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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
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.
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
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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