Callister - An introduction - 8th edition

516 • Chapter 13 / Applications and Processing of Ceramics
Combustion
gases
Controlled
atmosphere
Raw
materials
Heater
Molten glass
Melting furnace
Heating
zone
Liquid tin
Fire
polishing
zone
Cooling
zone
Annealing
furnace (Lehr)
Cutting
section
Float Bath Furnace
Figure 13.9 Schematic diagram showing the float process for making sheet glass.
(Courtesy of Pilkington Group Limited.)
mechanical pressing in a mold. This piece is inserted into a finishing or blow mold
and forced to conform to the mold contours by the pressure created from a blast of air.
Drawing is used to form long glass pieces such as sheet, rod, tubing, and fibers,
which have a constant cross section.
Up until the late 1950s, sheet glass (or plate) was produced by casting (or drawing)
the glass into a plate shape, grinding both faces to make them flat and parallel,
and finally, polishing the faces to make the sheet transparent—a procedure that
was relatively expensive. A more economical float process was patented in 1959 in
England. With this technique (represented schematically in Figure 13.9), the molten
glass passes (on rollers) from one furnace onto a bath of liquid tin located in a second
furnace. Thus, as this continuous glass ribbon “floats” on the surface of the
molten tin, gravitational and surface tension forces cause the faces to become
perfectly flat and parallel and the resulting sheet to be of uniform thickness.
Furthermore, sheet faces acquire a bright “fire-polished” finish in one region of
the furnace. The sheet next passes into an annealing furnace (lehr), and is finally
cut into sections (Figure 13.9). Of course, the success of this operation requires rigid
control of both temperature and chemistry of the gaseous atmosphere.
Continuous glass fibers are formed in a rather sophisticated drawing operation.
The molten glass is contained in a platinum heating chamber. Fibers are formed by
drawing the molten glass through many small orifices at the chamber base.The glass
viscosity, which is critical, is controlled by chamber and orifice temperatures.
thermal shock
Heat-Treating Glasses
Annealing
When a ceramic material is cooled from an elevated temperature, internal stresses,
called thermal stresses, may be introduced as a result of the difference in cooling
rate and thermal contraction between the surface and interior regions. These thermal
stresses are important in brittle ceramics, especially glasses, because they may
weaken the material or, in extreme cases, lead to fracture, which is termed thermal
shock (see Section 19.5). Normally, attempts are made to avoid thermal stresses,
which may be accomplished by cooling the piece at a sufficiently slow rate. Once
such stresses have been introduced, however, elimination, or at least a reduction in
their magnitude, is possible by an annealing heat treatment in which the glassware
is heated to the annealing point, then slowly cooled to room temperature.