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Commercial Glass

The main constituent of practically all commercial glasses is sand. Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700° C. Adding other chemicals to sand can considerably reduce the temperature of the fusion. The addition of sodium carbonate (Na₂O), known as soda ash, in a quantity to produce a fused mixture of 75% silica (SiO₂) and 25% of sodium oxide (Na₂O), will reduce the temperature of fusion to about 800° C. However, a glass of this composition is water soluble and is known as water glass. In order to give the glass stability, other chemicals like calcium oxide (CaO) and magnesium oxide (MgO) are needed. The raw materials used for introducing CaO and MgO are their carbonates CaCO3 (limestone) and MgCO₃ (dolomite), which when subjected to high temperatures give off carbon dioxide leaving the oxides in the glass.

Soda-lime glass: The chemical and physical properties of soda-lime glasses make them suitable for a visible light and hence applications. The nominally colorless types transmit a very high percentage of visible light and hence have been used for windows since at least the time of the Romans. Soda-lime glass containers are virtually inert, and so cannot contaminate the contents inside. Their resistance to chemical attack from aqueous solutions is good enough to withstand repeated boiling (as in the case of preserving jars) without any significant changes in the glass surface. One of the main disadvantages of soda-lime is their relatively high thermal expansion. Silica does not expand very greatly when heated but the addition of soda has dramatic effect in increasing the expansion rate and, in general, the higher the soda content of a glass, the poorer will be its resistance to sudden changes of temperature (thermal shock). Thus, care is needed when soda-lime containers are filled with hot liquids to prevent breakage due to rapid thermal expansion.

Borosilicate glass: As the name implies, borosilicate glasses are composed mainly of silica (70-80%) and boric oxide (7-13%) with smaller amounts of the alkalis (sodium and potassium oxides) and aluminum oxide. They are characterized by the relatively low alkali content and consequently have good chemical durability and thermal shock resistance. Thus they are permanently suitable for process plants in the chemical industry, for laboratory apparatus, for ampoules and other pharmaceutical containers, for various high intensity lighting applications and as glass fibers for textile and plastic reinforcement. In the home they are familiar in the form of ovenware and other heat-resisting ware, possibly better known under the trade name of the first glass of this type to be placed on the consumer market- Pyrex.

The Float Glass Process: The float glass process, invented by Pilkington Brothers PLC and introduced in 1959, is now the principal method of producing flat glass throughout the world. The glass is held in a chemically controlled atmosphere at a high enough temperature (1000° C) for a long enough time for irregularities to melt out and for the surface to become flat and parallel. Because the surface of the molten tin is flat, the glass becomes flat and the thickness of the ribbon, in the range 2.5mm to 25mm, is controlled at this stage. The ribbon is cooled down while still advancing along the molten tin until th surfaces are hard enough (600° C) for it to be lifted onto the conveyor rollers without marking the bottom surface. The ribbon passes through the annealing lehr to the automatic warehouse where computers govern the cutting of the ribbon to match custome orders. A large modern float glass plant will produce 5000 tons of glass per week. It operates continuously 24 hours a day, 365 days a year for several years. The glass produced has a uniform thickness and bright fire-polished surfaces without the need for grinding and polishing.

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