How Glass is Shaped and Finished

There are four main methods of shaping glass: blowing, pressing, drawing, and casting. After the shaping process, annealing is used to increase the strength of the glass. Tempering and other finishing techniques may also be used to further strengthen the glass. At the cold end of the plant, finished glass products are inspected and boxed for sale. The glass may also be decorated before packaging. See the section How glass is decorated later in this article.

Pressing is accomplished by dropping a hot gob of glass into a mold, then pressing it with a plunger until it spreads and fills the inside of the mold. To be pressed, an article must be of such a shape that the plunger can be withdrawn. Baking dishes, glass blocks, and lenses are often pressed. As with blowing, pressing can be done by hand or by machine, and with single or multiple molds. Press-and-blow machines use a combination of the pressing and the blowing methods to form the article. Such machines can produce hundreds of glass containers per hour.

Drawing is the method used for shaping flat glass, glass rod, glass tubing, and fiberglass. Almost all flat glass produced today is float glass. It is shaped by drawing a wide sheet of molten glass into a furnace containing a bath of molten tin. This furnace is called a float bath because the glass “floats” in an even layer on the perfectly smooth surface of the molten tin. Heating in the float bath is carefully controlled to melt out any roughness in the glass. Because glass turns solid at a higher temperature than tin, it can be moved from the molten tin for further cooling. When flat glass is shaped in a float bath, both sides come out with a brilliant finish that requires no grinding or polishing.

Glass rod is made by drawing a stream of molten glass out of the furnace. Tubing is made by drawing the molten glass around a rotating cylinder or cone called a mandrel. Air blowing through the mandrel causes the glass to form a continuous tube. Fiberglass is made by drawing the molten glass through tiny holes in the bottom of the furnace.

Casting involves filling molds with molten glass. The glass may be poured either from ladles or directly from the furnace, or drained from the bottom of the furnace. Casting is used in the production of architectural glass pieces, art glass, laser glass, and telescope mirrors.

Annealing is a process that removes the stresses and strains remaining in glass after shaping. Most glassware is annealed just after it has been formed. If it is not annealed, glass may shatter from tension caused by uneven cooling. Annealing is done by reheating the glass and gradually cooling it according to a planned time-and-temperature schedule.

Tempering is a process in which a glass article that is already formed is reheated until almost soft. Then, under carefully controlled conditions, it is chilled suddenly by blasts of cold air or by plunging it in oil or certain chemicals in a liquid state. This tempering treatment makes the glass much stronger than ordinary glass. Glass articles can also be tempered with chemicals.

Inspection. In almost every glass plant, engineers take frequent samples directly from the furnace and test them for quality and desired properties. Samples of finished glass products also are tested for size, proper annealing, and other qualities.


Contributor:
• Steve W. Martin, Ph.D., Professor of Materials Science and Engineering, Iowa State University.

Steve W. Martin, "Glass," Discovery Channel School, original content provided by World Book Online, http://www.discoveryschool.com/homeworkhelp/
worldbook/atozscience/g/225740.html
, August 2001.

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