The main constituent of practically all-commercial glass is sand. Sand by itself can be fused to produce glass but the temperature at which this can be achieved is about 1700C. 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 gives off carbon dioxide leaving the oxides in the glass.
The chemical and physical properties of soda-lime glass make it suitable for visible light and hence applications. The nominally colorless types transmit a very high percentage of visible light and have been used for windows since at least the time of the Romans. Soda-lime glass containers are virtually inert, and therefore do not contaminate the contents inside. Their resistance to chemical attack from aqueous solutions is good enough to withstand repeated boiling (as in the cast of preserving jars) without any significant changes in the glass surface.
One of the main disadvantages of soda-lime glass is their relatively high thermal expansion. Silica does not expand very greatly when heated but the addition of soda has a dramatic effect in increasing the expansion rate. In general, the higher the soda content of glass, the poorer its resistance will be 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.
As the name implies, borosilicate glass is composed mainly of silica (70-80%) and boric oxide (7-13%) with smaller amounts of 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 apparatuses, 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 Pyrex, the first glass of this type to be placed on the consumer market.
The float glass process, invented by Pilkington Brothers PLC and introduced in 1959, is now the principal method of producing flat glass around the world.
The glass is held in a chemically controlled atmosphere at a high enough temperature (1000C) for a long enough time for irregularities to melt out and for the surface to become flat and parallel. Because the surface of molten tin is flat, the glass becomes flat and the thickness of the ribbon, in the range of 2.5mm to 25mm, is controlled at this stage. The ribbon is cooled down while still advancing along the molten tin until the surface is hard enough (600C) for it to be lifted onto the conveyor rollers without marking the bottom surface. The ribbon passes through the annealing chamber to the automatic warehouse where computers govern the cutting of the ribbon to match custom orders. A large modern float glass plant will produce 5000 tons of glass per week. The glass produced has a uniform thickness and bright fire-polished surfaces.
Creative Art Glass Inc., 525 Denison Street, Unit 3, Markham, Ontario, Canada L3R 1B8