The Glass Making Process

Written by michelle miley Google
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The Glass Making Process
("Colourful Bottle" Photo by Matthew Bowden (www.digitallyrefreshing.com))

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Melting the Ingredients

Glass can be made from three ingredients. Silica, the most prevalent ingredient in the Earth's crust, is found as quartz and sand. Soda ash, or sodium carbonate, is also used to make glass. This material can be obtained from the ashes of certain plants and from processing salt. Calcium oxide, also known as lime, is another key ingredient of glass and is found in limestone. Silica, soda ash and lime are the main ingredients in glass, but many other chemicals can be added to the mixture to create certain properties. For example, barium is sometimes used to increase glass' brilliance, and boron can be added to increase heat resistance. Chemicals that reduce the melting point of the silica are also frequently used. Sometimes recycled glass, called cullet, is also added. The desired ingredients are mixed and then melted into liquid in large furnaces.

Blowing, Pressing, and Drawing

Once glass has been melted, it can be shaped and used in various ways. When blowing glass, a ball of melted glass is placed on the end of a hollow iron tube. Air is then blown into the tube to shape the glass on the other end. Air can be blown in manually or by machine. The glass is reheated often during this process so that it stays pliable. When the glass blower is finished with the piece, it is allowed to cool and is then broken free from the blowing tube. In the pressing method of glass making, the liquid glass is poured into a mould and then pressed into shape by either a machine or a special tool. This is how bowls and ashtrays are made. Glass drawing is used to make flat and tubular pieces, such as glass for windows and test tubes. To make the glass flat, it is placed into a tank along with melted tin. The glass floats on top of the tin. Because the tin has a very smooth surface, the glass floats on top and forms sheets. Tube-shaped glass is poured into a spinning mould. Air is blown through the mould as it spins to keep the cooling glass on the outside edge of the tube.

Annealing and Tempering

Once the glass is finished, it goes through an annealing or tempering process to increase its strength. The annealing process involves making the glass hot again and then allowing it to cool slowly. When glass is tempered it is also reheated, but tempered glass is cooled very quickly with air. Depending on the desired properties, the finished glass can be coated with different materials to further increase strength, reduce glare or add other desired traits.

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