Callister - An introduction - 8th edition

13.10 Fabrication and Processing of Clay Products • 519
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 12.14. When water is added, the water molecules fit 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.
hydroplastic forming
slip casting
Fabrication Techniques
The as-mined raw materials usually have to go through a milling or grinding operation
in which particle size is reduced; this is followed by screening or sizing to yield
a powdered product having a desired range of particle sizes. For multicomponent
systems, powders must be thoroughly mixed with water and perhaps other ingredients
to give flow characteristics that are compatible with the particular forming technique.
The formed piece must have sufficient mechanical strength to remain intact
during transporting, drying, and firing operations. Two common shaping techniques
are used to form clay-based compositions: hydroplastic forming and slip casting.
Hydroplastic Forming
As mentioned previously, clay minerals, when mixed with water, become highly plastic
and pliable and may be molded without cracking; however, they have extremely
low yield strengths.The consistency (water–clay ratio) of the hydroplastic mass must
give a yield strength sufficient to permit a formed ware to maintain its shape during
handling and drying.
The most common hydroplastic forming technique is extrusion, in which a stiff
plastic ceramic mass is forced through a die orifice having the desired cross-sectional
geometry; it is similar to the extrusion of metals (Figure 11.8c). Brick, pipe, ceramic
blocks, and tiles are all commonly fabricated using hydroplastic forming. Usually
the plastic ceramic is forced through the die by means of a motor-driven auger, and
often air is removed in a vacuum chamber to enhance the density. Hollow internal
columns in the extruded piece (e.g., building brick) are formed by inserts situated
within the die.
3
Flux, in the context of clay products, is a substance that promotes the formation of a
glassy phase during the firing heat treatment.