Complete the following text by changing the words in the brackets in suitable form of active or passive! Glass (can make) from three ingredients: silica, soda ash, and calcium oxide which is known as lime. Silica, soda ash and lime (be) the main ingredients in glass, but many other chemicals (can add) 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. Once glass (has melt), 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 (blow)into the tube to shape the glass on the other end. Air can be blown in manually or by machine. The glass (reheat) 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 (pour) into a mold and then pressed into shape by either a machine or a special tool. This is how bowls and ashtrays are made. Glass drawing (use)to make flat and tubular pieces, such as glass for windows and test tubes. To make the glass flat, it (place) into a tank along with melted tin. The glass floats on top of the tin. Because the tin (have) a very smooth surface, the glass (float) on top and (form) sheets. Tube-shaped glass is poured into a spinning mold. Air is blown through the mold as it spins to keep the cooling glass on the outside edge of the tube. Once the glass is finished, it (go) through an annealing or tempering process to increase its strength. The annealing process (involve) 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 coat) with different materials to further increase strength, reduce glare or add other desired traits.
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can be added
has been melted
blowed
reheats
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floats
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can be coated
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