GTBL042-14 GTBL042-Callister-v2 August 29, 2007 8:59
14.8 Fabrication and Processing of Clay Products • 59516001250900800650Working ptSoftening ptAnnealing ptTi m eMeltingFormingNucleationGrowthViscosity (Pa-s)T(°C)~2.5 × 1013~4 × 107~10^3Figure 14.21 Typical time-versus-temperature processing cycle for a Li 2 O–Al 2 O 3 –SiO 2
glass–ceramic. (Adapted from Y. M. Chiang, D. P. Birnie, III, and W. D. Kingery,Physical
Ceramics—Principles for Ceramic Science and Engineering. Copyright©c1997 by John Wiley
& Sons, New York. Reprinted by permission of John Wiley & Sons, Inc.)maintained. This fusion temperature range, of course, depends on the composition
of the clay.
Clays are aluminosilicates, being composed of alumina (Al 2 O 3 ) and silica (SiO 2 )
that contain chemically bound water. They have a broad range of physical characteris-
tics, chemical compositions, and structures; common impurities include compounds
(usually oxides) of barium, calcium, sodium, potassium, and iron, and also some
organic matter. Crystal structures for the clay minerals are relatively complicated;
however, one prevailing characteristic is a layered structure. The most common clay
minerals that are of interest have what is called the kaolinite structure. Kaolinite
clay [Al 2 (Si 2 O 5 )(OH) 4 ] has the crystal structure shown in Figure 3.14. When water
is added, the water molecules fit in between these layered sheets and form a thin
film around the clay particles. The particles are thus free to move over one another,
which accounts for the resulting plasticity of the water–clay mixture.Compositions of Clay Products
In addition to clay, many of these products (in particular the whitewares) also contain
some nonplastic ingredients; the nonclay minerals include flint, or finely ground
quartz, and a flux such as feldspar.^3 The quartz is used primarily as a filler material,
being inexpensive, relatively hard, and chemically unreactive. It experiences little
change during high-temperature heat treatment because it has a melting temperature
well above the normal firing temperature; when melted, however, quartz has the
ability to form a glass.
When mixed with clay, a flux forms a glass that has a relatively low melting point.
The feldspars are some of the more common fluxing agents; they are a group of
aluminosilicate materials that contain K+,Na+, and Ca^2 +ions.
As would be expected, the changes that take place during drying and firing
processes, and also the characteristics of the finished piece, are influenced by the
proportions of these three constituents: clay, quartz, and flux. A typical porcelain
might contain approximately 50% clay, 25% quartz, and 25% feldspar.(^3) Flux, in the context of clay products, is a substance that promotes the formation of a glassy
phase during the firing heat treatment.