Ceramic and Glass Materials

7 Clays 127

and low-volume hand techniques such as throwing and wheel turning [27]. The imposed

pressure range for soft plastic forming is 0.1–0.75 MPa (15–110 psi) [26]. The soft plastic

forming techniques of jiggering and jollying are used to form objects that have a center

of radial symmetry conducive to forming by rotation, such as dinner plates, cups, mugs,

and flowerpots. During jiggering, the plastic mass is placed on a rotating form that

determines the profile of the product and a tool is brought down to cut away excess

material from the back (Fig. 11a) [31]. Jollying is similar to jiggering, except that in this

case the tool determines the inner profile of the product and the form serves as a physical

support [26]. Typically, objects such as dinner plates are formed by jiggering, while

objects such as teacups and mugs are formed by jollying. Similarly, ram pressing

employs two forms that are pressed together, but without rotating tooling [26]. Ram

pressing can be used to form a variety of shapes including dinner plates (Fig. 11b).

3.5 Casting

Slip casting is used to produce clay-based ceramics from clay–water slurries contain-

ing 25 wt% water or more [26]. For casting slips, the water content is high enough so

that all of the particles in the system are separated by free water. Most often, slip cast-

ing requires no applied pressure, although many industrial shops have switched to

pressure casting (slip casting with an applied pressure) to improve productivity and

reproducibility. Slip casting requires a well-dispersed, stable suspension of ceramic

particles and a porous mold, which is most often gypsum (hydrated plaster of Paris)

[22]. When the slurry is poured into the mold, the pores in the mold draw water out

of the slurry, causing particles to deposit on the mold surface (Fig. 12) [15]. When the

cast layer has sufficient thickness, the excess slip is poured out, leaving a thin, nega-

tive replica of the mold. The replica is partially dried in the mold until it pulls away

Fig. 10 Augur (a) and piston (b) type extruders used for stiff plastic forming of clay-based ceramics.
(a reproduced by permission of John Wiley from W.D. Kingery, Introduction to Ceramics, 1st
Edition, John Wiley, New York, 1960; b reproduced by permission of the McGraw-Hill Companies
from F.H. Norton, Fine Ceramics, McGraw Hill, New York, 1970) [25,3]