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In order to better understand the glass and glazing industry, a brief history of glass may be helpful.

Glass was discovered over 4000 years ago. It was considered precious and used by royalty and for religious purposes. During the Roman Empire, glass making reached a high degree of quality and use, but declined significantly during the Middle Ages when the main achievement was “stained glass.” In the 7th century, Syrians developed the “crown” method for forming flat glass, whereby the molten glass was taken in lump form and spun on a cylindrical disc to flatten the glass. Interestingly, this represented the most common method to produce flat glass for the next 1000 years.

In the early part of the 20th century, inexpensive sheet glass was formed by drawing the glass ribbon vertically out of the molten glass pool. Unfortunately, sheet glass still suffered from distortion because of the differences in viscosity of the molten glass. In order to obtain relatively distortion-free glass for use in coach windows or mirrors, the plate glass process was developed. Plate glass was made by pouring molten glass onto a table and rolling it until flattened, then grinding and polishing it into a plate. This process eventually advanced by feeding the molten glass though continuous rollers, grinders and polishers. Sheet glass is no longer commercially produced in the United States.

In 1959, the float glass process was introduced. This unique glass making process revolutionized the flat glass industry. In the float process, molten glass from the furnace flows by gravity and displacement onto a bath of molten tin where a continuous ribbon is formed. This glass ribbon is pulled or drawn through the tin bath and upon exiting is guided on rollers through an annealing lehr where it is cooled, under controlled conditions, until it emerges at essentially room temperature. The product is now flat, fire-finished and has virtually parallel surfaces. Automatic cutters generally are used to trim the edges and cut across the width of the moving ribbon. This creates sizes, which can be shipped or handled for further processing. The float glass process accounts for almost all of the flat glass presently produced in the United States.

Commercial float glass is nearly colorless with a visible light transmittance ranging from 75% to 92% depending on thickness. With the exception of specialty low-iron glass, a faint green or blue-green color may be noticeable in glazing applications where the glass thickness approaches or exceeds 3/8" (10 mm). Specialty low-iron glass has a higher visible transmittance than commercial float glass of the same thickness.

Tinted or Heat-Absorbing Glass is made by adding various colorants to the normal, clear glass batch to create a desired color. The typical colors produced domestically include bronze, gray, dark gray, aquamarine, green, deep green, blue, deep blue and black. Some companies in Europe produce other colors, for instance rose and emerald green. Visible light transmittance will vary from 14% to 85%, depending on color and thickness. The color density is also a function of thickness. As the thickness increases, visible light transmittance will decrease.

Tinting reduces the solar transmittance of glass and increases solar heat absorption. Because of this heat buildup, heat-treating (heat-strengthening or tempering) is sometimes required for tinted glass.

Color of tinted/heat-absorbing glass is a major consideration for either design and aesthetic reasons or for color matching requirements. Tinted heat-absorbing glass should be viewed as installed for color comparison. Colors may vary considerably among different manufacturers and from run to run. No published color standard exists; the manufacturer should be consulted for color information.

Annealed Glass
Upon exiting the tin bath, the float glass ribbon is guided on rollers through an annealing lehr where it is cooled, under controlled conditions, until it emerges at essentially room temperature. The product is now flat, fire-finished and has virtually parallel surfaces. Automatic cutters generally are used to trim the edges and cut across the width of the moving ribbon. This creates sizes that can be shipped or handled for further processing. This glass is referred to as annealed glass.

Annealed glass may be used in its original state or it can be further fabricated by cutting, heat-treating, coating, laminating, or insulating. Annealed glass provides the least resistance to mechanical and thermal stresses when compared with heat-strengthened and fully tempered glass.

Industry production quality requirements, product tolerances and test procedures for annealed glass are defined in the ASTM International (ASTM) document C 1036 Standard Specification for Flat Glass.

The above information is from the GANA Glazing Manual, 2004 Edition - the most frequently referenced resource in the architectural glass and glazing industry. The Glazing Manual is an excellent addition to any technical library. Go here to order a copy of the manual or CD-ROM. For further information on this and other GANA reference documents visit the PUBLICATIONS section of the GANA website.

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Last modified: 11/09/08