GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59
14.7 Fabrication and Processing of Glasses and Glass–Ceramics • 591400200 400 600 800 1000 1200 1400 1600 1800800 1200 1600 2000 2400 2800 3200
10181016101410121010108106104102110161014101210101081061041021Borosilicate
glass 96% silica
glassFused
silica
Strain pointAnnealing pointSoftening pointWorking pointMelting point
Soda-lime glassWorking rangeViscosity (Pa-s) Viscosity (P)Temperature (°F)Temperature (°C)Figure 14.17
Logarithm of
viscosity versus
temperature for
fused silica and three
silica glasses. (From
E. B. Shand,
Engineering Glass,
Modern Materials,
Vol. 6, Academic
Press, New York,
1968, p. 262.)Of course, the temperature at which each of these points occurs depends on
glass composition. For example, the softening points for soda–lime and 96% silica
glasses from Figure 14.17 are about 700 and 1550◦C (1300 and 2825◦F), respectively.
That is, forming operations may be carried out at significantly lower temperatures
for the soda–lime glass. The formability of a glass is tailored to a large degree by its
composition.Glass Forming
Glass is produced by heating the raw materials to an elevated temperature above
which melting occurs. Most commercial glasses are of the silica–soda–lime variety;
the silica is usually supplied as common quartz sand, whereas Na 2 O and CaO are
added as soda ash (Na 2 CO 3 ) and limestone (CaCO 3 ). For most applications, espe-
cially when optical transparency is important, it is essential that the glass product
be homogeneous and pore free. Homogeneity is achieved by complete melting and
mixing of the raw ingredients. Porosity results from small gas bubbles that are pro-
duced; these must be absorbed into the melt or otherwise eliminated, which requires
proper adjustment of the viscosity of the molten material.
Five different forming methods are used to fabricate glass products: pressing,
blowing, drawing, and sheet and fiber forming. Pressing is used in the fabrication of
relatively thick-walled pieces such as plates and dishes. The glass piece is formed by
pressure application in a graphite-coated cast iron mold having the desired shape;
the mold is ordinarily heated to ensure an even surface.
Although some glass blowing is done by hand, especially for art objects, the
process has been completely automated for the production of glass jars, bottles, and
light bulbs. The several steps involved in one such technique are illustrated in Figure
14.18. From a raw gob of glass, aparison, or temporary shape, is formed by mechanical