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Types and Chemistry of Glasses
Glass is a state of matter. Glasses combine some properties of crystals and some of liquids but are distinctly different from both. Glasses have the mechanical rigidity of crystals, but the random disordered arrangement of molecules that characterizes liquids
Glass is made today by melting quartz or sand at a very high temperature and molding it into shape. Glass without any additives are called quatrz glass. They have very high melting points and requires extra hot flames to work with during galssblowing. But melting sand by itself is too expensive because of the high temperatures required (about 1850°C, or 3360°F).
Therefore, FLUXES are added which let the FORMER melt more readily and at lower temperatures (1300°C, or 2370°). These include: Soda Ash (Na2O), Potash (K2O), lime (CaO) and Lithium Carbonate(Li2CO3.) However, FLUXES also make the glass chemically unstable, liable to dissolve in water or form unwanted crystals. Therefore, STABILIZERS are added to make the glass uniform and keep its special structure intact. These include: Limestone, Litharge, Alumina,Magnesia, Barium Carbonate, Strontium Carbonate,Zinc Oxide, Zirconia
Glass has no specific melting point and there is a wide temperature range in which the glass can be shaped. This way of making glass is very expensive because of the high temperature needed in melting. Soda ash (Na2CO3) and lime (CaO) can be added to the quartz to lower the melting temperature and creating the most common form of glass called soda-lime glass. Which is what windows and bottles are made of. Making glass this way can result in a very impure glass. Glass consists of covalently bonded Silicon and Oxygen atoms with positively charged metal oxides bonded within the oxygen-silicon matrix. (tetrahedrally bonded)
Nearly all commercial glasses fall into one of six basic categories or types. These categories are based on chemical composition. Within each type, except for fused silica, there are numerous distinct compositions.
- Soda-lime glassis the most common (90% of glass made), and least expensive form of glass. It usually contains 60-75% silica, 12-18% soda, 5-12% lime. Resistance to high temperatures and sudden changes of temperature are not good and resistance to corrosive chemicals is only fair.
- Lead glasshas a high percentage of lead oxide (at least 20% of the batch). It is relatively soft, and its refractive index gives a brilliance that may be exploited by cutting. It is somewhat more expensive than soda-lime glass and is favored for electrical applications because of its excellent electrical insulating properties. Thermometer tubing and art glass are also made from lead-alkali glass, commonly called lead glass. This glass will not withstand high temperatures or sudden changes in temperature.
- Borosilicate glassis any silicate glass having at least 5% of boric oxide in its composition. It has high resistance to temperature change and chemical corrosion. Not quite as convenient to fabricate as either lime or lead glass, and not as low in cost as lime, borosilicate’s cost is moderate when measured against its usefulness. Pipelines, light bulbs, photochromic glasses, sealed-beam headlights, laboratory ware, and bake ware are examples of borosilicate products.
- Aluminosilicate glasshas aluminum oxide in its composition. It is similar to borosilicate glass but it has greater chemical durability and can withstand higher operating temperatures. Compared to borosilicate, aluminosilicates are more difficult to fabricate. When coated with an electrically conductive film, aluminosilicate glass is used as resistors for electronic circuitry
- Pyrax or Borosilicate: Another property of the glasses is varying thermal heat of expansion rates. Knowing these rates can tell you what types of glass will shatter when heated or cooled rapidly. One that you probably all ready know of is Pyrex, which is a borosilicate glass. This glass has the compound B2O3in the matrix.Ninety-six percent silica glass is a borosilicate glass, melted and formed by conventional means, then processed to remove almost all the non-silicate elements from the piece. By reheating to 1200°C the resulting pores are consolidated. This glass is resistant to heat shock up to 900°C.
- Quartz is fused silica glassis pure silicon dioxide in the non-crystalline state. It is very difficult to fabricate, so it is the most expensive of all glasses. It can sustain operating temperatures up to 1200°C for short periods.