e.g. Pyrex, Duran) have as main constituents silica and boron trioxide. They have fairly low coefficients of thermal expansion (7740 Pyrex CTE is 3.25×10−6/°C as compared to about 9×10−6/°C for a typical soda-lime glass), making them more dimensionally stable. The lower coefficient of thermal expansion (CTE) also makes them less subject to stress caused by thermal expansion, thus less vulnerable to cracking from thermal shock. They are commonly used for reagent bottles, optical components and household cookware.
- Lead-oxide glass, crystal glass, lead glass: silica + lead oxide (PbO) + potassium oxide (K2O) + soda (Na2O) + zinc oxide (ZnO) + alumina. Because of its high density (resulting in a high electron density), it has a high refractive index, making the look of glassware more brilliant (called “crystal”, though of course it is a glass and not a crystal). It also has a high elasticity, making glassware “ring”. It is also more workable in the factory, but cannot stand heating very well. This kind of glass is also more fragile than other glasses (and is easier to cut).
- Aluminosilicate glass: silica + alumina + lime + magnesia + barium oxide (BaO) + boric oxide (B2O3). Extensively used for fiberglass, used for making glass-reinforced plastics (boats, fishing rods, etc.) and for halogen bulb glass. Aluminosilicate glasses are also resistant to weathering and water erosion.
- Germanium-oxide glass: alumina + germanium dioxide (GeO2). Extremely clear glass, used for fiber-optic waveguides in communication networks. Light loses only 5% of its intensity through 1 km of glass fiber.
Another common glass ingredient is crushed alkali glass or “cullet” ready for recycled glass. The recycled glass saves on raw materials and energy. Impurities in the cullet can lead to product and equipment failure. Fining agents such as sodium sulfate, sodium chloride, or antimony oxide may be added to reduce the number of air bubbles in the glass mixture. Glass batch calculation is the method by which the correct raw material mixture is determined to achieve the desired glass composition.