Glass has been made since 3500 BC, with the traditional method still using sand, soda charcoal, and limestone lime. Soda ash was added to lower melting points, but lime is now used for greater chemical stability. Glass is shaped by blowing, pressing, or vacuuming, and annealed for strength. Float glass is the largest production method, and specialized glass includes lead, heat resistant, and stained glass.
The glass making process can be traced back to 3500 BC when pottery was coated with a glass surface glaze. While there are many modern variations on the process of making glass for industrial and scientific uses, the traditional glass used to make windows, food and drink containers, and ornamental shapes is still made with the same three ingredients used in ancient times. These are sand, alkali-based baking soda referred to as “soda charcoal,” and limestone lime. These three main ingredients are fused together in a furnace at a temperature of approximately 2,500° Fahrenheit (1,371° Celsius), after which various minor ingredients may be added to alter the color or brilliance of the glass. It is then cooled to several hundred degrees so it can be shaped by blowing, pressing or vacuuming into pipes, bowls, mirrors, windows and more.
The introduction of soda ash into the glassmaking process was discovered early on as a way to lower the initial melting point from a peak of about 4,100° Fahrenheit (2,200° Celsius) to 2,700° Fahrenheit (1,482° Celsius). Soda ash however makes glass somewhat soluble in water so lime has been added from limestone to prevent this and other chemicals such as aluminum oxide are now being replaced by lime as they offer greater chemical stability. Soda-lime glass still accounts for 90% of all glass produced.
When glass is cooled, it becomes easier to shape, but it must be continuously heated during the glassmaking process to add strength and prevent shattering. This procedure is known as annealing. A further tempering process is also employed to strengthen the glass by heating it and then suddenly cooling it with jets of cold air.
The most common shaping method in the ancient glassmaking process was to blow air into a ball of molten glass at the end of a long hollow tube, which the Romans perfected to make beautiful vases and bowls. Float glass manufacturing, invented in 1959 by a British company, is the largest current production method by pouring molten glass into a tank with molten tin on the surface. The glass floats on top of the tin and forms into extremely smooth sheets which can then be cut into the shape of a window. Dolomite is often included in the manufacture of glass plates at levels below 0.1% to maintain the colorless quality of the glass. Drawing a stream of molten glass around the inside of a metal cylinder as it rotates and air is pumped inside, is another frequent glass making process used to make a wide variety of glass tubes for light bulbs, tubes vacuum cleaners and glassware used in laboratories.
Other types of specialized glass include lead glass, which contains at least 20% lead oxide and is brilliant when cut and faceted. Heat resistant glass products used in kitchen bowls and laboratory ware are made by adding 5% boric oxide or aluminosilicate compounds. Stained glass products are made by adding iron sulphides for brown and amber colors, iron chromate for green hues, and cobalt white for blue hues. Natural blown glass has a red to orange color without the need for chemical additives.
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