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Glass, hard, brittle, transparent substance composed mainly of silicates. A natural black glass called obsidian occurs when the molten rock from an erupting volcano cools rapidly, androck crystals, made of quartz, are another type of naturally occurring glass. The ancient Egyptians and Romans manufactured glass, and stained glass became important in medieval art and architecture in Europe. The mass production of glass is carried out in long “tank” furnaces that may hold over 100 tons. The raw materials are melted and fused at high temperatures, then poured into molds or drawn into shape. For glassware of the highest quality, handblowing is still practiced. The glassblower dips a long blowpipe into the furnace and withdraws a “gob” of molten glass that is blown down the pipe and inflated like a balloon. By deft manipulation of the pipe, shapes are formed. Bottles are blown mechanically. Ordinary window glass, or sheet glass, is made by drawing a wide ribbon of glass upwards from the furnace and through tall cooling towers, at the top of which it is cut into sheets. After glass has been shaped, it usually has to be reheated and cooled at a carefully controlled rate in a special oven called a leer. This process, called annealing, reduces the stress produced in the glass by uneven cooling.

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