- Education and Science
How Glass is Made
Glass in General
To start of, glass is a hard transparent material from which windows, bottles, jars and glasses, lenses and laboratory items is made. There are numerous types of glass but the main ingredient in all is 'silica' (SiO2).
The man-made glasses appeared around 4000 BC in Egypt and by 1500 BC it had developed into both an art and a technology. Glass science was then not developed until the work of numerous scientists including Michael Faraday (1791-1867), then later Carl Zeiss (1816-1888), Ernst Karl Abbe (1840-1905) and Friedrich Otto Schott (1851-1935). Over 30 elements had been used in experimental glasses by the turn of this century 20th century.
The raw materials are heated in a furnace forming a red-hot liquid. A blob of this molten glass on the end of a blow pipe can be blown into intricate and lovely shapes by glass-blowers. Thus, most glassware is made from molds, for bottles etc, and sheet glass is made by floating molten glass on a bed of molten tin. Glass is used in numerous ways and its properties can be altered by the processing or by adding of other materials.
Toughened or hardened glass is manufactured by cooling glass swiftly under cold air; laminated glass has a plastic sandwiched by two layers of glass, increasing strength and glass fibers in a resin form a useful composite, in other words a mix or combination of materials, for making strong but light structures such boat and car bodies for example.
The Sand Glass
Five thousand years ago, on some beach in the Middle East, someone probably lit a fire and later found shiny, transparent globules like jewels among the sand. So how were these new discoveries transformed into one of the major household and building materials of the modern world - glass?
The raw material from which glass is made is silica, the most abundant of all the earth's minerals. Milky white in color, it is found in many forms of rock, including granite. And as every beach in the world has been formed by water pounding rocks into tiny particles, sand is the major source of silica.
When you are next at the seaside, try examining a handful of sand. Any grain which is partially transparent, rather than black, yellow, red or some other definite color, is a grain of silica. You may notice it when examining it. Sand also contains other minerals, but silica is the main component because it is hard, insoluble and does not decompose, so it's durable than the others.
Pure silica has such a high melting point that no ordinary fire would convert it into glass. So the first Middle Eastern glass-makers must have lit their fire on sand which was impregnated with soda (compounds of sodium) left behind by evaporated water from a lake or sea. The soda reduces silica's melting point. Today, lime and soda are combined with silica to produce soda-lime glass, used for making bottles, window panes and standard drinking glasses. When glass cools, its structure does not return to the crystalline structure of silica, which is opaque. Instead, it forms a disordered structure rather like a frozen liquid, which is transparent.
Other materials may be added to provide color, or to improve the quality of the finished glass. Glass containing 10-15 percent of boric (boracic) oxide, for example, is resistant to sudden heating or cooling and is used for ovenware. Adding lead oxide, a technique discovered in the 17th century, produces a heavy glass with a brilliant glitter - lead crystal.
Modern sheet glass is made by heating the mixed ingredients in long tanks. The mixture always contains broken glass, known as cullet, a term which also means recycled waste glass for use in glass making. The cullet then melts at a lower temperature than the other materials and helps them to combine thoroughly. As newly made glass is taken out from one end of the tank, in a sheet up to 10ft (3m) wide, raw materials are poured in at the other, so that the level in the tank always remains constant. The tanks are lined with heat-proof bricks and remain in continuous production for as long as their linings last, which may be several years.
Laminated Bulletproof Glass
Facts on the Bulletproof Glass
Bullet-proof glass can be stronger than steel. Mainly, glass is thought of as a fragile material, but actually it is very strong and tough. If it is pulled lengthways, a flawless fiber of glass is five times stronger than the best steel ever made. Glass fibers set in plastic produce a tough and resilient material suitable for boats or car bodies called glass-fiber reinforced plastic, or GRP.
Extra-strong glass is produced by heat toughening or by lamination. In toughening, the glass is heated to just below its melting point, then suddenly chilled with jets of air. This makes the surface of the glass cool and shrink before the inner part. As a result, the surface is compressed inwards. This built-in compression has to be overcome before the toughened glass will break. So toughened glass can be bent more, or struck harder, before it breaks. When it does, it disintegrates into tiny fragments, rather than the dangerous shards of ordinary glass.
Laminated glass is a sandwich of two layers of glass and one of plastic. Although the plastic layer may be very thin, it is tough. Impacts may shatter the glass, but it will remain sticking to the plastic and does not form splinters, which makes it particularly suitable for the windscreens of cars. Aircraft windscreens must be able to withstand high pressure, extreme temperatures and impacts from flying birds. Three or four layers of glass are interleaved with layers of vinyl, then bonded together. This produces a windscreen which is able to withstand the impact of a large bird while the plane is flying at up to 400mph (650km/h). The same glass also gives the pilots of military aircraft protection against bullets.