Glass is an inorganic product of fusion that has cooled to a rigid condition without crystallizing. Its basic ingredients are silica (from sand, which has a melting point of over 2,000 °C), calcium monoxide (extracted from limestone), and sodium monoxide (from soda ash, or sodium carbonate, which lowers the melting point of silica to about 1,000 °C). In addition to the basic raw materials, small quantities of feldspar and salt cake are used to produce soda-lime glass. Feldspar provides a source of metallic oxide and retards devitrification of the glass, whereas salt cake acts as a “sponge” in furnaces by absorbing “scum.”
Some types of glass use other ingredients, such as oxides of aluminum lead, and boron. A material that is always included in the manufacture of glass is cullet, composed of crushed glass from imperfect production, trim, and other in-house waste glass. Cullet aids the melting process because it liquefies at a lower temperature than the virgin materials, providing a molten bath that helps transfer heat to other materials. To assure themselves of its composition and purity, manufacturers generally prefer using internally generated cullet over purchased cullet.
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